Astronomical Curiosities: Facts and Fallacies
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Title: Astronomical Curiosities: Facts and Fallacies
Author: J. Ellard Gore
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ASTRONOMICAL CURIOSITIES
FACTS AND FALLACIES
ASTRONOMICAL CURIOSITIES
FACTS AND FALLACIES
BY J. ELLARD GORE
MEMBER OF THE ROYAL IRISH ACADEMY FELLOW OF THE
ROYAL ASTRONOMICAL SOCIETY CORRESPONDING MEMBER
OF THE ROYAL ASTRONOMICAL SOCIETY OF CANADA ETC.
AUTHOR OF "ASTRONOMICAL ESSAYS," "STUDIES IN
ASTRONOMY," "THE VISIBLE UNIVERSE," ETC.
LONDON
CHATTO & WINDUS
1909
PRINTED BY
WILLIAM CLOWES AND SONS, LIMITED
LONDON AND BECCLES
_All rights reserved_
PREFACE
The curious facts, fallacies, and paradoxes contained in the following
pages have been collected from various sources. Most of the information
given will not, I think, be found in popular works on astronomy, and will,
it is hoped, prove of interest to the general reader.
J. E. G.
_September, 1909._
CONTENTS
PAGE
CHAPTER
I. THE SUN 1
II. MERCURY 10
III. VENUS 17
IV. THE EARTH 32
V. THE MOON 48
VI. MARS 59
VII. THE MINOR PLANETS 68
VIII. JUPITER 74
IX. SATURN 84
X. URANUS AND NEPTUNE 91
XI. COMETS 97
XII. METEORS 117
XIII. THE ZODIACAL LIGHT AND GEGENSCHEIN 127
XIV. THE STARS 135
XV. DOUBLE AND BINARY STARS 160
XVI. VARIABLE STARS 170
XVII. NEBULÆ AND CLUSTERS 191
XVIII. HISTORICAL 217
XIX. THE CONSTELLATIONS 239
XX. THE VISIBLE UNIVERSE 313
XXI. GENERAL 329
INDEX 359
ILLUSTRATIONS
PAGE
AL-SUFI'S "EARTHEN JAR" 247
AL-SUFI'S "FISHES" IN ANDROMEDA 249
ASTRONOMICAL CURIOSITIES
CHAPTER I
The Sun
Some observations recently made by Prof. W. H. Pickering in Jamaica, make
the value of sunlight 540,000 times that of moonlight. This makes the
sun's "stellar magnitude" minus 26·83, and that of moonlight minus 12·5.
Prof. Pickering finds that the light of the full moon is equal to 100,000
stars of zero magnitude. He finds that the moon's "albedo" is about
0·0909; or in other words, the moon reflects about one-tenth of the light
which falls on it from the sun. He also finds that the light of the full
moon is about twelve times the light of the half moon: a curious and
rather unexpected result.
M. C. Fabry found that during the total eclipse of the sun on August 30,
1905, the light of the corona at a distance of five minutes of arc from
the sun's limit, and in the vicinity of the sun's equator, was about 720
candle-power. Comparing this with the intrinsic light of the full moon
(2600 candle-power) we have the ratio of 0·28 to 1. He finds that the
light of the sun in the zenith, and at its mean distance from the earth,
is 100,000 times greater than the light of a "decimal candle" placed at a
distance of one metre from the eye.[1] He also finds that sunlight is
equal to 60,000 million times the light of Vega. This would make the sun's
"stellar magnitude" minus 26·7, which does not differ much from Prof.
Pickering's result, given above, and is probably not far from the truth.
From experiments made in 1906 at Moscow, Prof. Ceraski found that the
light of the sun's limb is only 31·4 to 38·4 times brighter than the
illumination of the earth's atmosphere very near the limb. This is a very
unexpected result; and considering the comparative faintness of the sun's
corona during a total eclipse, it is not surprising that all attempts to
photograph it without an eclipse have hitherto failed.[2]
From Paschen's investigations on the heat of the sun's surface, he finds a
result of 5961° (absolute), "assuming that the sun is a perfectly black
body."[3] Schuster finds that "There is a stratum near the sun's surface
having an average temperature of approximately 5500° C., to which about
0·3 of the sun's radiation is due. The remaining portion of the radiation
has an intensity equal to that due to a black body having a temperature of
about 6700° C." The above results agree fairly well with those found by
the late Dr. W. E. Wilson.[4] The assumption of the sun being "a black
body" seems a curious paradox; but the simple meaning of the statement is
that the sun is assumed to act as a radiator as _if it were a perfectly
black body heated to the high temperature given above_.
According to Prof. Langley, the sun's photosphere is 5000 times brighter
than the molten metal in a "Bessemer convertor."[5]
Observations of the sun even with small telescopes and protected by dark
glasses are very dangerous to the eyesight. Galileo blinded himself in
this way; Sir William Herschel lost one of his eyes; and some modern
observers have also suffered. The present writer had a narrow escape from
permanent injury while observing the transit of Venus, in 1874, in India,
the dark screen before the eyepiece of a 3-inch telescope having
blistered--that is, partially fused during the observation. Mr. Cooper,
Markree Castle, Ireland, in observing the sun, used a "drum" of alum water
and dark spectacles, and found this sufficient protection against the
glare in using his large refracting telescope of 13·3-inches aperture.
Prof. Mitchell, of Columbia University (U.S.A.), finds that lines due to
the recently discovered atmospherical gases argon and neon are present in
the spectrum of the sun's chromosphere. The evidence for the existence of
krypton and xenon is, however, inconclusive. Prof. Mitchell suggests that
these gases may possibly have reached the earth's atmosphere from the sun.
This would agree with the theory advanced by Arrhenius that "ionised
particles are constantly being repulsed by the pressure of light, and thus
journey from one sun to another."[6]
Prof. Young in 1870, and Dr. Kreusler in June, 1904, observed the helium
line D_{3} as a _dark_ line "in the spectrum of the region about a
sun-spot."[7] This famous line, from which helium was originally
discovered in the sun, and by which it was long afterwards detected in
terrestrial minerals, usually appears as a _bright_ line in the spectrum
of the solar chromosphere and "prominences." It has also been seen _dark_
by Mr. Buss in sun-spot regions.[8]
The discovery of sun-spots was claimed by Hariotte, in 1610, and by
Galileo, Fabricius, and Scheiner, in 1611. The latter wrote 800 pages on
them, and thought they were small planets revolving round the sun! This
idea was also held by Tardè, who called them _Astra Borbonia_, and by C.
Malapert, who termed them _Sydera Austricea_. But they seem to have been
noticed by the ancients.
Although in modern times there has been no extraordinary development of
sun-spots at the epoch of maximum, it is not altogether impossible that in
former times these spots may have occasionally increased to such an
extent, both in number and size, as to have perceptibly darkened the sun's
light. A more probable explanation of recorded sun-darkenings seems,
however, to be the passing of a meteoric or nebulous cloud between the sun
and the earth. A remarkable instance of sun-darkening recorded in Europe
occurred on May 22, 1870, when the sun's light was observed to be
considerably reduced in a cloudless sky in the west of Ireland, by the
late John Birmingham; at Greenwich on the 23rd; and on the same date, but
at a later hour, in North-Eastern France--"a progressive manifestation,"
Mr. Birmingham says, "that seems to accord well with the hypothesis of
moving nebulous matter." A similar phenomenon was observed in New England
(U.S.A.), on September 6, 1881.
One of the largest spots ever seen on the sun was observed in June, 1843.
It remained visible for seven or eight days. According to Schwabe--the
discoverer of the sun-spot period--its diameter was 74,000 miles, so that
its area was many times that of the earth's surface. The most curious
thing about this spot was that it appeared near a _minimum_ of the
sun-spot cycle! and was therefore rather an anomalous phenomenon. It was
suggested by the late Daniel Kirkwood that this great spot was caused by
the fall of meteoric matter into the sun; and that it had possibly some
connection with the great comet of 1843, which approached the sun nearer
than any other recorded comet, its distance from the sun at perihelion
being about 65,000 miles, or less than one-third of the moon's distance
from the earth. This near approach of the comet to the sun occurred about
three months before the appearance of the great sun-spot; and it seems
probable that the spot was caused by the downfall of a large meteorite
travelling in the wake of the comet.[9] The connection between comets and
meteors is well known.
The so-called blackness of sun-spots is merely relative. They are really
very bright. The most brilliant light which can be produced artificially
looks like a black spot when projected on the sun's disc.
According to Sir Robert Ball a pound of coal striking a body with a
velocity of five miles a second would develop as much heat as it would
produce by its combustion. A body falling into the sun from infinity would
have a velocity of 450 miles a second when it reached the sun's surface.
Now as the momentum varies as the square of the velocity we have a pound
of coal developing 90{2} (=450/5){2}, or 8,100 times as much heat as would
be produced by its combustion. If the sun were formed of coal it would be
consumed in about 3000 years. Hence it follows that the contraction of the
sun's substance from infinity would produce a supply of heat for 3000 ×
8100, or 24,300,000 years.
The late Mr. Proctor and Prof. Young believed "that the contraction theory
of the sun's heat is the true and only available theory." The theory is,
of course, a sound one; but it may now be supplemented by supposing the
sun to contain a certain small amount of radium. This would bring physics
and geology into harmony. Proctor thought the "sun's real globe is very
much smaller than the globe we see. In other words the process of
contraction has gone on further than, judging from the sun's apparent
size, we should suppose it to have done, and therefore represents more sun
work" done in past ages.
With reference to the suggestion, recently made, that a portion, at least,
of the sun's heat may be due to radium, and the experiments which have
been made with negative results, Mr. R. T. Strutt--the eminent
physicist--has made some calculations on the subject and says, "even if
all the sun's heat were due to radium, there does not appear to be the
smallest possibility that the Becquerel radiation from it could ever be
detected at the earth's surface."[10]
The eminent Swedish physicist Arrhenius, while admitting that a large
proportion of the sun's heat is due to contraction, considers that it is
probably the chemical processes going on in the sun, and not the
contraction which constitute the _chief_ source of the solar heat.[11]
As the centre of gravity of the sun and Jupiter lies at a distance of
about 460,000 miles from the sun's centre, and the sun's radius is only
433,000 miles, it follows that the centre of gravity of the sun and planet
is about 27,000 miles _outside_ the sun's surface. The attractions of the
other planets perpetually change the position of the centre of gravity of
the solar system; but in some books on astronomy it is erroneously stated
that the centre of gravity of the system is _always_ within the sun's
surface. If _all_ the planets lay on the same side of the sun at the same
time (as might possibly happen), then the centre of gravity of the whole
system would lie considerably more than 27,000 miles outside the sun's
surface.
With reference to the sun's great size, Carl Snyder has well said, "It was
as if in Vulcan's smithy the gods had moulded one giant ball, and the
planets were but bits and small shot which had spattered off as the
glowing ingot was cast and set in space. Little man on a little part of a
little earth--a minor planet, a million of which might be tumbled into the
shell of the central sun--was growing very small; his wars, the
convulsions of a state, were losing consequence. Human endeavour, human
ambitions could now scarce possess the significance they had when men
could regard the earth as the central fact of the universe."[12]
With reference to the late Prof. C. A. Young (U.S.A.)--a great authority
on the sun--an American writer has written the following lines:--
"The destined course of whirling worlds to trace,
To plot the highways of the universe,
And hear the morning stars their song rehearse,
And find the wandering comet in his place;
This is the triumph written in his face,
And in the gleaming eye that read the sun
Like open book, and from the spectrum won
The secrets of immeasurable space."[13]
CHAPTER II
Mercury
As the elongation of Mercury from the sun seldom exceeds 18°, it is a
difficult object, at least in this country, to see without a telescope. As
the poet says, the planet--
"Can scarce be caught by philosophic eye
Lost in the near effulgence of its blaze."
Tycho Brahé, however, records several observations of Mercury with the
unaided vision in Denmark.
It can be occasionally caught with the naked eye in this country after
sunset, when it is favourably placed for observation, and I have so seen
it several times in Ireland. On February 19, 1888, I found it very visible
in strong twilight near the western horizon, and apparently brighter than
an average star of the first magnitude would be in the same position. In
the clear air of the Punjab sky I observed Mercury on November 24-29,
1872, near the western horizon after sunset. Its appearance was that of a
reddish star of the first magnitude. On November 29 I compared its
brilliancy with that of Saturn, which was some distance above it, and
making allowance for the glare near the horizon in which Mercury was
immersed, its brightness appeared to me to be quite equal to that of
Saturn. In June, 1874, I found it equal to Aldebaran, and of very much the
same colour. Mr. W. F. Denning, the famous observer of meteors, states
that he observed Mercury with the naked eye about 150 times during the
years 1868 to 1905.[14]
He found that the duration of visibility after sunset is about 1{h} 40{m}
when seen in March, 1{h} 30{m} in April, and 1{h} 20{m} in May. He thinks
that the planet is, at its brightest, "certainly much brighter than a
first magnitude star."[15] In February, 1868, he found that its brightness
rivalled that of Jupiter, then only 2° or 3° distant. In November, 1882,
it seemed brighter than Sirius. In 1876 it was more striking than Mars,
but the latter was then "faint and at a considerable distance from the
earth."
In 1878, when Mercury and Venus were in the same field of view of a
telescope, Nasmyth found that the surface brightness (or "intrinsic
brightness," as it is called) of Venus was at least twice as great as that
of Mercury; and Zöllner found that from a photometric point of view the
surface of Mercury is comparable with that of the moon.
With reference to the difficulty of seeing Mercury, owing to its proximity
to the sun, Admiral Smyth says, "Although Mercury is never in _opposition_
to the earth, he was, when in the house of Mars, always viewed by
astrologers as a most malignant planet, and one full of evil influences.
The sages stigmatized him as a false deceitful star (_sidus dolosum_), the
eternal torment of astronomers, eluding them as much as terrestrial
mercury did the alchemists; and Goad, who in 1686 published a whole folio
volume full of astro-meteorological aphorisms, unveiling the choicest
secrets of nature, contemptuously calls Mercury a 'squinting lacquey of
the sun, who seldom shows his head in these parts, as if he was in debt.'
His extreme mobility is so striking that chemists adopted his symbol to
denote quicksilver."[16]
Prof. W. H. Pickering thinks that the shortness of the cusps (or "horns")
of Mercury's disc indicates that the planet's atmosphere is of small
density--even rarer than that of Mars.
The diameter of Mercury is usually stated at about 3000 miles; but a long
series of measures made by Prof. See in the year 1901 make the real
diameter about 2702 miles. This would make the planet smaller than some of
the satellites of the large planets, probably smaller than satellites III.
and IV. of Jupiter, less than Saturn's satellite Titan, and possibly
inferior in size to the satellite of Neptune. Prof. Pickering thinks that
the density of Mercury is about 3 (water = 1). Dr. See's observations show
"no noticeable falling off in the brightness of Mercury near the limb."
There is therefore no evidence of any kind of atmospheric absorption in
Mercury, and the observer "gets the impression that the physical condition
of the planet is very similar to that of our moon."[17]
Schröter (1780-1815) observed markings on Mercury, from which he inferred
that the planet's surface was mountainous, and one of these mountains he
estimated at about 11 miles in height![18] But this seems very doubtful.
To account for the observed irregularities in the motion of Mercury in its
orbit, Prof. Newcomb thinks it possible that there may exist a ring or
zone of "asteroids" a little "outside the orbit of Mercury" and having a
combined mass of "one-fiftieth to one-three-hundredth of the mass of
Venus, according to its distance from Mercury." Prof. Newcomb, however,
considers that the existence of such a ring is extremely improbable, and
regards it "more as a curiosity than a reality."[19]
M. Léo Brenner thinks that he has seen the dark side of Mercury, in the
same way that the dark side of Venus has been seen by many observers. In
the case of Mercury the dark side appeared _darker_ than the background of
the sky. Perhaps this may be due to its being projected on the zodiacal
light, or outer envelope of the sun.[20]
Mercury is said to have been occulted by Venus in the year 1737.[21] But
whether this was an actual occultation, or merely a near approach does not
seem to be certain.
The first transit of Mercury across the sun's disc was observed by
Gassendi on November 6, 1631, and Halley observed one on November 7, 1677,
when in the island of St. Helena.
Seen from Mercury, Venus would appear brighter than even we see it, and as
it would be at its brightest when in opposition to the sun, and seen on a
dark sky with a full face, it must present a magnificent appearance in the
midnight sky of Mercury. The earth will also form a brilliant object, and
the moon would be distinctly visible. The other planets would appear very
much as they do to us, but with somewhat less brilliancy owing to their
greater distance.
As the existence of an intra-Mercurial planet (that is a planet revolving
round the sun within the orbit of Mercury) seems now to be very
improbable, Prof. Perrine suggests that possibly "the finely divided
matter which produces the zodiacal light when considered in the aggregate
may be sufficient to cause the perturbations in the orbit of Mercury."[22]
Prof. Newcomb, however, questions the exact accuracy of Newton's law, and
seems to adopt Hall's hypothesis that gravity does not act _exactly_ as
the inverse square of the distance, and that the exponent of the distance
is not 2, but 2·0000001574.[23]
Voltaire said, "If Newton had been in Portugal, and any Dominican had
discovered a heresy in his inverse ratio of the squares of the distances,
he would without hesitation have been clothed in a _san benito_, and burnt
as a sacrifice to God at an _auto da fé_."[24]
An occultation of Mercury by Venus was observed with a telescope on May
17, 1737.[25]
May transits of Mercury across the sun's disc will occur in the years
1924, 1957, and 1970; and November transits in the years 1914, 1927, and
1940.[26]
From measurements of the disc of Mercury during the last transit, M. R.
Jonckheere concludes that the _polar_ diameter of the planet is greater
than the _equatorial_! His result, which is very curious, if true, seems
to be supported by the observations of other observers.[27]
The rotation period of Mercury, or the length of its day, seems to be
still in doubt. From a series of observations made in the years 1896 to
1909, Mr. John McHarg finds a period of 1·0121162 day, or 1{d} 0{h} 17{m}
26{s}·8. He thinks that "the planet possesses a considerable atmosphere
not so clear as that of Mars"; that "its axis is very considerably
tilted"; and that it "has fairly large sheets of water."[28]
CHAPTER III
Venus
Venus was naturally--owing to its brightness--the first of the planets
known to the ancients. It is mentioned by Hesiod, Homer, Virgil, Martial,
and Pliny; and Isaiah's remark about "Lucifer, son of the morning" (Isaiah
xiv. 12) probably refers to Venus as a "morning star." An observation of
Venus is found on the Nineveh tablets of date B.C. 684. It was observed in
daylight by Halley in July, 1716.
In _very_ ancient times Venus, when a morning star, was called Phosphorus
or Lucifer, and when an evening star Hesperus; but, according to Sir G. C.
Lewis, the identity of the two objects was known so far back as 540 B.C.
When Venus is at its greatest brilliancy, and appears as a morning star
about Christmas time (which occurred in 1887, and again in 1889), it has
been mistaken by the public for a return of the "Star of Bethlehem."[29]
But whatever "the star of the Magi" was it certainly was _not_ Venus. It,
seems, indeed absurd to suppose that "the wise men" of the East should
have mistaken a familiar object like Venus for a strange apparition. There
seems to be nothing whatever in the Bible to lead us to expect that the
star of Bethlehem will reappear.
Mr. J. H. Stockwell has suggested that the "Star of Bethlehem" may perhaps
be explained by a conjunction of the planets Venus and Jupiter which
occurred on May 8, B.C. 6, which was two years before the death of Herod.
From this it would follow that the Crucifixion took place on April 3, A.D.
33. But it seems very doubtful that the phenomenon recorded in the Bible
refers to any conjunction of planets.
Chacornac found the intrinsic brightness of Venus to be ten times greater
than the most luminous parts of the moon.[30] But this estimate is
probably too high.
When at its brightest, the planet is visible in broad daylight to good
eyesight, if its exact position in the sky is known. In the clear air of
Cambridge (U.S.A.) it is said to be possible to see it in this way in all
parts of its orbit, except when the planet is within 10° of the sun.[31]
Mr. A. Cameron, of Yarmouth, Nova Scotia, has, however, seen Venus with
the naked eye three days before conjunction when the planet was only
6-1/4° from the sun.[32] This seems a remarkable observation, and shows
that the observer's eyesight must have been very keen. In a private letter
dated October 22, 1888, the late Rev. S. J. Johnson informed the present
writer that he saw Venus with the naked eye only four days before
conjunction with the sun in February, 1878, and February, 1886.
The crescent shape of Venus is said to have been seen with the naked eye
by Theodore Parker in America when he was only 12 years old. Other
observers have stated the same thing; but the possibility of such an
observation has been much disputed in recent years.
In the Chinese Annals some records are given of Venus having been seen in
the Pleiades. On March 16, A.D. 845, it is said that "Venus eclipsed the
Pleiades." This means, of course, that the cluster was apparently effaced
by the brilliant light of the planet. Computing backwards for the above
date, Hind found that on the evening of March 16, 845, Venus was situated
near the star Electra; and on the following evening the planet passed
close to Maia; thus showing the accuracy of the Chinese record. Another
"eclipse" of the Pleiades by Venus is recorded in the same annals as
having occurred on March 10, A.D. 1002.[33]
When Venus is in the crescent phase, that is near "Inferior conjunction"
with the sun, it will be noticed, even by a casual observer, that the
crescent is not of the same shape as that of the crescent moon. The horns
or "cusps" of the planetary crescent are more prolonged than in the case
of the moon, and extend beyond the hemisphere. This appearance is caused
by refraction of the sun's light through the planetary atmosphere, and is,
in fact, a certain proof that Venus has an atmosphere similar to that of
the earth. Observations further show that this atmosphere is denser than
ours.
Seen from Venus, the earth and moon, when in opposition, must present a
splendid spectacle. I find that the earth would shine as a star about half
as bright again as Venus at her brightest appears to us, and the moon
about equal in brightness to Sirius! the two forming a superb "naked eye
double star"--perhaps the finest sight of its kind in the solar
system.[34]
Some of the earlier observers, such as La Hire, Fontana, Cassini, and
Schröter, thought they saw evidence of mountains on Venus. Schröter
estimated some of these to be 27 or 28 miles in height! but this seems
very doubtful. Sir William Herschel severely attacked these supposed
discoveries. Schröter defended himself, and was supported by Beer and
Mädler, the famous lunar observers. Several modern observers seem to
confirm Schröter's conclusions; but very little is really known about the
topography of Venus.
The well-known French astronomer Trouvelot--a most excellent observer--saw
white spots on Venus similar to those on Mars. These were well seen and
quite brilliant in July and August, 1876, and in February and November,
1877. The observations seem to show that these spots do not (unlike Mars)
increase and decrease with the planet's seasons. These white spots had
been previously noticed by former observers, including Bianchini, Derham,
Gruithuisen, and La Hire; but these early observers do not seem to have
considered them as snow caps, like those of Mars. Trouvelot was led by his
own observations to conclude that the period of rotation of Venus is
short, and the best result he obtained was 23{h} 49{m} 28{s}. This does
not differ much from the results previously found by De Vico, Fritsch, and
Schröter.[35]
A white spot near the planet's south pole was seen on several occasions by
H. C. Russell in May and June, 1876.[36]
Photographs of Venus taken on March 18 and April 29, 1908, by M. Quénisset
at the Observatory of Juvissy, France, show a white polar spot. The spot
was also seen at the same observatory by M. A. Benoit on May 20, 1903.
The controversy on the period of rotation of Venus, or the length of its
day, is a very curious one and has not yet been decided. Many good
observers assert confidently that it is short (about 24 hours); while
others affirm with equal confidence that it is long (about 225 days, the
period of the planet's revolution round the sun). Among the observers who
favour the short period of rotation are: D. Cassini (1667), J. Cassini
(1730), Schröter (1788-93), Mädler (1836), De Vico (1840?) Trouvelot
(1871-79), Flammarion, Léo Brenner, Stanley Williams, and J. McHarg; and
among those who support the long period are: Bianchini (1727),
Schiaparelli, Cerulli, Tacchini, Mascari, and Lowell. Some recent
spectroscopic observations seem to favour the short period.
Flammarion thinks that "nothing certain can be descried upon the surface
of Venus, and that whatever has hitherto been written regarding its period
of rotation must be considered null and void"; and again he says, "Nothing
can be affirmed regarding the rotation of Venus, inasmuch as the
absorption of its immense atmosphere certainly prevents any detail on its
surface from being perceived."[37]
The eminent Swedish physicist Arrhenius thinks, however, that the dense
atmosphere and clouds of Venus are in favour of a rapid rotation on its
axis.[38] He thinks that the mean temperature of Venus may "not differ
much from the calculated temperature 104° F." "Under these circumstances
the assumption would appear plausible that a very considerable portion of
the surface of Venus, and particularly the districts about the poles,
would be favourable to organic life."[39]
The "secondary light of Venus," or the visibility of the dark side, seems
to have been first mentioned by Derham in his _Astro Theology_ published
in 1715. He speaks of the visibility of the dark part of the planet's disc
"by the aid of a light of a somewhat dull and ruddy colour." The date of
Derham's observation is not given, but it seems to have been previous to
the year 1714. The light seems to have been also seen by a friend of
Derham. We next find observations by Christfried Kirch, assistant
astronomer to the Berlin Academy of Sciences, on June 7, 1721, and March
8, 1726. These observations are found in his original papers, and were
printed in the _Astronomische Nachrichten_, No. 1586. On the first date
the telescopic image of the planet was "rather tremulous," but in 1726 he
noticed that the dark part of the circle seemed to belong to a smaller
circle than the illuminated portion of the disc.[40] The same effect was
also noted by Webb.[41] A similar illusion is seen in the case of the
crescent moon, and this has given rise to the saying, "the old moon in the
new moon's arms."
We next come, in order of date, to an observation made by Andreas Mayer,
Professor of Mathematics at Griefswald in Prussia. The observation was
made on October 20, 1759, and the dark part of Venus was seen distinctly
by Mayer. As the planet's altitude at the time was not more than 14° above
the horizon, and its apparent distance from the sun only 10°, the
phenomenon--as Professor Safarik has pointed out--"must have had a most
unusual intensity."
Sir William Herschel makes no mention of having ever seen the "secondary
light" of Venus, although he noticed the extension of the horns beyond a
semicircle.
In the spring and summer of the year 1793, Von Hahn of Remplin in
Mecklenburg, using excellent telescopes made by Dollond and Herschel, saw
the dark part of Venus on several occasions, and describes the light as
"grey verging upon brown."
Schröter of Lilienthal--the famous observer of the moon--saw the horns of
the crescent of Venus extended many degrees beyond the semicircle on
several occasions in 1784 and 1795, and the border of the dark part
faintly lit up by a dusky grey light. On February 14, 1806, at 7 P.M. he
saw the whole of the dark part visible with an ash-coloured light, and he
was satisfied that there was no illusion. On January 24 of the same year,
1806, Harding at Göttingen, using a reflector of 9 inches aperture and
power 84, saw the dark side of Venus "shining with a pale ash-coloured
light," and very visible against the dark background of the sky. The
appearance was seen with various magnifying powers, and he thought that
there could be no illusion. In fact the phenomenon was as evident as in
the case of the moon. Harding again saw it on February 28 of the same
year, the illumination being of a reddish grey colour, "like that of the
moon in a total eclipse."
The "secondary light" was also seen by Pastorff in 1822, and by
Gruithuisen in 1825. Since 1824 observations of the "light" were made by
Berry, Browning, Guthrie, Langdon, Noble, Prince, Webb, and others. Webb
saw it with powers of 90 and 212 on a 9·38-inch mirror, and found it
"equally visible when the bright crescent was hidden by a field bar."[42]
Captain Noble's observation was rather unique. He found that the dark side
was "always distinctly and positively _darker_ than the background upon
which it is projected."
The "light" was also seen by Lyman in America in 1867, and by Safarik at
Prague. In 1871 the whole disc of Venus was seen by Professor
Winnecke.[43] On the other hand, Winnecke stated that he only saw it twice
in 24 years; and the great observers Dawes and Mädler never saw it at
all![44]
Various attempts have been made to explain the visibility--at times--of
the "dark side" of Venus. The following may be mentioned[45]:--(1)
Reflected earth-light, analogous to the dark side of the crescent moon.
This explanation was advocated by Harding, Schröter, and others. But,
although the earth is undoubtedly a bright object in the sky of Venus, the
explanation is evidently quite inadequate. (2) Phosphorescence of the
planet's atmosphere. This has been suggested by some observers. (3)
Visibility by contrast, a theory advanced by the great French astronomer
Arago. (4) Illumination of the planet's surface by an aurora borealis.
This also seems rather inadequate, but would account for the light being
sometimes visible and sometimes not. (5) Luminosity of the oceans--if
there be any--on Venus. But this also seems inadequate. (6) A planetary
surface glowing with intense heat. But this seems improbable. (7) The
Kunstliche Feuer (artificial fire) of Gruithuisen, a very fanciful theory.
Flammarion thinks that the visibility of the dark side may perhaps be
explained by its projection on a somewhat lighter background, such as the
zodiacal light, or an extended solar envelope.[46]
It will be seen that none of these explanations are entirely satisfactory,
and the phenomenon, if real, remains a sort of astronomical enigma. The
fact that the "light" is visible on some occasions and not on others would
render some of the explanations improbable or even inadmissible. But the
condition of the earth's atmosphere at times might account for its
invisibility on many occasions.
A curious suggestion was made by Zöllner, namely, that if the secondary
light of Venus could be observed with the spectroscope it would show
bright lines! But such an observation would be one of extreme difficulty.
M. Hansky finds that the visibility of the "light" is greater during
periods of maximum solar activity--that is, at the maxima of sun spots.
This he explains by the theory of Arrhenius, in which electrified "ions
emitted by the sun cause the phenomena of terrestrial magnetic storms and
auroras." "In the same way the dense atmosphere of Venus is rendered more
phosphorescent, and therefore more easily visible by the increased solar
activity."[47] This seems a very plausible hypothesis.
On the whole the occasional illumination of the night side of Venus by a
very brilliant aurora (explanation (4) above) seems to the present writer
to be the most probable explanation. Gruithuisen's hypothesis (7) seems
utterly improbable.
There is a curious apparent anomaly about the motion of Venus in the sky.
Although the planet's period of revolution round the sun is 224·7 days, it
remains on the same side of the sun, as seen from the earth, for 290 days.
The reason of this is that the earth is going at the same time round the
sun in the same direction, though at a slower pace; and Venus must
continue to appear on the same side of the sun until the excess of her
daily motion above that of the earth amounts to 179°, and this at the
daily rate of 37' will be about 290 days.
Several observations have been recorded of a supposed satellite of Venus.
But the existence of such a body has never been verified. In the year
1887, M. Stroobant investigated the various accounts, and came to the
conclusion that in several at least of the recorded observations the
object seen was certainly a star. Thus, in the observation made by
Roedickoer and Boserup on August 4, 1761, a satellite and star are
recorded as having been seen near the planet. M. Stroobant finds that the
supposed "satellite" was the star [Greek: ch]_{4} Orionis, and the "star"
[Greek: ch]_{3} Orionis. A supposed observation of a satellite made by
Horrebow on January 3, 1768, was undoubtedly [Greek: th] Libræ. M.
Stroobant found that the supposed motion of the "satellite" as seen by
Horrebow is accurately represented by the motion of Venus itself during
the time of observation. In most of the other supposed observations of a
satellite a satisfactory identification has also been found. M. Stroobant
finds that with a telescope of 6 inches aperture, a star of the 8th or
even the 9th magnitude can be well seen when close to Venus.[49]
On the night of August 13, 1892, Professor Barnard, while examining Venus
with the great 36-inch telescope of the Lick Observatory, saw a star of
the 7th magnitude in the same field with the planet. He carefully
determined the exact position of this star, and found that it is not in
Argelander's great catalogue, the _Durchmusterung_. Prof. Barnard finds
that owing to elongation of Venus from the sun at the time of observation
the star could not possibly be an intra-Mercurial planet (that is, a
planet revolving round the sun inside the orbit of Mercury); but that
possibly it might be a planet revolving between the orbits of Venus and
Mercury. As the brightest of the minor planets--Ceres, Pallas, Juno, and
Vesta--were not at the time near the position of the observed object, the
observation remains unexplained. It might possibly have been a _nova_, or
temporary star.[50]
Scheuten is said to have seen a supposed satellite of Venus following the
planet across the sun at the end of the transit of June 6, 1761.[51]
Humboldt speaks of the supposed satellite of Venus as among "the
astronomical myths of an uncritical age."[52]
An occultation of Venus by the moon is mentioned in the Chinese Annals as
having occurred on March 19, 361 A.D., and Tycho Brahé observed another on
May 23, 1587.[53]
A close conjunction of Venus and Regulus ([Greek: a] Leonis) is recorded
by the Arabian astronomer, Ibn Yunis, as having occurred on September 9,
885 A.D. Calculations by Hind show that the planet and star were within 2'
of arc on that night, and consequently would have appeared as a single
star to the naked eye. The telescope had not then been invented.[54]
Seen from Venus, the maximum apparent distance between the earth and moon
would vary from about 5' to 31'.[55]
It is related by Arago that Buonaparte, when going to the Luxembourg in
Paris, where the Directory were giving a fête in his honour, was very
much surprised to find the crowd assembled in the Rue de Touracour "pay
more attention to a region of the heavens situated above the palace than
to his person or the brilliant staff that accompanied him. He inquired the
cause and learned that these curious persons were observing with
astonishment, although it was noon, a star, which they supposed to be that
of the conqueror of Italy--an allusion to which the illustrious general
did not seem indifferent, when he himself, with his piercing eyes,
remarked the radiant body." The "star" in question was Venus.[56]
CHAPTER IV
The Earth
The earth being our place of abode is, of course, to us the most important
planet in the solar system. It is a curious paradox that the moon's
surface (at least the visible portion) is better known to us than the
surface of the earth. Every spot on the moon's visible surface equal in
size to say Liverpool or Glasgow is well known to lunar observers, whereas
there are thousands of square miles on the earth's surface--for example,
near the poles and in the centre of Australia--which are wholly unknown to
the earth's inhabitants; and are perhaps likely to remain so.
Many attempts have been made by "paradoxers" to show that the earth is a
flat plane and not a sphere. But M. Ricco has found by actual experiment
that the reflected image of the setting sun from a smooth sea is an
elongated ellipse. This proves mathematically beyond all doubt that the
surface of the sea is spherical; for the reflection from a plane surface
would be necessarily _circular_. The theory of a "flat earth" is
therefore proved to be quite untenable, and all the arguments (?) of the
"earth flatteners" have now been--like the French Revolution--"blown into
space."
The pole of minimum temperature in the northern hemisphere, or "the pole
of cold," as it has been termed, is supposed to lie near Werchojansk in
Siberia, where a temperature of nearly -70° has been observed.
From a series of observations made at Annapolis (U.S.A.) on the gradual
disappearance of the blue of the sky after sunset, Dr. See finds that the
extreme height of the earth's atmosphere is about 130 miles. Prof. Newcomb
finds that meteors first appear at a mean height of about 74 miles.[57]
An aurora seen in Canada on July 15, 1893, was observed from stations 110
miles apart, and from these observations the aurora was found to lie at a
height of 166 miles above the earth's surface. It was computed that if the
auroral "arch maintained an equal height above the earth its ends were
1150 miles away, so that the magnificent sight was presented of an auroral
belt in the sky with 2300 miles between its two extremities."[58]
"Luminous clouds" are bright clouds sometimes seen at night near the end
of June and beginning of July. They appear above the northern horizon
over the sun's place about midnight, and evidently lie at a great height
above the earth's surface. Observations made in Germany by Dr. Jesse, and
in England by Mr. Backhouse, in the years 1885-91, show that the height of
these clouds is nearly constant at about 51 miles.[59] The present writer
has seen these remarkable clouds on one or two occasions in County Sligo,
Ireland, during the period above mentioned.
M. Montigny has shown that "the approach of violent cyclones or other
storms is heralded by an increase of scintillation" (or twinkling of the
stars). The effect is also very evident when such storms pass at a
considerable distance. He has also made some interesting observations
(especially on the star Capella), which show that, not only does
scintillation increase in rainy weather, but that "it is very evident, at
such times, in stars situated at an altitude at which on other occasions
it would not be perceptible at all; thus confirming the remark of
Humboldt's with regard to the advent of the wet season in tropical
countries."[60]
In a paper on the subject of "Optical Illusions" in _Popular Astronomy_,
February, 1906, Mr. Arthur K. Bartlett, of Batter Creek, Michigan
(U.S.A.), makes the following interesting remarks:--
"The lunar halo which by many persons is regarded as a remarkable and
unexplained luminosity associated with the moon, is to meteorological
students neither a mysterious nor an anomalous occurrence. It has been
frequently observed and for many years thoroughly understood, and at
the present time admits of an easy scientific explanation. It is an
atmospheric exhibition due to the refraction and dispersion of the
moon's light through very minute ice crystals floating at great
elevations above the earth, and it is explained by the science of
meteorology, to which it properly belongs; for it is not of cosmical
origin, and in no way pertains to astronomy, as most persons suppose,
except as it depends on the moon, whose light passing through the
atmosphere, produces the luminous halo, which as will be seen, is
simply an optical illusion, originating, not in the vicinity of the
moon--two hundred and forty thousand miles away--but just above the
earth's surface, and within the aqueous envelope that surrounds it on
all sides.... A halo may form round the sun as well as the moon ...
but a halo is more frequently noticed round the moon for the reason
that we are too much dazzled by the sun's light to distinguish faint
colours surrounding its disc, and to see them it is necessary to look
through smoked glass, or view the sun by reflection from the surface
of still water, by which its brilliancy is very much reduced."...
"A 'corona' is an appearance of faintly coloured rings often seen around
the sun and moon when a light fleecy cloud passes over them, and should
not be mistaken for a halo, which is much larger and more complicated in
its structure. These two phenomena are frequently confounded by
inexperienced observers." With these remarks the present writer fully
concurs.
Mr. Bartlett adds--
"As a halo is never seen except when the sky is hazy, it indicates
that moisture is accumulating in the atmosphere which will form
clouds, and usually result in a storm. But the popular notion that the
number of bright stars visible within the circle indicates the number
of days before the storm will occur, is without any foundation
whatever, and the belief is almost too absurd to be refuted. In
whatever part of the sky a lunar halo is seen, one or more bright
stars are always sure to be noticed inside the luminous ring, and the
number visible depends entirely upon the position of the moon.
Moreover, when the sky within the circle is examined with even a small
telescope, hundreds of stars are visible where only one, or perhaps
two or three, are perceived with the naked eye."
It is possible to have five Sundays in February (the year must of course
be a "leap year"). This occurred in the year 1880, Sunday falling on
February 1, 8, 15, 22, and 29. But this will not happen again till the
year 1920. No century year (such as 1900, 2000, etc.) could possibly have
five Sundays in February, and the Rev. Richard Campbell, who investigated
this matter, finds the following sequence of years in which five Sundays
occur in February: 1604, 1632, 1660, 1688, 1728, 1756, 1784, 1824, 1852,
1880, 1920, 1948, 1976.[61]
In an article on "The Last Day and Year of the Century: Remarks on Time
Reckoning," in _Nature_, September 10, 1896, Mr. W. T. Lynn, the eminent
astronomer, says, "The late Astronomer Royal, Sir George Airy, once
received a letter requesting him to settle a dispute which had arisen in
some local debating society, as to which would be the first day of the
next century. His reply was, 'A very little consideration will suffice to
show that the first day of the twentieth century will be January 1, 1901.'
Simple as the matter seems, the fact that it is occasionally brought into
question shows that there is some little difficulty connected with it.
Probably, however, this is in a great measure due to the circumstance that
the actual figures are changed on January 1, 1900, the day preceding being
December 31, 1899. A century is a very definite word for an interval
respecting which there is no possible room for mistake or difference of
opinion. But the date of its ending depends upon that of its beginning.
Our double system of backward and forward reckoning leads to a good deal
of inconvenience. Our reckoning supposes (what we know was not the case,
but as an era the date does equally well) that Christ was born at the end
of B.C. 1. At the end of A.D. 1, therefore, one year had elapsed from the
event, at the end of A.D. 100, one century, and at the end of 1900,
nineteen centuries.... It is clear, then, that the year, as we call it, is
an ordinal number, and that 1900 years from the birth of Christ (reckoning
as we do from B.C. 1) will not be completed until the end of December 31
in that year, the twentieth century beginning with January 1, 1901, that
is (to be exact) at the previous midnight, when the day commences by civil
reckoning." With these remarks of Mr. Lynn I fully concur, and, so far as
I know, all astronomers agree with him. As the discussion will probably
again arise at the end of the twentieth century, I would like to put on
record here what the scientific opinion was at the close of the nineteenth
century.
Prof. E. Rutherford, the well-known authority on radium, suggests that
possibly radium is a source of heat from within the earth. Traces of
radium have been detected in many rocks and soils, and even in sea water.
Calculation shows that the total amount distributed through the earth's
crust is enormously large, although relatively small "compared with the
annual output of coal for the world." The amount of radium necessary to
compensate for the present loss of heat from the earth "corresponds to
only five parts in one hundred million millions per unit mass," and the
"observations of Elster and Gertel show that the radio-activity observed
in soils corresponds to the presence of about this proportion of
radium."[62]
The earth has 12 different motions. These are as follows:--
1. Rotation on its axis, having a period of 24 hours.
2. Revolution round the sun; period 365-1/4 days.
3. Precession; period of about 25,765 years.
4. Semi-lunar gravitation; period 28 days.
5. Nutation; period 18-1/2 years.
6. Variation in obliquity of the ecliptic; about 47" in 100 years.
7. Variation of eccentricity of orbit.
8. Change of line of apsides; period about 21,000 years.
9. Planetary perturbations.
10. Change of centre of gravity of whole solar system.
11. General motion of solar system in space.
12. Variation of latitude with several degrees of periodicity.[63]
"An amusing story has been told which affords a good illustration of
the ignorance and popular notions regarding the tides prevailing even
among persons of average intelligence. 'Tell me,' said a man to an
eminent living English astronomer not long ago, 'is it still
considered probable that the tides are caused by the moon?' The man of
science replied that to the best of his belief it was, and then asked
in turn whether the inquirer had any serious reason for questioning
the relationship. 'Well, I don't know,' was the answer; 'sometimes
when there is no moon there seems to be a tide all the same.'"![64]
With reference to the force of gravitation, on the earth and other bodies
in the universe, Mr. William B. Taylor has well said, "With each revolving
year new demonstrations of its absolute precision and of its universal
domination serves only to fill the mind with added wonder and with added
confidence in the stability and the supremacy of the power in which has
been found no variableness neither shadow of turning, but which--the same
yesterday, to-day and for ever--
"Lives through all life, extends through all extent,
Spreads undivided, operates unspent."[65]
With reference to the habitability of other planets, Tennyson has
beautifully said--
"Venus near her! smiling downwards at this earthlier earth of ours,
Closer on the sun, perhaps a world of never fading flowers.
Hesper, whom the poets call'd the Bringer home of all good things;
All good things may move in Hesper; perfect people, perfect kings.
Hesper--Venus--were we native to that splendour, or in Mars,
We should see the globe we groan in fairest of their evening stars.
Could we dream of war and carnage, craft and madness, lust and spite,
Roaring London, raving Paris, in that spot of peaceful light?
Might we not in glancing heavenward on a star so silver fair,
Yearn and clasp the hands, and murmur, 'Would to God that we were
there!'"
The ancient Greek writer, Diogenes Laertius, states that Anaximander
(610-547 B.C.) believed that the earth was a sphere. The Greek words are:
[Greek: misên te tên gên keisthai, kentry taxin epechousan ousan
sphairoeidê].[66]
With reference to the Aurora Borealis, the exact nature of which is not
accurately known, "a good story used to be told some years ago of a
candidate who, undergoing the torture of a _vivâ voce_ examination, was
unable to reply satisfactorily to any of the questions asked. 'Come, sir,'
said the examiner, with the air of a man asking the simplest question,
'explain to me the cause of the aurora borealis.' 'Sir,' said the unhappy
aspirant for physical honours, 'I could have explained it perfectly
yesterday, but nervousness has, I think, made me lose my memory.' 'This is
very unfortunate,' said the examiner; 'you are the only man who could have
explained this mystery, and you have forgotten it.'"[67] This was written
in the year 1899, and probably the phenomenon of the aurora remains
nearly as great a mystery to-day. In 1839, MM. Bravais and Lottin made
observations on the aurora in Norway in about N. latitude 70°. Bravais
found the height to be between 62 and 93 miles above the earth's surface.
The cause of the so-called Glacial Epoch in the earth's history has been
much discussed. The Russian physicist, Rogovsky, has advanced the
following theory--
"If we suppose that the temperature of the sun at the present time is
still increasing, or at least has been increasing until now, the
glacial epoch can be easily accounted for. Formerly the earth had a
high temperature of its own, but received a lesser quantity of heat
from the sun than now; on cooling gradually, the earth's surface
attained such a temperature as caused a great part of the surface of
the northern and southern hemispheres to be covered with ice; but the
sun's radiation increasing, the glaciers melted, and the climatic
conditions became as they are now. In a word, the temperature of the
earth's surface is a function of two quantities: one decreasing (the
earth's own heat), and the other increasing (the sun's radiation), and
consequently there may be a minimum, and this minimum was the glacial
epoch, which, as shown by recent investigations, those of Luigi de
Marchi (Report of _G. Schiaparelli, Meteorolog. Zeitschr._, 30,
130-136, 1895), are not local, but general for the whole earth" (see
also M. Neumahr, _Erdegeschicht_).[68]
Prof. Percival Lowell thinks that the life of geological palæozoic times
was supported by the earth's internal heat, which maintained the ocean at
a comparatively warm temperature.[69]
The following passage in the Book of the Maccabees may possibly refer to
an aurora--
"Now about this time Antiochus made his second inroad into Egypt. And
it _so_ befell that throughout all the city, for the space of almost
forty days, there appeared in the midst of the sky horsemen in swift
motion, wearing robes inwrought with gold and _carrying_ spears,
equipped in troops for battle; and drawing of swords; and _on the
other side_ squadrons of horse in array; and encounters and pursuits
of both armies; and shaking of shields, and multitudes of lances, and
casting of darts, and flashing of golden trappings, and girding on of
all sorts of armour. Wherefore all men besought that the vision might
have been given for food."[70]
According to Laplace "the decrease of the mean heat of the earth during a
period of 2000 years has not, taking the extremist limits, diminished as
much as 1/300th of a degree Fahrenheit."[71]
From his researches on the cause of the Precession of the Equinoxes,
Laplace concluded that "the motion of the earth's axis is the same as if
the whole sea formed a solid mass adhering to its surface."[72]
Laplace found that the major (or longer) axis of the earth's orbit
coincided with the line of Equinoxes in the year 4107 B.C. The earth's
perigee then coincided with the autumnal equinox. The epoch at which the
major axis was perpendicular to the line of equinoxes fell in the year
1250 A.D.[73]
Leverrier has found the minimum eccentricity of the earth's orbit round
the sun to be 0·0047; so that the orbit will never become absolutely
circular, as some have imagined.
Laplace says--
"Astronomy considered in its entirety is the finest monument of the
human mind, the noblest essay of its intelligence. Seduced by the
illusions of the senses and of self-pride, for a long time man
considered himself as the centre of the movement of the stars; his
vain-glory has been punished by the terrors which his own ideas have
inspired. At last the efforts of several centuries brushed aside the
veil which concealed the system of the world. We discover ourselves
upon a planet, itself almost imperceptible in the vast extent of the
solar system, which in its turn is only an insensible point in the
immensity of space. The sublime results to which this discovery has
led should suffice to console us for our extreme littleness, and the
rank which it assigns to the earth. Let us treasure with solicitude,
let us add to as we may, this store of higher knowledge, the most
exquisite treasure of thinking beings."[74]
With reference to probable future changes in climate, the great physicist,
Arrhenius, says--
"We often hear lamentation that the coal stored up in the earth is
wasted by the present generation without any thought of the future,
and we are terrified by the awful destruction of life and property
which has followed the volcanic eruptions of our days. We may find a
kind of consolation in the consideration that here, as in every other
case, there is good mixed with evil. By the influence of the
increasing percentage of carbonic acid in the atmosphere, we may hope
to enjoy ages with more equable and better climates, especially as
regards the colder regions of the earth, ages when the earth will
bring forth much more abundant crops than at present, for the benefit
of rapidly propagating mankind."[75]
The night of July 1, 1908, was unusually bright. This was noticed in
various parts of England and Ireland, and by the present writer in Dublin.
Humboldt states that "at the time of the new moon at midnight in 1743, the
phosphorescence was so intense that objects could be distinctly recognized
at a distance of more than 600 feet."[76]
An interesting proof of the earth's rotation on its axis has recently been
found.
"In a paper in the _Proceedings_ of the Vienna Academy (June, 1908) by
Herr Tumlirz, it is shown mathematically that if a liquid is flowing
outwards between two horizontal discs, the lines of flow will be
strictly straight only if the discs and vessel be at rest, and will
assume certain curves if that vessel and the discs are in rotation,
as, for example, due to the earth's rotation. An experimental
arrangement was set up with all precautions, and the stream lines were
marked with coloured liquids and photographed. These were in general
accord with the predictions of theory and the supposition that the
earth is rotating about an axis."[77]
In a book published in 1905 entitled _The Rational Almanac_, by Moses B.
Cotsworth, of York, the author states that (p. 397), "The explanation is
apparent from the Great Pyramid's Slope, which conclusively proves that
when it was built the latitude of that region was 7°·1 more than at
present. Egyptian Memphis now near Cairo was then in latitude 37°·1, where
Asia Minor now ranges, whilst Syria would then be where the Caucasus
regions now experience those rigorous winters formerly experienced in
Syria." But the reality of this comparatively great change of latitude in
the position of the Great Pyramid can be easily disproved. Pytheas of
Marseilles--who lived in the time of Alexander the Great, about 330
B.C.--measured the latitude of Marseilles by means of a gnomon, and found
it to be about 42° 56'-1/2. As the present latitude of Marseilles is 43°
17' 50", no great change in the latitude could have taken place in over
2000 years.[78] From this we may conclude that the latitude of the Great
Pyramid has _not_ changed by 7°·1 since its construction. There is, it is
true, a slow diminution going on in the obliquity of the ecliptic (or
inclination of the earth's axis), but modern observations show that this
would not amount to as much as one degree in 6000 years. Eudemus of
Rhodes--a disciple of Aristotle (who died in 322 B.C.)--found the
obliquity of the ecliptic to be 24°, which differs but little from its
present value, 23° 27'. Al-Sufi in the tenth century measured the latitude
of Schiraz in Persia, and found it 29° 36'. Its present latitude is 29°
36' 30",[79] so that evidently there has been no change in the latitude in
900 years.
CHAPTER V
The Moon
The total area of the moon's surface is about equal to that of North and
South America. The actual surface visible at any one time is about equal
to North America.
The famous lunar observer, Schröter, thought that the moon had an
atmosphere, but estimated its height at only a little over a mile. Its
density he supposed to be less than that of the vacuum in an air-pump.
Recent investigations, however, seem to show that owing to its small mass
and attractive force the moon could not retain an atmosphere like that of
the earth.
Prof. N. S. Shaler, of Harvard (U.S.A.), finds from a study of the moon
(from a geological point of view) with the 15-inch refractor of the
Harvard Observatory, that our satellite has no atmosphere nor any form of
organic life, and he believes that its surface "was brought to its present
condition before the earth had even a solid crust."[80]
There is a curious illusion with reference to the moon's apparent
diameter referred to by Proctor.[81] If, when the moon is absent in the
winter months, we ask a person whether the moon's diameter is greater or
less than the distance between the stars [Greek: d] and [Greek: e], and
[Greek: e] and [Greek: z] Orionis, the three well-known stars in the "belt
of Orion," the answer will probably be that the moon's apparent diameter
is about equal to each of these distances. But in reality the apparent
distance between [Greek: d] and [Greek: e] Orionis (or between [Greek: e]
and [Greek: z], which is about the same) is more than double the moon's
apparent diameter. This seems at first sight a startling statement; but
its truth is, of course, beyond all doubt and is not open to argument.
Proctor points out that if a person estimates the moon as a foot in
diameter, as its apparent diameter is about half a degree, this would
imply that the observer estimates the circumference of the star sphere as
about 720 feet (360° × 2), and hence the radius (or the moon's distance
from the earth) about 115 feet. But in reality all such estimates have no
scientific (that is, accurate) meaning. Some of the ancients, such as
Aristotle, Cicero, and Heraclitus, seem to have estimated the moon's
apparent diameter at about a foot.[82] This shows that even great minds
may make serious mistakes.
It has been stated by some writer that the moon as seen with the highest
powers of the great Yerkes telescope (40 inches aperture) appears "just
as it would be seen with the naked eye if it were suspended 60 miles over
our heads." But this statement is quite erroneous. The moon as seen with
the naked eye or with a telescope shows us nearly a whole hemisphere of
its surface. But if the eye were placed only 60 miles from the moon's
surface, we should see only a small portion of its surface. In fact, it is
a curious paradox that the nearer the eye is to a sphere the less we see
of its surface! The truth of this will be evident from the fact that on a
level plain an eye placed at a height, say 5 feet, sees a very small
portion indeed of the earth's surface, and the higher we ascend the more
of the surface we see. I find that at a distance of 60 miles from the
moon's surface we should only see a small portion of its visible
hemisphere (about 1/90th). The lunar features would also appear under a
different aspect. The view would be more of a landscape than that seen in
any telescope. This view of the matter is not new. It has been previously
pointed out, especially by M. Flammarion and Mr. Whitmell, but its truth
is not, I think, generally recognized. Prof. Newcomb doubts whether with
any telescope the moon has ever been seen so well as it would be if
brought within 500 miles of the earth.
A relief map of the moon 19 feet in diameter was added, in 1898, to the
Field Columbian Museum (U.S.A.). It was prepared with great care from the
lunar charts of Beer and Mädler, and Dr. Schmidt of the Athens
Observatory, and it shows the lunar features very accurately. Its
construction took five years.
On a photograph of a part of the moon's surface near the crater
Eratosthenes, Prof. William H. Pickering finds markings which very much
resemble the so-called "canals" of Mars. The photograph was taken in
Jamaica, and a copy of it is given in Prof. Pickering's book on the Moon,
and in _Popular Astronomy_, February, 1904.
Experiments made in America by Messrs. Stebbins and F. C. Brown, by means
of selenium cells, show that the light of the full moon is about nine
times that of the half moon;[83] and that "the moon is brighter between
the first quarter and full than in the corresponding phase after full
moon." They also find that the light of the full moon is equal to "0·23
candle power,"[83] that is, according to the method of measurement used in
America, its light is equal to 0·23 of a standard candle placed at a
distance of one metre (39·37 inches) from the eye.[84]
Mr. H. H. Kimball finds that no less than 52 per cent. of the observed
changes in intensity of the "earth-shine" visible on the moon when at or
near the crescent phase is due to the eccentricity of the lunar orbit,
and "this is probably much greater than could be expected from any
increase or diminution in the average cloudiness over the hemisphere of
the earth reflecting light to the moon."[85]
The "moon maiden" is a term applied to a fancied resemblance of a portion
of the Sinus Iridum to a female head. It forms the "promontory" known as
Cape Heraclides, and may be looked for when the moon's "age" is about 11
days. Mr. C. J. Caswell, who observed it on September 29, 1895, describes
it as resembling "a beautiful silver statuette of a graceful female figure
with flowing hair."
M. Landerer finds that the angle of polarization of the moon's
surface--about 33°--agrees well with the polarizing angle for many
specimens of igneous rocks (30° 51' to 33° 46'). The polarizing angle for
ice is more than 37°, and this fact is opposed to the theories of lunar
glaciation advanced by some observers.[86]
Kepler states in his _Somnium_ that he saw the moon in the crescent phase
on the morning and evening of the _same_ day (that is, before and after
conjunction with the sun). Kepler could see 14 stars in the Pleiades with
the naked eye, so his eyesight must have been exceptionally keen.
Investigations on ancient eclipses of the moon show that the eclipse
mentioned by Josephus as having occurred before the death of Herod is
probably that which took place on September 15, B.C. 5. This occurred
about 9.45 p.m.; and probably about six months before the death of Herod
(St. Matthew ii. 15).
The total lunar eclipse which occurred on October 4, 1884, was remarkable
for the almost total disappearance of the moon during totality. One
observer says that "in the open air, if one had not known exactly where to
look for it, one might have searched for some time without discovering it.
I speak of course of the naked eye appearance."[87] On the other hand the
same observer, speaking of the total eclipse of the moon on August 23,
1877, which was a bright one, says--
"The moon even in the middle of the total phase was a conspicuous
object in the sky, and the ruddy colour was well marked. In the very
middle of the eclipse the degree of illumination was as nearly as
possible equal all round the edge of the moon, the central parts being
darker than those near the edge."
In Roger de Hovedin's _Chronicle_ (A.D. 756) an account is given of the
occultation of "a bright star," by the moon during a total eclipse. This
is confirmed by Simeon of Durham, who also dates the eclipse A.D. 756.
This is, however, a mistake, the eclipse having occurred on the evening of
November 23, A.D. 755. Calvisius supposed that the occulted "star" might
have been Aldebaran. Pingré, however, showed that this was impossible, and
Struyck, in 1740, showed that the planet Jupiter was the "star" referred
to by the early observer. Further calculations by Hind (1885) show
conclusively that Struyck was quite correct, and that the phenomenon
described in the old chronicles was the occultation of Jupiter by a
totally eclipsed moon--a rather unique phenomenon.[88]
An occultation of Mars by the moon is recorded by the Chinese, on February
14, B.C. 69, and one of Venus, on March 30, A.D. 361. These have also been
verified by Hind, and his calculations show the accuracy of these old
Chinese records.
It has been suggested that the moon may possibly have a satellite
revolving round it, as the moon itself revolves round the earth. This
would, of course, form an object of great interest. During the total lunar
eclipses of March 10 and September 3, 1895, a careful photographic search
was made by Prof. Barnard for a possible lunar satellite. The eclipse of
March 10 was not very suitable for the purpose owing to a hazy sky, but
that of September 3 was "entirely satisfactory," as the sky was very
clear, and the duration of totality was very long. On the latter occasion
"six splendid" photographs were obtained of the total phase with a 6-inch
Willard lens. The result was that none of these photographs "show
anything which might be taken for a lunar satellite," at least any
satellite as bright as the 10th or 12th magnitude. It is, of course, just
possible that the supposed satellite might have been behind the moon
during the totality.
With reference to the attraction between the earth and moon, Sir Oliver
Lodge says--
"The force with which the moon is held in its orbit would be great
enough to tear asunder a steel rod 400 miles thick, with a tenacity of
30 tons to the square inch, so that if the moon and earth were
connected by steel instead of gravity, a forest of pillars would be
necessary to whirl the system once a month round their common centre
of gravity. Such a force necessarily implies enormous tensure or
pressure in the medium. Maxwell calculates that the gravitational
stress near the earth, which we must suppose to exist in the invisible
medium, is 3000 times greater than what the strongest steel can stand,
and near the sun it should be 2500 times as great as that."[89]
With reference to the names given to "craters" on the moon, Prof. W. H.
Pickering says,[90] "The system of nomenclature is, I think, unfortunate.
The names of the chief craters are generally those of men who have done
little or nothing for selenography, or even for astronomy, while the men
who should be really commemorated are represented in general by small and
unimportant craters," and again--
"A serious objection to the whole system of nomenclature lies in the
fact that it has apparently been used by some selenographers, from the
earliest times up to the present, as a means of satisfying their spite
against some of their contemporaries. Under the guise of pretending to
honour them by placing their names in perpetuity upon the moon, they
have used their names merely to designate the smallest objects that
their telescopes were capable of showing. An interesting illustration
of this point is found in the craters of Galileo and Riccioli, which
lie close together on the moon. It will be remembered that Galileo was
the discoverer of the craters on the moon. Both names were given by
Riccioli, and the relative size and importance of the craters
[Riccioli large, and Galileo very small] probably indicates to us the
relative importance that he assigned to the two men themselves. Other
examples might be quoted of craters named in the same spirit after men
still living.... With the exception of Maedler, one might almost say,
the more prominent the selenographer the more insignificant the
crater."
The mathematical treatment of the lunar theory is a problem of great
difficulty. The famous mathematician, Euler, described it as _incredibile
stadium atque indefessus labor_.[91]
With reference to the "earth-shine" on the moon when in the crescent
phase, Humboldt says, "Lambert made the remarkable observation (14th of
February, 1774) of a change of the ash-coloured moonlight into an
olive-green colour, bordering upon yellow. The moon, which then stood
vertically over the Atlantic Ocean, received upon its night side the green
terrestrial light, which is reflected towards her when the sky is clear by
the forest districts of South America."[92] Arago said, "Il n'est donc pas
impossible, malgré tout ce qu'un pareil résultat exciterait de surprise au
premier coup d'oeil qu'un jour les météorologistes aillent puiser dans
l'aspect de la Lune des notions précieuses sur _l'etat moyen_ de
diaphanité de l'atmosphère terrestre, dans les hemisphères qui
successivement concurrent à la production de la lumière cendrée."[93]
The "earth-shine" on the new moon was successfully photographed in
February, 1895, by Prof. Barnard at the Lick Observatory, with a 6-inch
Willard portrait lens. He says--
"The earth-lit globe stands out beautifully round, encircled by the
slender crescent. All the 'seas' are conspicuously visible, as are
also the other prominent features, especially the region about
_Tycho_. _Aristarchus_ and _Copernicus_ appear as bright specks, and
the light streams from _Tycho_ are very distinct."[94]
Kepler found that the moon completely disappeared during the total eclipse
of December 9, 1601, and Hevelius observed the same phenomenon during the
eclipse of April 25, 1642, when "not a vestige of the moon could be
seen."[95] In the total lunar eclipse of June 10, 1816, the moon during
totality was not visible in London, even with a telescope![95]
The lunar mountains are _relatively_ much higher than those on the earth.
Beer and Mädler found the following heights: Dörfel, 23,174 feet; Newton,
22,141; Casatus, 21,102; Curtius, 20,632; Callippus, 18,946; and Tycho,
18,748 feet.[96]
Taking the earth's diameter at 7912 miles, the moon's diameter, 2163
miles, and the height of Mount Everest as 29,000 feet, I find that
Everest 1 Dörfel 1
---------------- = ----, and --------------- = ---
Earth's diameter 1440 moon's diameter 492
From which it follows that the lunar mountains are _proportionately_ about
three times higher than those on the earth.
According to an hypothesis recently advanced by Dr. See, all the
satellites of the solar system, including our moon, were "captured" by
their primaries. He thinks, therefore, that the "moon came to earth from
heavenly space."[97]
CHAPTER VI
Mars
Mars was called by the ancients "the vanishing star," owing to the long
periods during which it is practically invisible from the earth.[98] It
was also called [Greek: puroeis] and Hercules.
I have seen it stated in a book on the "Solar System" by a well-known
astronomer that the _axis_ of Mars "is inclined to the plane of the orbit"
at an angle of 24° 50'! But this is quite erroneous. The angle given is
the angle between _the plane of the planet's equator_ and the plane of its
orbit, which is quite a different thing. This angle, which may be called
the obliquity of Mars' ecliptic, does not differ much from that of the
earth. Lowell finds it 23° 13' from observations in 1907.[99]
The late Mr. Proctor thought that Mars is "far the reddest star in the
heavens; Aldebaran and Antares are pale beside him."[100] But this does
not agree with my experience. Antares is to my eye quite as red as Mars.
Its name is derived from two Greek words implying "redder than Mars." The
colour of Aldebaran is, I think, quite comparable with that of the "ruddy
planet." In the telescope the colour of Mars is, I believe, more yellow
than red, but I have not seen the planet very often in a telescope. Sir
John Herschel suggested that the reddish colour of Mars may possibly be
due to red rocks, like those of the Old Red Sandstone, and the red soil
often associated with such rocks, as I have myself noticed near Torquay
and other places in Devonshire.
The ruddy colour of Mars was formerly thought to be due to the great
density of its atmosphere. But modern observations seem to show that the
planet's atmosphere is, on the contrary, much rarer than that of the
earth. The persistent visibility of the markings on its surface shows that
its atmosphere cannot be cloud-laden like ours; and the spectroscope shows
that the water vapour present is--although perceptible--less than that of
our terrestrial envelope.
The existence of water vapour is clearly shown by photographs of the
planet's spectrum taken by Mr. Slipher at the Lowell Observatory in 1908.
These show that the water vapour bands _a_ and near D are stronger in the
spectrum of Mars than in that of the moon at the same altitude.[101]
The dark markings on Mars were formerly supposed to represent water and
the light parts land. But this idea has now been abandoned. Light
reflected from a water surface is polarized at certain angles. Prof. W. H.
Pickering, in his observations on Mars, finds no trace of polarization in
the light reflected from the dark parts of the planet. But under the same
conditions he finds that the bluish-black ring surrounding the white polar
cap shows a well-marked polarization of light, thus indicating that this
dark ring is probably water.[102]
Projections on the limb of the planet have frequently been observed in
America. These are known _not_ to be mountains, as they do not reappear
under similar conditions. They are supposed to be clouds, and one seen in
December, 1900, has been explained as a cloud lying at a height of some 13
miles above the planet's surface and drifting at the rate of about 27
miles an hour. If there are any mountains on Mars they have not yet been
discovered.
The existence of the so-called "canals" of Mars is supposed to be
confirmed by Lowell's photographs of the planet. But what these "canals"
really represent, that is the question. They have certainly an artificial
look about them, and they form one of the most curious and interesting
problems in the heavens. Prof. Lowell says--
"Most suggestive of all Martian phenomena are the canals. Were they
more generally observable the world would have been spared much
scepticism and more theory. They may of course not be artificial, but
observations here [Flagstaff] indicate that they are; as will, I
think, appear from the drawings. For it is one thing to see two or
three canals and quite another to have the planet's disc mapped with
them on a most elaborate system of triangulation. In the first place
they are this season (August, 1894) bluish-green, of the same colour
as the seas into which the longer ones all eventually debouch. In the
next place they are almost without exception geodetically straight,
supernaturally so, and this in spite of their leading in every
possible direction. Then they are of apparently nearly uniform width
throughout their length. What they are is another matter. Their mere
aspect, however, is enough to cause all theories about glaciation
fissures or surface cracks to die an instant and natural death."[103]
Some of the observed colour-changes on Mars are very curious. In April,
1905, Mr. Lowell observed that the marking known as Mare Erythræum, just
above Syrtis, had "changed from a blue-green to a chocolate-brown colour."
The season on Mars corresponded with our February.
Signor V. Cerulli says that, having observed Mars regularly for ten years,
he has come to the conclusion that the actual existence of the "canals" is
as much a subject for physiological as for astronomical investigation. He
states that "the phenomena observed are so near the limit of the range of
the human eye that in observing them one really experiences an effect
accompanying the 'birth of vision.' That is to say, the eye sees more and
more as it becomes accustomed, or strained, to the delicate markings, and
thus the joining up of spots to form 'canals' and the gemination of the
latter follow as a physiological effect, and need not necessarily be
subjective phenomena seen by the unaccustomed eye."[104]
The possibility of life on Mars has been recently much discussed; some
denying, others asserting. M. E. Rogovsky says--
"As free oxygen and carbonic dioxide may exist in the atmosphere of
_Mars_, vegetable and animal life is quite possible. If the
temperature which prevails upon _Mars_ is nearer to -36° C. than to
-73° C., the existence of living beings like ourselves is possible. In
fact, the ice of some Greenland and Alpine glaciers is covered by red
algæ (_Sphærella nivalis_); we find there also different species of
rotaloria, variegated spiders, and other animals on the snow fields
illuminated by the sun; at the edges of glacier snows in the Tyrol we
see violet bells of _Soldanella pusilla_, the stalks of which make
their way through the snow by producing heat which melts it round
about them. Finally the Siberian town Verkhociansk, near Yakutsk,
exists, though the temperature there falls to -69°·8 C. and the mean
temperature of January to -51°·2, and the mean pressure of the vapour
of water is less than 0·05mm. It is possible, therefore, that living
beings have become adapted to the conditions now prevailing upon
_Mars_ after the lapse of many ages, and live at an even lower
temperature than upon the earth, developing the necessary heat
themselves."
M. Rogovsky adds, "Water in organisms is mainly a liquid or solvent, and
many other liquids may be the same. We have no reason to believe that life
is possible only under the same conditions and with the same chemical
composition of organisms as upon the earth, although indeed we cannot
affirm that they actually exist on Mars."[105] With the above views the
present writer fully concurs.
Prof. Lowell thinks that the polar regions of Mars, both north and south,
are actually warmer than the corresponding regions of the earth, although
the mean temperature of the planet is probably twelve degrees lower than
the earth's mean temperature.[106]
A writer in _Astronomy and Astrophysics_ (1892, p. 748) says--
"Whether the planet Mars is inhabited or not seems to be the
all-absorbing question with the ordinary reader. With the astronomer
this query is almost the last thing about the planet that he would
think of when he has an opportunity to study its surface markings ...
no astronomer claims to know whether the planet is inhabited or not."
Several suggestions have been made with reference to the possibility of
signalling to Mars. But, as Mr. Larkin of Mount Lowe (U.S.A.) points out,
all writers on this subject seem to forget the fact that the night side of
two planets are never turned towards each other. "When the sun is between
them it is day on the side of Mars which is towards us, and also day on
the side of the earth which is towards Mars. When they are on the same
side of the sun, it is day on Mars when night on the earth, and for this
reason they could never see our signals. This should make it apparent that
the task of signalling to Mars is a more difficult one than the most
hopeful theorist has probably considered. All this is under the
supposition that the Martians (if there are such) are beings like
ourselves. If they are not like us, we cannot guess what they are
like."[107] These views seem to me to be undoubtedly correct, and show the
futility of visual signals. Electricity might, however, be conceivably
used for the purpose; but even this seems highly improbable.
Prof. Newcomb, in his work _Astronomy for Everybody_, says with reference
to this question, "The reader will excuse me from saying nothing in this
chapter about the possible inhabitants of Mars. He knows just as much
about the subject as I do, and that is nothing at all."
It is, however, quite possible that life _in some form_ may exist on Mars.
As Lowell well says, "Life but waits in the wings of existence for its cue
to enter the scene the moment the stage is set."[108] With reference to
the "canals" he says--
"It is certainly no exaggeration to say that they are the most
astonishing objects to be viewed in the heavens. There are celestial
sights more dazzling, spectacles that inspire more awe, but to the
thoughtful observer who is privileged to see them well, there is
nothing in the sky so profoundly impressive as these canals of
Mars."[109]
The eminent Swedish physicist Arrhenius thinks that the mean annual
temperature on Mars may possibly be as high as 50° F. He says, "Sometimes
the snow-caps on the poles of Mars disappear entirely during the Mars
summer; this never happens on our terrestrial poles. The mean temperature
of Mars must therefore be above zero, probably about +10° [Centigrade =
50° Fahrenheit]. Organic life may very probably thrive, therefore, on
Mars."[110] He thinks that this excess of mean temperature above the
calculated temperature may be due to an increased amount of carbonic acid
in the planet's atmosphere, and says "any doubling of the percentage of
carbon dioxide in the air would raise the temperature of the earth's
surface by 4°; and if the carbon dioxide were increased fourfold, the
temperature would rise by 8°."[111]
Denning says,--[112]
"A few years ago, when christening celestial formations was more in
fashion than it is now, a man simply had to use a telescope for an
evening or two on Mars or the moon, and spice the relation of his
seeings with something in the way of novelty, when his name would be
pretty certainly attached to an object and hung in the heavens for all
time! A writer in the _Astronomical Register_ for January, 1879,
humorously suggested that 'the matter should be put into the hands of
an advertising agent,' and 'made the means of raising a revenue for
astronomical purposes.' Some men would not object to pay handsomely
for the distinction of having their names applied to the seas and
continents of Mars or the craters of the moon."
An occultation of Mars by the moon is recorded by Aristotle as having
occurred on April 4, 357 B.C.[113]
Seen from Mars the maximum apparent distance between the earth and moon
would vary from 3-1/2' to nearly 17'.[114]
CHAPTER VII
The Minor Planets
Up to 1908 the number of minor planets (or asteroids) certainly known
amounted to over 650.
From an examination of the distribution of the first 512 of these small
bodies, Dr. P. Stroobant finds that a decided maximum in number occurs
between the limits of distance of 2·55 and 2·85 (earth's mean distance
from sun = 1), "199 of the asteroids considered revolving in this
annulus." He finds that nearly all the asteroidal matter is concentrated
near to the middle of the ring in the neighbourhood of the mean distance
of 2·7, and the smallest asteroids are relatively less numerous in the
richest zones.[115]
There are some "striking similarities" in the orbits of some of the
asteroids. Thus, in the small planets Sophia (No. 251 in order of
discovery) and Magdalena (No. 318) we have the mean distance of Sophia
3·10, and that of Magdalena 3·19 (earth's mean distance = 1). The
eccentricities of the orbits are 0·09 and 0·07; and the inclinations of
the orbits to the plane of the ecliptic 10° 29' and 10° 33'
respectively.[116] This similarity may be--and probably is--merely
accidental, but it is none the less curious and interesting.
Some very interesting discoveries have recently been made among the minor
planets. The orbit of Eros intersects the orbit of Mars; and the following
have nearly the same mean distance from the sun as Jupiter:--
Achilles (1906 TG), No. 588,
Patrocles (1906 XY), No. 617,
Hector (1907 XM), No. 624,
and another (No. 659) has been recently found. Each of these small planets
"moves approximately in a vertex of an equilateral triangle that it forms
with Jupiter and the sun."[117] The minor planet known provisionally as HN
is remarkable for the large eccentricity of its orbit (0·38), and its
small perihelion distance (1·6). When discovered it had a very high South
Declination (61-1/2°), showing that the inclination of the plane of its
orbit to the plane of the ecliptic is considerable.[118]
Dr. Bauschinger has made a study of the minor planets discovered up to the
end of 1900. He finds that the ascending nodes of the orbits show a
marked tendency to cluster near the ascending node of Jupiter's orbit, a
fact which agrees well with Prof. Newcomb's theoretical results. There
seems to be a slight tendency for large inclinations and great
eccentricities to go together; but there appears to be no connection
between the eccentricity and the mean distance from the sun. The
longitudes of the perihelia of these small planets "show a well-marked
maximum near the longitude of _Jupiter's_ perihelion, and equally
well-marked minimum near the longitude of his aphelion," which is again in
good agreement with Newcomb's calculations.[119] Dr. Bauschinger's
diameter for Eros is 20 miles. He finds that the whole group, including
those remaining to be discovered, would probably form a sphere of about
830 miles in diameter.
The total mass of the minor planets has been frequently estimated, but
generally much too high. Mr. B. M. Roszel of the John Hopkins University
(U.S.A.) has made a calculation of the probable mass from the known
diameter of Vesta (319 miles, Pickering), and finds the volume of the
first 216 asteroids discovered. From this calculation it appears that it
would take 310 asteroids of the 6th magnitude, or 1200 of the 7th to equal
the moon in volume. Mr. Roszel concludes that the probable mass of the
whole asteroidal belt is between 1/50th and 1/100th of that of the
moon.[120] Subsequently Mr. Roszel extended his study to the mass of 311
asteroids,[121] and found a combined mass of about 1/40th of the moon's
mass.
Dr. Palisa finds that the recently discovered minor planet (1905 QY)
varies in light to a considerable extent.[122] This planet was discovered
by Dr. Max Wolf on August 23, 1905; but it was subsequently found that it
is identical with one previously known, (167) Urda.[123] The light
variation is said to be from the 11th to the 13th magnitude.[124]
Variation in some of the other minor planets has also been suspected.
Prof. Wendell found a variation of about half a magnitude in the planet
Eunomia (No. 15). He also found that Iris (No. 7) varies about a quarter
of a magnitude in a period of about 6{h} 12{m}.[125] But these variations
are small, and perhaps doubtful. The variability of Eros is well known.
The planet Eros is a very interesting one. The perihelion portion of its
orbit lies between the orbits of Mars and the earth, and the aphelion part
is outside the orbit of Mars. Owing to the great variation in its distance
from the earth the brightness of Eros varies from the 6th to the 12th
magnitude. That is, when brightest, it is 250 times brighter than when it
is faintest.[126] This variation of light, is of course, merely due to the
variation of distance; but some actual variation in the brightness of the
planet has been observed.
It has been shown by Oeltzen and Valz that Cacciatore's supposed distant
comet, mentioned by Admiral Smyth in his _Bedford Catalogue_, must have
been a minor planet.[127]
Dr. Max Wolf discovered 36 new minor planets by photography in the years
1892-95. Up to the latter year he had never seen one of these through a
telescope! His words are, "Ich selsbt habe noch nie einen meinen kleinen
Planeten am Himmel gesehen."[128]
These small bodies have now become so numerous that it is a matter of much
difficulty to follow them. At the meeting of the Royal Astronomical
Society on January 8, 1909, Mr. G. F. Chambers made the following
facetious remarks--
"I would like to make a suggestion that has been in my mind for
several years past--that it should be made an offence punishable by
fine or imprisonment to discover any more minor planets. They seem to
be an intolerable nuisance, and are a great burden upon the literary
gentlemen who have to keep pace with them and record them. I have
never seen, during the last few years at any rate, any good come from
them, or likely to come, and I should like to see the supply stopped,
and the energies of the German gentlemen who find so many turned into
more promising channels."
Among the minor planets numbered 1 to 500, about 40 "have not been seen
since the year of their discovery, and must be regarded as lost."[129]
CHAPTER VIII
Jupiter
This brilliant planet--only inferior to Venus in brightness--was often
seen by Bond (Jun.) with the naked eye in "high and clear sunshine"; also
by Denning, who has very keen eyesight. Its brightness on such occasions
is so great, that--like Venus--it casts a distinct shadow in a dark
room.[130]
The great "red spot" on Jupiter seems to have been originally discovered
by Robert Hooke on May 9, 1664, with a telescope of 2 inches aperture and
12 feet focus. It seems to have existed ever since; at least the evidence
is, according to Denning, in favour of the identity of Hooke's spot with
the red spot visible in recent years. The spot was also observed by
Cassini in the years 1665-72, and is sometimes called "Cassini's spot."
But the real discoverer was Hooke.[131]
The orbit of Jupiter is so far outside the earth's orbit that there can
be little visible in the way of "phase"--as in the case of Mars, where the
"gibbous" phase is sometimes very perceptible. Some books on astronomy
state that Jupiter shows no phase. But this is incorrect. A distinct,
although very slight, gibbous appearance is visible when the planet is
near quadrature. Webb thought it more conspicuous in twilight than in a
dark sky. With large telescopes, Jupiter's satellites II. and III. have
been seen--in consequence of Jupiter's phase--to emerge from occultation
"at a sensible distance from the limb."[132]
According to M. E. Rogovsky, the high "albedo of Jupiter, the appearance
of the clear (red) and dark spots on its surface and their continual
variation, the different velocity of rotation of the equatorial and other
zones of its surface, and particularly its small density (1·33, water as
unity), all these facts afford irrefragable proofs of the high temperature
of this planet. The dense and opaque atmosphere hides its glowing surface
from our view, and we see therefore only the external surface of its
clouds. The objective existence of this atmosphere is proved by the bands
and lines of absorption in its spectrum. The interesting photograph
obtained by Draper, September 27, 1879, in which the blue and green parts
are more brilliant for the equatorial zone than for the adjacent parts of
the surface, appears to show that _Jupiter_ emits its proper light. It is
possible that the constant red spot noticed on its surface by several
observers, as Gledhill, Lord Rosse, and Copeland (1873), Russel and
Bredikhin (1876), is the summit of a high glowing mountain. G. W. Hough
considers Jupiter to be gaseous, and A. Ritter inferred from his formulæ
that in this case the temperature at the centre would be 600,000° C."[133]
The four brighter satellites of Jupiter are usually known by numbers I.,
II., III., and IV.; I. being the nearest to the planet, and IV. the
farthest. III. is usually the brightest, and IV. the faintest, but
exceptions to this rule have been noticed.
With reference to the recently discovered sixth and seventh satellites of
Jupiter, Prof. Perrine has suggested that the large inclination of their
orbits to the plane of the planet's equator seems to indicate that neither
of these bodies was originally a member of Jupiter's family, but has been
"captured by the planet." This seems possible as the orbits of some of the
minor planets lie near the orbit of Jupiter (see "Minor Planets"). A
similar suggestion has been made by Prof. del Marmol.[134]
Many curious observations have been recorded with reference to Jupiter's
satellites; some very difficult of explanation. In 1711 Bianchini saw
satellite IV. so faint for more than an hour that it was hardly visible! A
similar observation was made by Lassell with a more powerful telescope on
June 13, 1849. Key, T. T. Smyth, and Denning have also recorded unusual
faintness.[135] A very remarkable phenomenon was seen by Admiral Smyth,
Maclear, and Pearson on June 26, 1828. Satellite II., "having fairly
entered on Jupiter, was found 12 or 13 minutes afterwards _outside the
limb_, where it remained visible for at least 4 minutes, and then suddenly
vanished." As Webb says, "Explanation is here set at defiance;
demonstrably neither in the atmosphere of the earth, nor Jupiter, where
and what could have been the cause? At present we can get no answer."[136]
When Jupiter is in opposition to the sun--that is, on the meridian at
midnight--satellite I. has been seen projected on its own shadow, the
shadow appearing as a dark ring round the satellite.
On January 28, 1848, at Cambridge (U.S.A.) satellite III. was seen in
transit lying between the shadows of I. and II. and so black that it could
not be distinguished from the shadows, "except by the place it occupied."
This seems to suggest inherent light in the planet's surface, as the
satellite was at the time illuminated by full sunshine; its apparent
blackness being due to the effect of contrast. Cassini on one occasion
failed to find the shadow of satellite I. when it should have been on the
planet's disc,[137] an observation which again points to the glowing light
of Jupiter's surface. Sadler and Trouvelot saw the shadow of satellite I.
double! an observation difficult to explain--but the same phenomenon was
again seen on the evening of September 19, 1891, by Mr. H. S. Halbert of
Detroit, Michigan (U.S.A.). He says that the satellite "was in transit
nearing egress, and it appeared as a white disc against the dark southern
equatorial belt; following it was the usual shadow, and at an equal
distance from this was a second shadow, smaller and not so dark as the
true one, and surrounded by a faint penumbra."[138]
A dark transit of satellite III. was again seen on the evening of December
19, 1891, by two observers in America. One observer noted that the
satellite, when on the disc of the planet, was intensely black. To the
other observer (Willis L. Barnes) it appeared as an ill-defined _dark_
image.[139] A similar observation was made on October 9 of the same year
by Messrs. Gale and Innes.[140]
A "black transit" of satellite IV. was seen by several observers in 1873,
and by Prof. Barnard on May 4, 1886. The same phenomenon was observed on
October 30, 1903, in America, by Miss Anne S. Young and Willis S. Barnes.
Miss Young says--
"The ingress of the satellite took place at 8{h} 50{m} (E. standard
time) when it became invisible upon the background of the planet. An
hour later it was plainly visible as a dark round spot upon the
planet. It was decidedly darker than the equatorial belt."[141]
The rather rare phenomenon of an occultation of one of Jupiter's
satellites by another was observed by Mr. Apple, director of the Daniel
Scholl Observatory, Franklin and Marshall College, Lancaster, Pa.
(U.S.A.), on the evening of March 16, 1908. The satellites in question
were I. and II., and they were so close that they could not be separated
with the 11·5-inch telescope of the Observatory.[142] One of the present
writer's first observations with a telescope is dated May 17, 1873, and is
as follows: "Observed one of Jupiter's satellites occulted (or very nearly
so) by another. Appeared as one with power 133" (on 3-inch refractor in
the Punjab). These satellites were probably I. and II.
Jupiter has been seen on several occasions apparently without his
satellites; some being behind the disc, some eclipsed in his shadow, and
some in transit across the disc. This phenomenon was seen by Galileo,
March 15, 1611; by Molyneux, on November 12, 1681; by Sir William
Herschel, May 23, 1802; by Wallis, April 15, 1826; by Greisbach, September
27, 1843; and by several observers on four occasions in the years
1867-1895.[143] The phenomenon again occurred on October 3, 1907, No. 1
being eclipsed and occulted, No. 2 in transit, No. 3 eclipsed, and No. 4
occulted.[144] It was not, however, visible in Europe, but could have been
seen in Asia and Oceania.[144] The phenomenon will occur again on October
22, 1913.[145]
On the night of September 19, 1903, a star of magnitude 6-1/2 was occulted
by the disc of Jupiter. This curious and rare phenomenon was photographed
by M. Lucien Rudaux at the Observatory of Donville, France.[146] The star
was Lalande 45698 (= BAC 8129).[147]
Prof. Barnard, using telescopes with apertures from 5 inches up to 36
inches (Lick), has failed to see a satellite through the planet's limb (an
observation which has been claimed by other astronomers). He says, "To my
mind this has been due to either poor seeing, a poor telescope, or an
excited observer."[148] He adds--
"I think it is high time that the astronomers reject the idea that the
satellites of Jupiter can be seen through his limb at occultation.
When the seeing is bad there is a spurious limb to Jupiter that well
might give the appearance of transparency at the occultation of a
satellite. But under first-class conditions the limb of Jupiter is
perfectly opaque. It is quibbling and begging the question altogether
to say the phenomenon of transparency may be a rare one and so have
escaped my observations. Has any one said that the moon was
transparent when a star has been seen projected on it when it ought to
have been behind it?"
Prof. Barnard and Mr. Douglass have seen white polar caps on the third and
fourth satellites of Jupiter. The former says they are "exactly like those
on Mars." "Both caps of the fourth satellite have been clearly
distinguished, that at the north being sometimes exceptionally large,
covering a surface equal to one-quarter or one-third of the diameter of
the satellite."[149] This was confirmed on November 23, 1906, when Signor
J. Comas Sola observed a brilliant white spot surrounded by a dark marking
in the north polar region of the third satellite. There were other dark
markings visible, and the satellite presented the appearance of a
miniature of Mars.[150]
An eighth satellite of Jupiter has recently been discovered by Mr. Melotte
at the Greenwich Observatory by means of photography. It moves in a
retrograde direction round Jupiter in an orbit inclined about 30° to that
of the planet. The period of revolution is about two years. The orbit is
very eccentric, the eccentricity being about one-third, or greater than
that of any other satellite of the solar system. When nearest to Jupiter
it is about 9 millions of miles from the planet, and when farthest about
20 millions.[151] It has been suggested by Mr. George Forbes that this
satellite may possibly be identical with the lost comet of Lexell which at
its return in the year 1779 became entangled in Jupiter's system, and has
not been seen since. If this be the case, we should have the curious
phenomenon of a comet revolving round a planet!
According to Humboldt the four bright satellites of Jupiter were seen
almost simultaneously and quite independently by Simon Marius at Ausbach
on December 29, 1609, and by Galileo at Padua on January 7, 1610.[152] The
actual priority, therefore, seems to rest with Simon Marius, but the
publication of the discovery was first made by Galileo in his _Nuncius
Siderius_ (1610).[153] Grant, however, in his _History of Physical
Astronomy_, calls Simon Marius an "impudent pretender"! (p. 79).
M. Dupret at Algiers saw Jupiter with the naked eye on September 26, 1890,
twenty minutes before sunset.[154]
Humboldt states that he saw Jupiter with the naked eye when the sun was
from 18° to 20° above the horizon.[155] This was in the plains of South
America near the sea-level.
CHAPTER IX
Saturn
To show the advantages of large telescopes over small ones, Mr. C. Roberts
says that "with the 25-inch refractor of the Cambridge Observatory the
view of the planet Saturn is indescribably glorious; everything I had ever
seen before was visible at a glance, and an enormous amount of detail that
I had never even glimpsed before, after a few minutes' observation."[156]
Chacornac found that the illumination of Saturn's disc is the reverse of
that of Jupiter, the edges of Saturn being brighter than the centre of the
disc, while in the case of Jupiter--as in that of the sun--the edges are
fainter than the centre.[157] According to Mr. Denning, Saturn bears
satisfactorily "greater magnifying power than either Mars or
Jupiter."[158]
At an occultation of Saturn by the moon, which occurred on June 13, 1900,
M. M. Honorat noticed the great contrast between the slightly yellowish
colour of the moon and the greenish tint of the planet.[159]
In the year 1892, when the rings of Saturn had nearly disappeared, Prof.
L. W. Underwood, of the Underwood Observatory, Appleton, Wisconsin
(U.S.A.), saw one of Saturn's satellites (Titan) apparently moving along
the needlelike appendage to the planet presented by the rings. "The
apparent diameter of the satellite so far exceeded the apparent thickness
of the ring that it gave the appearance of a beautiful golden bead moving
very slowly along a fine golden thread."[160]
In 1907, when the rings of Saturn became invisible in ordinary telescopes,
Professor Campbell, observing with the great Lick telescope, noticed
"prominent bright knots, visible ... in Saturn's rings. The knots were
symmetrically placed, two being to the east and two to the west." This was
confirmed by Mr. Lowell, who says, "Condensations in Saturn's rings
confirmed here and measured repeatedly. Symmetric and permanent." This
phenomenon was previously seen by Bond in the years 1847-56. Measures of
these light spots made by Prof. Barnard with the 40-inch Yerkes telescope
show that the outer one corresponded in position with the outer edge of
the middle ring close to the Cassini division, and the inner condensation,
curious to say, seemed to coincide in position with the "crape ring."
Prof. Barnard thinks that the thickness of the rings "must be greatly
under 100 miles, and probably less than 50 miles," and he says--
"The important fact clearly brought out at this apparition of _Saturn_
is that the bright rings are not opaque to the light of the sun--and
this is really what we should expect from the nature of their
constitution as shown by the theory of Clerk Maxwell, and the
spectroscopic results of Keeler."[161]
Under certain conditions it would be theoretically possible, according to
Mr. Whitmell, to see the globe of Saturn through the Cassini division in
the ring. But the observation would be one of great difficulty and
delicacy. The effect would be that, of the arc of the division which
crosses the planet's disc, "a small portion will appear bright instead of
dark, and may almost disappear."[162]
A remarkable white spot was seen on Saturn on June 23, 1903, by Prof.
Barnard, and afterwards by Mr. Denning.[163] Another white spot was seen
by Denning on July 9 of the same year.[164] From numerous observations of
these spots, Denning found a rotation period for the planet of about
10{h} 39{m} 21{s}.[165] From observations of the same spots Signor Comas
Sola found a period 10{h} 38{m}·4, a close agreement with Denning's
result. For Saturn's equator, Prof. Hill found a rotation period of 10{h}
14{m} 23{s}·8, so that--as in the case of Jupiter--the rotation is faster
at the equator than in the northern latitudes of the planet. A similar
phenomenon is observed in the sun. Mr. Denning's results were fully
confirmed by Herr Leo Brenner, and other German astronomers.[166]
Photographs taken by Prof. V. M. Slipher in America show that the spectrum
of Saturn is similar to that of Jupiter. None of the bands observed in the
planet's spectrum are visible in the spectrum of the rings. This shows
that if the rings possess an atmosphere at all, it must be much rarer than
that surrounding the ball of the planet. Prof. Slipher says that "none of
the absorption bands in the spectrum of _Saturn_ can be identified with
those bands due to absorption in the earth's atmosphere," and there is no
trace of aqueous vapour.[167]
In September, 1907, M. G. Fournier suspected the existence of a "faint
transparent and luminous ring" outside the principal rings of Saturn. He
thinks that it may possibly be subject to periodical fluctuations of
brightness, sometimes being visible and sometimes not.[168] This dusky
ring was again suspected at the Geneva Observatory in October, 1908.[169]
M. Schaer found it a difficult object with a 16-inch Cassegrain reflector.
Prof. Stromgen at Copenhagen, and Prof. Hartwig at Bamberg, however,
failed to see any trace of the supposed ring.[170] It was seen at
Greenwich in October, 1908.
A "dark transit" of Saturn's satellite Titan across the disc of the planet
has been observed on several occasions. It was seen by Mr. Isaac W. Ward,
of Belfast, on March 27, 1892, with a 4·3-inch Wray refractor. The
satellite appeared smaller than its shadow. The phenomenon was also seen
on March 12 of the same year by the Rev. A. Freeman, Mr. Mee, and M. F.
Terby; and again on November 6, 1907, by Mr. Paul Chauleur and Mr. A. B.
Cobham.[171]
The recently discovered tenth satellite of Saturn, Themis, was discovered
by photography, and has never been seen by the eye even with the largest
telescopes! But its existence is beyond all doubt, and its orbit round the
planet has been calculated.
Prof. Hussey of the Lick Observatory finds that Saturn's satellite Mimas
is probably larger than Hyperion. He also finds from careful measurements
that the diameter of Titan is certainly overestimated, and that its
probable diameter is about 2500 miles.[172]
The French astronomer, M. Lucien Rudaux, finds the following variation in
the light of the satellites of Saturn:--
Japetus from 9th magnitude to 12th
Rhea " 9 " 10·6
Dione " 9·5 " 10·5
Tethys " 9·8 " 10·5
Titan " 8 " 8·6
The variation of light is, he thinks, due to the fact that the period of
rotation of each satellite is equal to that of their revolution round the
planet; as in the case of our moon.[173]
The names of the satellites of Saturn are derived from the ancient heathen
mythology. They are given in order of distance from the planet, the
nearest being Mimas and the farthest Themis.
1. Mimas was a Trojan born at the same time as Paris.
2. Enceladus was son of Tartarus and Ge.
3. Tethys was wife of Oceanus, god of ocean currents. She became mother of
all the chief rivers in the universe, as also the Oceanides or sea nymphs.
4. Dione was one of the wives of Zeus.
5. Rhea was a daughter of Uranus. She married Saturn, and became the
mother of Vesta, Ceres, Juno, and Pluto.
6. Titan was the eldest son of Uranus.
7. Hyperion was the god of day, and the father of sun and moon.
8. Japetus was the fifth son of Uranus, and father of Atlas and
Prometheus.[174]
9. Phoebe was daughter of Uranus and Ge.
10. Themis was daughter of Uranus and Ge, and, therefore, sister of
Phoebe.
In a review of Prof. Comstock's _Text Book of Astronomy_ in _The
Observatory_, November, 1901, the remark occurs, "We are astonished to see
that Mr. Comstock alludes with apparent seriousness to the _nine_
satellites of Saturn. As regards the ninth satellite, we thought that all
astronomers held with Mrs. Betsy Prig on the subject of this astronomical
Mrs. Harris." This reads curiously now (1909) when the existence of the
ninth satellite (Phoebe) has been fully confirmed, and a tenth satellite
discovered.
CHAPTER X
Uranus and Neptune
From observations of Uranus made in 1896, M. Leo Brenner concluded that
the planet rotates on its axis in about 8-1/2 hours (probably 8{h} 27{m}).
This is a short period, but considering the short periods of Jupiter and
Saturn there seems to be nothing improbable about it.
Prof. Barnard finds that the two inner satellites of Uranus are difficult
objects even with the great 36-inch telescope of the Lick Observatory!
They have, however, been photographed at Cambridge (U.S.A.) with a 13-inch
lens, although they are "among the most difficult objects known."[175]
Sir William Huggins in 1871 found strong absorption lines (six strong
lines) in the spectrum of Uranus. One of these lines indicated the
presence of hydrogen, a gas which does not exist in our atmosphere. Three
of the other lines seen were situated near lines in the spectrum of
atmospheric air. Neither carbonic acid nor sodium showed any indications
of their presence in the planet's spectrum. A photograph by Prof. Slipher
of Neptune's spectrum "shows the spectrum of this planet to contain many
strong absorption bands. These bands are so pronounced in the part of the
spectrum between the Fraunhofer lines F and D, as to leave the solar
spectrum unrecognizable.... Neptune's spectrum is strikingly different
from that of _Uranus_, the bands in the latter planet all being reinforced
in _Neptune_. In this planet there are also new bands which have not been
observed in any of the other planets. The F line of hydrogen is remarkably
dark ... this band is of more than solar strength in the spectrum of
Uranus also. Thus free hydrogen seems to be present in the atmosphere of
both these planets. This and the other dark bands in these planets bear
evidence of an enveloping atmosphere of gases which is quite unlike that
which surrounds the earth."[176]
With the 18-inch equatorial telescope of the Strasburgh Observatory, M.
Wirtz measured the diameter of Neptune, and found from forty-nine measures
made between December 9, 1902, and March 28, 1903, a value of 2"·303 at a
distance of 30·1093 (earth's distance from sun = 1). This gives a diameter
of 50,251 kilometres, or about 31,225 miles,[177] and a mean density of
1·54 (water = 1; earth's mean density = 5·53). Prof. Barnard's measures
gave a diameter of 32,900 miles, a fairly close agreement, considering the
difficulty of measuring so small a disc as that shown by Neptune.
The satellite of Neptune was photographed at the Pulkown Observatory in
the year 1899. The name Triton has been suggested for it. In the old Greek
mythology Triton was a son of Neptune, so the name would be an appropriate
one.
The existence of a second satellite of Neptune is suspected by Prof.
Schaeberle, who thinks he once saw it with the 36-inch telescope of the
Lick Observatory "on an exceptionally fine night" in 1895.[178] But this
supposed discovery has not yet been confirmed. Lassell also thought he had
discovered a second satellite, but this supposed discovery was never
confirmed.[178]
The ancient Burmese mention eight planets, the sun, the moon, Mercury,
Venus, Mars, Jupiter, Saturn, and another named Râhu, which is invisible.
It has been surmised that "Râhu" is Uranus, which is just visible to the
naked eye, and may possibly have been discovered by keen eyesight in
ancient times. The present writer has seen it several times without
optical aid in the West of Ireland, and with a binocular field-glass of 2
inches aperture he found it quite a conspicuous object.
When Neptune was _visually_ discovered by Galle, at Berlin, he was
assisted in his observation by Prof. d'Arrest. The incident is thus
described by Dr. Dreyer, "On the night of June 14, 1874, while observing
Coggia's comet together, I reminded Prof. d'Arrest how he had once said in
the course of a lecture, that he had been present at the finding of
Neptune, and that 'he might say it would not have been found without him.'
He then told me (and I wrote it down the next day), how he had suggested
the use of Bremiker's map (as first mentioned by Dr. Galle in 1877) and
continued, 'We then went back to the dome, where there was a kind of desk,
at which I placed myself with the map, while Galle, looking through the
refractor, described the configurations of the stars he saw. I followed
them on the map one by one, until he said: "And then there is a star of
the 8th magnitude, in such and such a position," whereupon I immediately
exclaimed: "That star is not on the map."'"[179] This was the planet. But
it seems to the present writer that if Galle or d'Arrest had access to
Harding's Atlas (as they probably had) they might easily have found the
planet with a good binocular field-glass. As a matter of fact Neptune is
shown in Harding's Atlas (1822) as a star of the 8th magnitude, having
been mistaken for a star by Lalande on May 8 and 10, 1795; and the present
writer has found Harding's 8th magnitude stars quite easy objects with a
binocular field-glass having object-glasses of two inches diameter, and a
power of about six diameters.
SUPPOSED PLANET BEYOND NEPTUNE.--The possible existence of a planet beyond
Neptune has been frequently suggested. From considerations on the aphelia
of certain comets, Prof. Forbes in 1880 computed the probable position of
such a body. He thought this hypothetical planet would be considerably
larger than Jupiter, and probably revolve round the sun at a distance of
about 100 times the earth's mean distance from the sun. The place
indicated was between R.A. 11{h} 24{m} and 12{h} 12{m}, and declination 0°
0' to 6° 0' north. With a view to its discovery, the late Dr. Roberts took
a series of eighteen photographs covering the region indicated. The result
of an examination of these photographs showed, Dr. Roberts says, that "no
planet of greater brightness than a star of the 15th magnitude exists on
the sky area herein indicated." Prof. W. H. Pickering has recently revived
the question, and has arrived at the following results: Mean distance of
the planet from the sun, 51·9 (earth's mean distance = 1); period of
revolution, 373-1/2 years; mass about twice the earth's mass; probable
position for 1909 about R.A. 7{h} 47{m}, north declination 21°, or about
5° south-east of the star [Greek: k] Geminorum. The supposed planet would
be faint, its brightness being from 11-1/2 to 13-1/2, according to the
"albedo" (or reflecting power) it may have.[180]
Prof. Forbes has again attacked the question of a possible ultra-Neptunian
planet, and from a consideration of the comets of 1556, 1843 I, 1880 I,
and 1882 II, finds a mean distance of 105·4, with an inclination of the
orbit of 52° to the plane of the ecliptic. This high inclination implies
that "during the greatest part of its revolution it is beyond the zodiac,"
and this, Mr. W. T. Lynn thinks, "may partly account for its not having
hitherto been found by observation."[181]
From a consideration of the approximately circular shape of the orbits of
all the large planets of the solar system, Dr. See suggests the existence
of three planets outside Neptune, with approximate distances from the sun
of 42, 56, and 72 respectively (earth's distance = 1), and recommends a
photographic search for them. He says, "To suppose the planetary system to
terminate with an orbit so round as that of Neptune is as absurd as to
suppose that Jupiter's system terminates with the orbit of the fourth
satellite."[182]
According to Grant, even twenty years before the discovery of Neptune the
error of Prof. Adams' first approximation amounted to little more than
10°.[183]
CHAPTER XI
Comets
We learn from Pliny that comets were classified in ancient times,
according to their peculiar forms, into twelve classes, of which the
principal were: _Pogonias_, bearded; _Lampadias_, torch-like; _Xiphias_,
sword-like; _Pitheus_, tun-like; _Acontias_, javelin-like; _Ceratias_,
horn-like; _Disceus_, quoit-like; and _Hippias_, horse-mane-like.[184]
Of the numerous comets mentioned in astronomical records, comparatively
few have been visible to the naked eye. Before the invention of the
telescope (1610) only those which were so visible _could_, of course, be
recorded. These number about 400. Of the 400 observed since then, some 70
or 80 only have been visible by unaided vision; and most of these now
recorded could never have been seen without a telescope. During the last
century, out of 300 comets discovered, only 13 were very visible to the
naked eye. Hence, when we read in the newspapers that a comet has been
discovered the chances are greatly against it becoming visible to the
naked eye.[185]
Although comparatively few comets can be seen without a telescope, they
are sometimes bright enough to be visible in daylight! Such were those of
B.C. 43, A.D. 1106, 1402, 1532, 1577, 1744, 1843, and the "great September
comet" of 1882.
If we except the great comet of 1861, through the tail of which the earth
is supposed to have passed, the comet which came nearest to the earth was
that of 1770, known as Lexell's, which approached us within two millions
of miles, moving nearly in the plane of the ecliptic. It produced,
however, no effect on the tides, nor on the moon's motion, which shows
that its mass must have been very small. It was computed by Laplace that
if its mass had equalled that of the earth, the length of our year would
have been shortened by 2 hours 47 minutes, and as there was no perceptible
change Laplace concluded that the comet's mass did not exceed 1/5000th of
the earth's mass. This is the comet which passed so near to Jupiter that
its period was reduced to 5-1/2 years. Owing to another near approach in
1779 it became invisible from the earth, and is now lost.[186] Its
identity with the recently discovered eighth satellite of Jupiter has been
suggested by Mr. George Forbes (see under "Jupiter"). At the near approach
of Lexell's comet to the earth in 1770, Messier, "the comet ferret,"
found that its head had an apparent diameter of 2-1/2°, or nearly five
times that of the moon!
Another case of near approach to the earth was that of Biela's comet at
its appearance in 1805. On the evening of December 9 of that year, the
comet approached the earth within 3,380,000 miles.[187]
The comet of A.D. 1106 is stated to have been seen in daylight close to
the sun. This was on February 4 of that year. On February 10 it had a tail
of 60° in length, according to Gaubil.[188]
The comet of 1577 seems to have been one of the brightest on record.
According to Tycho Brahé, it was visible in broad daylight. He describes
the head as "round, bright, and of a yellowish light," with a curved tail
of a reddish colour.[189]
The comet of 1652 was observed for about three weeks only, and Hevelius
and Comiers state that it was equal to the moon in apparent size! This
would indicate a near approach to the earth. An orbit computed by Halley
shows that the least distance was about 12 millions of miles, and the
diameter of the comet's head rather less than 110,000 miles, or about 14
times the earth's diameter.
According to Mr. Denning, "most of the periodical comets at perihelion are
outside the earth's orbit, and hence it follows that they escape
observation unless the earth is on the same side of the sun as the
comet."[190]
It was computed by M. Faye that the _volume_ of the famous Donati's comet
(1858) was about 500 times that of the sun! On the other hand, he
calculated that its _mass_ (or quantity of matter it contained) was only a
fraction of the earth's mass. This shows how almost inconceivably tenuous
the material forming the comet must have been--much more rarefied, indeed,
than the most perfect vacuum which can be produced in an air-pump. This
tenuity is shown by the fact that stars were seen through the tail "as if
the tail did not exist." A mist of a few hundred yards in thickness is
sufficient to hide the stars from our view, while a thickness of thousands
of miles of cometary matter does not suffice even to dim their brilliancy!
At the time of the appearance of the great comet of 1843, it was doubtful
whether the comet had transited the sun's disc. But it is now known, from
careful calculations by Prof. Hubbard, that a transit really took place
between 11{h} 28{m} and 12{h} 29{m} on February 27, 1843, and might have
been observed in the southern hemisphere. The distance of this remarkable
comet from the sun at its perihelion passage was less than that of any
known comet. A little before 10 p.m. on February 27, the comet passed
within 81,500 miles of the sun's surface with the enormous velocity of
348 miles a second! It remained less than 2-1/4 hours north of the
ecliptic, passing from the ascending to the descending node of its orbit
in 2{h} 13{m}·4.[191] The great comet of 1882 transited the sun's disc on
Sunday, September 17, of that year, the ingress taking place at 4{h} 50{m}
58{s}, Cape mean time. When on the sun the comet was absolutely invisible,
showing that there was nothing solid about it. It was visible near the sun
with the naked eye a little before the transit took place.[192] This great
comet was found by several computors to have been travelling in an
elliptic orbit with a period of about eight centuries. Morrison found 712
years; Frisby, 794; Fabritius, 823; and Kreutz, 843 years.[193]
The great southern comet of 1887 may be described as a comet without a
head! The popular idea of a comet is a star with a tail. But in this case
there was no head visible--to the naked eye at least. Dr. Thome of the
Cordoba Observatory--its discoverer--describes it as "a beautiful
object--a narrow, straight, sharply defined, graceful tail, over 40° long,
shining with a soft starry light against a dark sky, beginning apparently
without a head, and gradually widening and fading as it extended
upwards."[194]
The great southern comet of 1901 had five tails on May 6 of that year. Two
were fairly bright, and the remaining three rather faint. Mr. Gale saw a
number of faint stars through the tails. The light of these seem to have
been "undimmed." Mr. Cobham noticed that the stars Rigel and [Greek: b]
Eridani shone through one of the faint tails, and "showed no perceptible
difference."[195]
Prof. W. H. Pickering says that "the head of a comet, as far as our
present knowledge is concerned, seems therefore to be merely a meteor
swarm containing so much gaseous material that when electrified by its
approach to the sun it will be rendered luminous" (_Harvard Annual_, vol.
xxxii. part ii. p. 295) "... if the meteors and their atmospheres are
sufficiently widely separated from one another, the comet may be brilliant
and yet transparent at the same time."
In the case of Swift's comet of 1892 some periodical differences of
appearance were due, according to Prof. W. H. Pickering, to a rotation of
the comet round an axis passing longitudinally through the tail, and he
estimated the period of rotation at about 94 to 97 hours. He computed that
in this comet the repulsive force exerted by the sun on the comet's tail
was "about 39·5 times the gravitational force."[196]
The comet known as 1902_b_ approached the planet Mercury within two
millions of miles on November 29 of that year. Prof. O. C. Wendell, of
Harvard Observatory, made some observations on the transparency of this
comet. He found with the aid of a photometer and the 15-inch telescope of
the observatory that in the case of two faint stars over which the comet
passed on October 14, 1902, the absorption of light by the comet was
insensible, and possibly did not exceed one or two hundredths of a
magnitude,[197] an amount quite imperceptible to the naked eye, and shows
conclusively how almost inconceivably rarefied the substance of this comet
must be.
The comet known as Morehouse (1908_c_) showed some curious and wonderful
changes. Mr. Borelly found that five tails are visible on a photographic
plate taken on October 3, 1908, and the trail of an occulted star
indicates a slight absorption effect. According to M. L. Rabourdin, great
changes took place from day to day, and even during the course of an hour!
Similar changes were recorded by G. M. Gauthier; and Prof. Barnard, who
photographed the comet on 30 nights from September 2 to October 13, states
that the photographs of September 30 "are unique, whilst the
transformation which took place between the taking of these and the taking
of the next one on October 1 was very wonderful."[198] The spectrum
showed the lines of cyanogen instead of the hydrocarbon spectrum shown by
most comets.
Prof. Barnard has suggested that all the phenomena of comets' tails cannot
be explained by a repulsive force from the sun. Short tails issuing from
the comet's nucleus at considerable angles with the main tail point to
eruptive action in the comet itself. The rapid changes and distortions
frequently observed in the tails of some comets suggest motion through a
resisting medium; and the sudden increase of light also occasionally
observed points in the same direction.[199]
It was computed by Olbers that if a comet having a mass of 1/2000th of the
earth's mass--which would form a globe of about 520 miles in diameter and
of the density of granite--collided with the earth, with a velocity of 40
miles a second, our globe would be shattered into fragments.[200] But that
any comet has a solid nucleus of this size seems very doubtful; and we may
further say that the collision of the earth with _any_ comet is highly
improbable.
It seems to be a common idea that harvests are affected by comets, and
even "comet wines" are sometimes spoken of. But we know that the earth
receives practically no heat from the brightest comet. Even in the case of
the brilliant comet of 1811, one of the finest on record, it was found
that "all the efforts to concentrate its rays did not produce the
slightest effect on the blackened bulb of the most sensitive thermometer."
Arago found that the year 1808, in which several comets were visible, was
a cold year, "and 1831, in which there was no comet, enjoyed a much higher
temperature than 1819, when there were three comets, one of which was very
brilliant."[201] We may, therefore, safely conclude that even a large
comet has no effect whatever on the weather.
From calculations on the orbit of Halley's comet, the next return of which
is due in 1910, Messrs. Cowell and Crommelin find that the identity of the
comet shown on the Bayeux Tapestry with Halley's comet is now "fully
established." They find that the date of perihelion passage was March 25,
1066, which differs by only 4 days from the date found by Hind. The
imposing aspect of the comet in 1066 described in European chronicles of
that time is confirmed by the Chinese Annals. In the latter records the
brightness is compared to that of Venus, and even with that of the moon!
The comparison with the moon was probably an exaggeration, but the comet
doubtless made a very brilliant show. In the Bayeux Tapestry the
inscription on the wall behind the spectators reads: "_isti mirant
stella_." Now, this is bad Latin, and Mr. W. T. Lynn has made the
interesting suggestion that some of the letters are hidden by the
buildings in front and that the real sentence is "_isti mirantur
stellam_."[202] The present writer has examined the copy of the Bayeux
Tapestry which is in the Dublin Museum, and thinks that Mr. Lynn's
suggestion seems very plausible. But the last letter of _stellam_ is
apparently hidden by the comet's tail, which does not seem very probable!
The conditions under which the comet will appear in 1910 are not unlike
those of 1066 and 1145. "In each year the comet was discovered as a
morning star, then lost in the sun's rays; on its emergence it was near
the earth and moved with great rapidity, finally becoming stationary in
the neighbourhood of Hydra, where it was lost to view."[203] In 1910 it
will probably be an evening star before March 17, and after May 11, making
a near approach to the earth about May 12. About this time its apparent
motion in the sky will be very rapid. As, however, periodical comets--such
as Halley's--seem to become fainter at each return, great expectations
with reference to its appearance in 1910 should not be indulged in.
The appearance of Halley's comet in A.D. 1222 is thus described by
Pingré--a great authority on comets--(quoting from an ancient writer)--
"In autumn, that is to say in the months of August and September, a
star of the first magnitude was seen, very red, and accompanied by a
great tail which extended towards the top of the sky in the form of a
cone extremely pointed. It appeared to be very near the earth. It was
observed (at first?) near the place of the setting sun in the month of
December."
With reference to its appearance in the year 1456, when it was of "vivid
brightness," and had a tail of 60° in length, Admiral Smyth says,[204] "To
its malign influence were imputed the rapid successes of Mahomet II.,
which then threatened all Christendom. The general alarm was greatly
aggravated by the conduct of Pope Callixtus III., who, though otherwise a
man of abilities, was a poor astronomer; for that pontiff daily ordered
the church bells to be rung at noon-tide, extra _Ave-Marias_ to be
repeated, and a special protest and excommunication was composed,
exorcising equally the Devil, the Turks, and the comet." With reference to
this story, Mr. G. F. Chambers points out[205] that it is probably based
on a passage in Platina's _Vitæ Pontificum_. But in this passage there is
no mention made of excommunication or exorcism, so that the story, which
has long been current, is probably mythical. In confirmation of this view,
the Rev. W. F. Rigge has shown conclusively[206] that no bull was ever
issued by Pope Callixtus III. containing a reference to _any_ comet. The
story would therefore seem to be absolutely without foundation, and should
be consigned to the limbo of all such baseless myths.
With reference to the appearance of Halley's comet, at his last return in
1835, Sir John Herschel, who observed it at the Cape of Good Hope, says--
"Among the innumerable stars of all magnitudes, from the ninth
downwards, which at various times were seen through it, and some
extremely near to the nucleus (though not _exactly on it_) there never
appeared the least ground for presuming any extinction of their light
in traversing it. Very minute stars indeed, on entering its brightest
portions, were obliterated, as they would have been by an equal
illumination of the field of view; but stars which before their entry
appeared bright enough to bear that degree of illumination, were in no
case, so far as I could judge, affected to a greater extent than they
would have been by so much lamp-light artificially introduced."[207]
It is computed by Prof. J. Holetschak that, early in October, 1909,
Halley's comet should have the brightness of a star of about 14-1/2
magnitude.[208] It should then--if not detected before--be discoverable
with some of the large telescopes now available.
According to the computations of Messrs. Cowell and Crommelin, the comet
should enter Pisces from Aries in January, 1910. "Travelling westward
towards the star [Greek: g] Piscium until the beginning of May, and then
turning eastward again, it will travel back through the constellations
Cetus, Orion, Monoceros, Hydra, and Sextans." From this it seems that
observers in the southern hemisphere will have a better view of the comet
than those in northern latitudes. The computed brightness varies from 1 on
January 2, 1910, to 1112 on May 10. But the actual brightness of a comet
does not always agree with theory. It is sometimes brighter than
calculation would indicate.
According to Prof. O. C. Wendell, Halley's comet will, on May 12, 1910,
approach the earth's orbit within 4·6 millions of miles; and he thinks
that possibly the earth may "encounter some meteors," which are presumably
connected with the comet. He has computed the "radiant point" of these
meteors (that is, the point from which they appear to come), and finds its
position to be R.A. 22{h} 42{m}·9, Decl. N. 1° 18'. This point lies a
little south-west of the star [Greek: b] Piscium.
According to Dr. Smart, the comet will, on June 2, "cross the Equator
thirteen degrees south of Regulus, and will then move slowly in the
direction of [Greek: ph] Leonis. The comet will be at its descending node
on the ecliptic in the morning of May 16, and the earth will pass through
the node on the comet's orbit about two and a half days later. The
comet's orbit at the node is about 13 million miles within that of the
earth. Matter repelled from the comet's nucleus by the sun with a velocity
of about 216,000 miles per hour, would just meet the earth when crossing
the comet's orbit plane. Matter expelled with a velocity of 80,000 miles
per hour, as in the case of Comet Morehouse, would require seven days for
the journey. Cometary matter is said to have acquired greater velocities
than this, for (according to Webb, who quotes Chacornac) Comet II., 1862,
shot luminous matter towards the sun, with a velocity of nearly 2200 miles
per second. It is therefore possible that matter thrown off by the comet
at the node may enter our atmosphere, in which case we must hope that
cyanogen, which so often appears in cometary spectra, may not be
inconveniently in evidence."[209]
Cyanogen is, of course, a poisonous gas, but cometary matter is so
rarefied that injurious effects on the earth need not be feared.
If we can believe the accounts which have been handed down to us, some
very wonderful comets were visible in ancient times. The following may be
mentioned:--
B.C. 165. The sun is said to have been "seen for several hours in the
night." If this was a comet it must have been one of extraordinary
brilliancy.[210]
B.C. 146. "After the death of Demetrius, king of Syria, the father of
Demetrius and Antiochus, a little before the war in Achaia, there appeared
a comet as large as the sun. Its disc was first red, and like fire,
spreading sufficient light to dissipate the darkness of night; after a
little while its size diminished, its brilliancy became weakened, and at
length it entirely disappeared."[211]
B.C. 134. It is recorded that at the birth of Mithridates a great comet
appeared which "occupied the fourth part of the sky, and its brilliancy
was superior to that of the sun." (?)[212]
B.C. 75. A comet is described as equal in size to the moon, and giving as
much light as the sun on a cloudy day. (!)[213]
A.D. 531. In this year a great comet was observed in Europe and China. It
is described as "a very large and fearful comet," and was visible in the
west for three weeks. Hind thinks that this was an appearance of Halley's
comet,[214] and this has been confirmed by Mr. Crommelin.
A.D. 813, August 4. A comet is said to have appeared on this date, of
which the following curious description is given: "It resembled two moons
joined together; they separated, and having taken different forms, at
length appeared like a man without a head." (!)[215]
A.D. 893. A great comet is said to have appeared in this year with a tail
100° in length, which afterwards increased to 200°![216]
A.D. 1402. A comet appeared in February of this year, which was visible in
daylight for eight days. "On Palm Sunday, March 19, its size was
prodigious." Another comet, visible in the daytime, was seen from June to
September of the same year.
When the orbit of the comet known as 1889 V was computed, it was found
that it had passed through Jupiter's system in 1886 (July 18-21). The
calculations show that it must have passed within a distance of 112,300
miles of the planet itself--or less than half the moon's distance from the
earth--and "its centre may possibly have grazed the surface of
Jupiter."[217]
Sir John Herschel thought that the great comet of 1861 was by far the
brightest comet he had ever seen, those of 1811 and 1858 (Donati's) not
excepted.[218] Prof. Kreutz found its period of revolution round the sun
to be about 409 years, with the plane of the orbit nearly at right angles
to the plane of the ecliptic.
* * * * *
On November 9, 1795, Sir William Herschel saw the comet of that year pass
centrally over a small double star of the 11th and 12th magnitudes, and
the fainter of the two components remained distinctly visible during the
comet's transit over the star. This comet was an appearance of the comet
now known as Encke's.[219] Struve saw a star of the 10th magnitude through
nearly the brightest part of Encke's comet on November 7, 1828, but the
star's light was not dimmed by the comet.
Sir John Herschel saw a cluster of stars of the 16th or 17th magnitude
through Biela's comet, although the interposed cometary matter must have
been at least 50,000 miles in thickness.[220]
Bessel found that on September 29, 1835, a star of the 10th magnitude
shone with undimmed lustre through the tail of Halley's comet within 8
seconds of arc of the central point of the head. At Dorpat (Russia) Struve
saw the same star "in conjunction only 2"·2 from the brightest point of
the comet. The star remained continuously visible, and its light was not
perceptibly diminished whilst the nucleus of the comet seemed to be almost
extinguished before the radiance of the small star of the 9th or 10th
magnitude."[221]
Webb says--
"Donati saw a 7 mg. star enlarged so as to show a sensible disc, when
the nucleus of comet III., 1860, passed very near it. Stars are said
to have started, or become tremulous, during occultations by comets.
Birmingham observed the comet of Encke illuminated by a star over
which it passed, August 23, 1868; and Klein, in 1861, remarked an
exceptional twinkling in 5 mg. stars involved in the tail."[222]
The comet of 1729 had the greatest perihelion distance of any known
comet;[223] that is, when nearest to the sun, it did not approach the
central luminary within four times the earth's distance from the sun!
Barnard's comet, 1889 I., although it never became visible to the naked
eye, was visible with a telescope from September 2, 1888, to August 18,
1890, or 715 days--the longest period of visibility of any comet on
record. When last seen it was 6-1/4 times the earth's distance from the
sun, or about 580 millions of miles,[224] or beyond the orbit of Jupiter!
Messier, who was called "the comet ferret," discovered "all his comets
with a small 2-foot telescope of 2-1/4 inches aperture, magnifying 5
times, and with a field of 4°."[225]
It is a very curious fact that Sir William Herschel, "during all his
star-gaugings and sweeps for nebulæ, never discovered a comet;"[226] that
is an object which was afterwards _proved_ to be a comet. Possibly,
however, some of his nebulæ which are now missing, may have been really
comets.
Sir William Herschel found the diameter of the head of the great comet of
1811 to be 127,000 miles. The surrounding envelope he estimated to be at
least 643,000 miles, or about three-fourths of the sun's diameter.
On a drawing of the tails of the great comet of 1744 given in a little
book printed in Berlin in that year, no less than 12 tails are shown!
These vary in length and brightness. A copy of this drawing is given in
_Copernicus_.[227] The observations were made by "einen geschichten
Frauenzimmer," who Dr. Dreyer identifies with Christian Kirch, or one of
her two sisters, daughters of the famous Gottfried and Maria Margaretta
Kirch (_Idem_, p. 107). Dr. Dreyer thinks that the drawing "seems to have
been carefully made, and not to be a mere rough sketch as I had at first
supposed" (_Idem_, p. 185).
The tails of some comets were of immense length. That of the comet of 1769
had an absolute length of 38 millions of miles. That of 1680, 96 million
of miles, or more than the sun's distance from the earth. According to Sir
William Herschel, the tail of the great comet of 1811 was over 100
millions of miles in length. That of the great comet of 1843--one of the
finest in history--is supposed to have reached a length of 150 millions of
miles![228]
In width the tails of comets were in some cases enormous. According to Sir
William Herschel, the tail of the comet of 1811 had a diameter of 15
millions of miles! Its volume was, therefore, far greater than that of the
sun![228]
According to Hevelius the comet of 1652 was of such a magnitude that it
"resembled the moon when half full; only it shone with a pale and dismal
light."[229]
Halley's comet at its next appearance will be examined with the
spectroscope for the first time in its history. At its last return in
1835, the spectroscope had not been invented.
For the great comet of 1811, Arago computed a period of 3065 years; and
Encke found a period of 8800 years for the great comet of 1680.[230]
The variation in the orbital velocity of some comets is enormous. The
velocity of the comet of 1680 when passing round the sun (perihelion) was
about 212 miles a second! Whereas at its greatest distance from the sun
(aphelion) the velocity is reduced to about 10 feet a second!
CHAPTER XII
Meteors
Mr. Denning thinks that the meteor shower of the month of May, known as
the Aquarids, is probably connected with Halley's comet. The meteors
should be looked for after 1 a.m. during the first week in May, and may
possibly show an enhanced display in May, 1910, when Halley's comet will
be near the sun and earth.[231]
On November 29, 1905, Sir David Gill observed a fireball with an apparent
diameter equal to that of the moon, which remained visible for 5 minutes
and disappeared in a hazy sky. Observed from another place, Mr. Fuller
found that the meteor was visible 2 hours later! Sir David Gill stated
that he does not know of any similar phenomenon.[232]
Mr. Denning finds that swiftly moving meteors become visible at a greater
height above the earth's surface than the slower ones. Thus, for the
Leonids and Perseids, which are both swift, it has been found that the
Leonids appear at an average height of 84 miles, and disappear at a height
of 56 miles; and the Perseids at 80 and 54 miles respectively. "On the
other hand, the mean height of the very slow meteors average about 65
miles at the beginning and 38 miles at the end of their appearance."[233]
During the night of July 21-22, 1896, Mr. William Brooks, the well-known
astronomer, and director of the Smith Observatory at Geneva (New York),
saw a round dark body pass slowly across the moon's bright disc, the moon
being nearly full at the time. The apparent diameter of the object was
about one minute of arc, and the duration of the transit 3 or 4 seconds,
the direction of motion being from east to west. On August 22 of the same
year, Mr. Gathman (an American observer) saw a meteor crossing the _sun's_
disc, the transit lasting about 8 seconds.[234]
A meteor which appeared in Italy on July 7, 1892, was shown by Prof. von
Niessl to have had an _ascending_ path towards the latter end of its
course! The length of its path was computed to be 683 miles. When first
seen, its height above the earth was about 42 miles, and when it
disappeared its height had increased to about 98 miles, showing that its
motion was directed upwards![235]
In the case of the fall of meteoric stones, which occasionally occur, it
has sometimes been noticed that the sound caused by the explosion of the
meteorite, or its passage through the air, is heard before the meteorite
is seen to fall. This has been explained by the fact that owing to the
resistance of the air to a body moving at first with a high velocity its
speed is so reduced that it strikes the earth with a velocity less than
that of sound. Hence the sound reaches the earth before the body strikes
the ground.[236]
The largest meteoric stone preserved in a museum is that known as the
Anighita, which weighs 36-1/2 tons, and was found at Cape York in
Greenland. It was brought to the American Museum of Natural History by
Commander R. E. Peary, the Arctic explorer.
The second largest known is that of Bacubirito in Mexico, the weight of
which is estimated at 27-1/2 tons.
The third largest is that known as the Williamette, which was found in
1902 near the town of that name in Western Oregon (U.S.A.). It is composed
of metallic nickel-iron, and weighs about 13-1/2 tons. It is now in the
American Museum of Natural History.
A large meteorite was actually seen, from the deck of the steamer _African
Prince_, to fall into the Atlantic Ocean, on October 7, 1906! The captain
of the vessel, Captain Anderson, describes it as having a train of light
resembling "an immense broad electric-coloured band, gradually turning to
orange, and then to the colour of molten metal. When the meteor came into
the denser atmosphere close to the earth, it appeared, as nearly as is
possible to describe it, like a molten mass of metal being poured out. It
entered the water with a hissing noise close to the ship."[237] This was a
very curious and perhaps unique phenomenon, and it would seem that the
vessel had a narrow escape from destruction.
In Central Arizona (U.S.A.) there is a hill called Coon Butte, or Coon
Mountain. This so-called "mountain" rises to a height of only 130 to 160
feet above the surrounding plain, and has on its top a crater of 530 to
560 feet deep; the bottom of the crater--which is dry--being thus 400 feet
below the level of the surrounding country. This so-called "crater" is
almost circular and nearly three-quarters of a mile in diameter. It has
been suggested that this "crater" was formed by the fall of an enormous
iron meteorite, or small asteroid. The "crater" has been carefully
examined by a geologist and a physicist. From the evidence and facts
found, the geologist (Mr. Barringer) states that "they do not leave, in my
mind, a scintilla of doubt that this mountain and its crater were produced
by the impact of a huge meteorite or small asteroid." The physicist (Mr.
Tilghmann) says that he "is justified, under due reserve as to
subsequently developed facts, in announcing that the formation at this
locality is due to the impact of a meteor of enormous and unprecedented
size." There are numerous masses of meteoric iron in the vicinity of the
"crater." The so-called Canyon Diabolo meteorite was found in a canyon of
that name about 2-1/2 miles from the Coon Mountain. The investigators
estimate that the great meteoric fall took place "not more than 5000 years
ago, perhaps much less." Cedar trees about 700 years old are now growing
on the rim of the mountain. From the results of artillery experiments, Mr.
Gilbert finds that "a spherical projectile striking solid limestone with a
velocity of 1800 feet a second will penetrate to a depth of something less
than two diameters," and from this Mr. L. Fletcher concludes "that a
meteorite of large size would not be prevented by the earth's atmosphere
from having a penetration effect sufficient for the production of such a
crater."[238]
The meteoric origin of this remarkable "crater" is strongly favoured by
Mr. G. P. Merrill, Head Curator of Geology, U.S. National Museum.
The Canyon Diabolo meteorite above referred to was found to contain
diamonds! some black, others transparent. So some have said that "the
diamond is a gift from Heaven," conveyed to earth in meteoric
showers.[239] But diamond-bearing meteorites would seem to be rather a
freak of nature. It does not follow that _all_ diamonds had their origin
in meteoric stones. The mineral known as periodot is frequently found in
meteoric stones, but it is also a constituent of terrestrial rocks.
In the year 1882 it was stated by Dr. Halm and Dr. Weinhand that they had
found fossil sponges, corals, and crinoids in meteoric stones! Dr.
Weinhand thought he had actually determined three genera![240] But this
startling result was flatly contradicted by Carl Vogt, who stated that the
supposed fossils are merely crystalline conformations.[241]
Some meteorites contain a large quantity of occluded gases, hydrogen,
helium, and carbon oxides. It is stated that Dr. Odling once "lighted up
the theatre of the Royal Institution with gas brought down from
interstellar space by meteorites"![242]
On February 10, 1896, a large meteorite burst over Madrid with a loud
report. The concussion was so great that many windows in the city were
broken, and some partitions in houses were shaken down![243]
A very brilliant meteor or fireball was seen in daylight on June 9, 1900,
at 2{h} 55{m} p.m. from various places in Surrey, Sussex, and near London.
Calculations showed that "the meteor began 59 miles in height over a point
10 miles east of Valognes, near Cherbourg, France. Meteor ended 23 miles
in height, over Calais, France. Length of path 175 miles. Radiant point,
280°, 12°."[244]
It was decided some years ago "in the American Supreme Court that a
meteorite, though a stone fallen from heaven, belongs to the owner of the
freehold interest in the land on which it falls, and not to the
tenant."[245]
With reference to the fall of meteoric matter on the earth, Mr. Proctor
says, "It is calculated by Dr. Kleiber of St. Petersburgh that 4250 lbs.
of meteoric dust fall on the earth every hour--that is, 59 tons a day, and
more than 11,435 tons a year. I believe this to be considerably short of
the truth. It sounds like a large annual growth, and the downfall of such
an enormous mass of meteoric matter seems suggestive of some degree of
danger. But in reality, Dr. Kleiber's estimate gives only about 25
millions of pounds annually, which is less than 2 ounces annually to each
square mile of the earth's surface,"[246] a quantity which is, of course,
quite insignificant.
According to Humboldt, Chladni states that a Franciscan monk was killed by
the fall of an aërolite at Milan in the year 1660.[247] Humboldt also
mentions the death by meteoric stones of a monk at Crema on September 4,
1511, and two Swedish sailors on board ship in 1674.[248]
It is a curious fact that, according to Olbers, "no fossil meteoric
stones" have ever been discovered.[249] Considering the number which are
supposed to have fallen to the earth in the course of ages, this fact
seems a remarkable one.
On May 10, 1879, a shower of meteorites fell at Eitherville, Iowa
(U.S.A.). Some of the fragments found weighed 437, 170, 92-1/2, 28,
10-1/2, 4 and 2 lbs. in weight. In the following year (1880) when the
prairie grass had been consumed by a fire, about "5000 pieces were found
from the size of a pin to a pound in weight."[250]
According to Prof. Silvestria of Catania, a shower of meteoric dust mixed
with rain fell on the night of March 29, 1880. The dust contained a large
proportion of iron in the metallic state. In size the particles varied
from a tenth to a hundredth of a millimetre.[251]
It is sometimes stated that the average mass of a "shooting star" is only
a few grains. But from comparisons with an electric arc light, Prof. W.
H. Pickering concludes that a meteor as bright as a third magnitude star,
composed of iron or stone, would probably have a diameter of 6 or 7
inches. An average bright fireball would perhaps measure 5 or 6 feet in
diameter.[252]
In the Book of Joshua we are told "that the LORD cast down great stones
from heaven upon them unto Azekah, and they died" (Joshua x. 11). In the
latter portion of the verse "hailstones" are mentioned, but as the
original Hebrew word means stones in general (not hailstones), it seems
very probable that the stones referred to were aërolites.[253]
The stone mentioned in the Acts of the Apostles, from which was found "the
_image_ which fell down from Jupiter" (Acts xix. 35), was evidently a
meteoric stone.[253]
The famous stone in the Caaba at Mecca, is probably a stone of meteoric
origin.[253]
I
"Stones from Heaven! Can you wonder,
You who scrutinize the Earth,
At the love and veneration
They received before the birth
Of our scientific methods?
II
"Stones from Heaven! we can handle
Fragments fallen from realms of Space;
Oh! the marvel and the mystery,
Could we understand their place
In the scheme of things created!
III
"Stones from Heaven! With a mighty
Comet whirling formed they part?
Fell they from their lofty station
Like a brilliant fiery dart,
Hurl'd from starry fields of Night?"[254]
CHAPTER XIII
The Zodiacal Light and Gegenschein
According to Gruson and Brugsch, the Zodiacal Light was known in ancient
times, and was even worshipped by the Egyptians. Strabo does not mention
it; but Diodorus Siculus seems to refer to it (B.C. 373), and he probably
obtained his information from some Greek writers before his time, possibly
from Zenophon, who lived in the sixth century B.C.[255] Coming to the
Christian era, it was noticed by Nicephorus, about 410 B.C. In the Koran,
it is called the "false Aurora"; and it is supposed to be referred to in
the "Rubáiyát" of Omar Khayyam, the Persian astronomical poet, in the
second stanza of that poem (Edward Fitzgerald's translation)--
"Dreaming when Dawn's Left Hand was in the Sky,[256]
I heard a voice within the Tavern cry,
Awake, my Little ones, and fill the Cup,
Before Life's Liquor in its Cup be dry."
It was observed by Cassini in 1668,[257] and by Hooke in 1705. A short
description of its appearance will be found in Childrey's _Britannia
Baconica_ (1661), p. 183.
The finest displays of this curious light seem to occur between the middle
of January and the middle of February. In February, 1856, Secchi found it
brighter than he had ever seen it before. It was yellowish towards the
axis of the cone, and it seemed to be brighter than the Milky Way in
Cygnus. He described it as "un grande spectacle." In the middle of
February, 1866, Mr. Lassell, during his last residence in Malta, saw a
remarkable display of the Zodiacal Light. He found it at least twice as
bright as the brightest part of the Milky Way, and much brighter than he
had previously seen it. He found that the character of its light differed
considerably from that of the Milky Way. It was of a much redder hue than
the Galaxy. In 1874 very remarkable displays were seen in the
neighbourhood of London in January and February of that year; and in 1875
on January 24, 25, and 30. On January 24 it was noticed that the "light"
was distinctly reddish and much excelled in brightness any portion of the
Milky Way.
Humboldt, who observed it from Andes (at a height of 13,000 to 15,000
feet), from Venezuela and from Cumana, tells us that he has seen the
Zodiacal Light equal in brightness to the Milky Way in Sagittarius.
As probably many people have never seen the "light," a caution may be
given to those who care to look for it. It is defined by the Rev. George
Jones, Chaplain to the "United States' Japan Expedition" (1853-55), as "a
brightness that appears in the western sky after sunset, and in the east
before sunrise; following nearly or quite the line of the ecliptic in the
heavens, and stretching upwards to various elevations according to the
season of the year." From the description some might suppose that the
light is visible _immediately_ after sunset. But this is not so; it never
appears until twilight is over and "the night has fully set in."
The "light" is usually seen after sunset or before sunrise. But attempts
have recently been made by Prof. Simon Newcomb to observe it north of the
sun. To avoid the effects of twilight the sun must be only slightly more
than 18° below the horizon (that is, a little before or after the longest
day). This condition limits the place of observation to latitudes not much
south of 46°; and to reduce atmospheric absorption the observing station
should be as high as possible above the level of the sea. Prof. Newcomb,
observing from the Brienzer Rothorn in Switzerland (latitude 46° 47' N.,
longitude 8° 3' E.), succeeded in tracing the "light" to a distance of 35°
north of the sun. It would seem, therefore, that the Zodiacal Light
envelops the sun on all sides, but has a greater extension in the
direction of the ecliptic.[258] From observations at the Lick Observatory,
Mr. E. A. Fath found an extension of 46° north of the sun.[259]
From observations of the "light" made by Prof. Barnard at the Yerkes
Observatory during the summer of 1906, he finds that it extends to at
least 65° north of the sun, a considerably higher value than that found by
Prof. Newcomb.[260] The difference may perhaps be explained by actual
variation of the meteoric matter producing the light. Prof. J. H. Poynting
thinks that possibly the Zodiacal Light is due to the "dust of long dead
comets."[261]
From careful observations of the "light," Mr. Gavin J. Burns finds that
its luminosity is "some 40 or 50 per cent. brighter than the background of
the sky. Prof. Newcomb has made a precisely similar remark about the
luminosity of the Milky Way, viz. that it is surprisingly small." This
agrees with my own observations during many years. It is only on the
finest and clearest nights that the Milky Way forms a conspicuous object
in the night sky. And this only in the country. The lights of a city
almost entirely obliterate it. Mr. Burns finds that the Zodiacal Light
appears "to be of a yellowish tint; or if we call it white, then the Milky
Way is comparatively of a bluish tint." During my residence in the Punjab
the Zodiacal Light seemed to me constantly visible in the evening sky in
the spring months. In the west of Ireland I have seen it nearly as bright
as the brightest portions of the Milky Way visible in this country
(February 20, 1890). The "meteoric theory" of the "light" seems to be the
one now generally accepted by astronomers, and in this opinion I fully
concur.
From observations made in Jamaica in the years 1899 and 1901, Mr. Maxwell
Hall arrived at the conclusion that "the Zodiacal Light is caused by
reflection of sunlight from masses of meteoric matter still contained in
the invariable plane, which may be considered the original plane of the
solar system."[262] According to Humboldt, Cassini believed that the
Zodiacal Light "consisted of innumerably small planetary bodies revolving
round the sun."[263]
THE GEGENSCHEIN, or COUNTER-GLOW.--This is a faint patch of light seen
opposite the sun's place in the sky, that is on the meridian at midnight.
It is usually elliptical in shape, with its longer axis lying nearly in
the plane of the ecliptic. It seems to have been first detected by Brorsen
(the discoverer of the short-period comet of 1846) about the middle of
the nineteenth century. But it is not easy to see, for the famous Heis of
Münster, who had very keen eyesight, did not succeed in seeing it for
several years after Brorsen's announcement.[264] It was afterwards
independently discovered by Backhouse, and Barnard.
Prof. Barnard's earlier observations seemed to show that the Gegenschein
does not lie exactly opposite to the sun, but very nearly so. He found its
longitude is within one degree of 180°, and its latitude about 1°·3 north
of the ecliptic.[265] But from subsequent observations he came to the
conclusion that the differences in longitude and apparent latitude are due
to atmospheric absorption, and that the object really lies in the ecliptic
and _exactly_ opposite to the sun.[266]
Barnard finds that the Gegenschein is not so faint as is generally
supposed. He says "it is best seen by averted vision, the face being
turned 60° or 70° to the right or left, and the eyes alone turned towards
it." It is invisible in June and December, while in September it is round,
with a diameter of 20°, and very distinct. No satisfactory theory has yet
been advanced to account for this curious phenomenon. Prof. Arthur Searle
of Harvard attributes it to a number of asteroids too small to be seen
individually. When in "opposition" to the sun these would be fully
illuminated and nearest to the earth. Its distance from the earth probably
exceeds that of the moon. Dr. Johnson Stoney thinks that the Gegenschein
may possibly be due to a "tail" of hydrogen and helium gases repelled from
the earth by solar action; this "tail" being visible to us by reflected
sunlight.
It was observed under favourable circumstances in January and February,
1903, by the French astronomer, M. F. Quénisset. He found that it was
better seen when the atmosphere was less clear, contrary to his experience
of the Zodiacal Light. Prof. Barnard's experience confirms this. M.
Quénisset notes that--as in the case of the Zodiacal Light--the southern
border of the Gegenschein is sharper than the northern. He found that its
brightness is less than that of the Milky Way between Betelgeuse and
[Greek: g] Geminorum; and thinks that it is merely a strengthening of the
Zodiacal Light.[267]
A meteoritic theory of the Gegenschein has been advanced by Prof. F. R.
Moulton, which explains it by light reflected from a swarm of meteorites
revolving round the sun at a distance of 930,240 miles outside the earth's
orbit.
Both the Zodiacal Light and Gegenschein were observed by Herr Leo Brenner
on the evening of March 4, 1896. He found the Zodiacal Light on this
evening to be "_perhaps eight times brighter_ than the Milky Way in
Perseus." The "_Gegenschein distinctly visible_ as a round, bright,
cloud-like nebula below Leo (Virgo), and about twice the brightness of the
Milky Way in Monoceros between Canis Major and Canis Minor."[268]
Humboldt thought that the fluctuations in the brilliancy of the Zodiacal
Light were probably due to a real variation in the intensity of the
phenomenon rather than to the elevated position of the observer.[269] He
says that he was "astonished in the tropical climates of South America, to
observe the variable intensity of the light."
CHAPTER XIV
The Stars
Pliny says that Hipparchus "ventured to count the stars, a work arduous
even for the Deity." But this was quite a mistaken idea. Those visible to
the naked eye are comparatively few in number, and the enumeration of
those visible in an opera-glass--which of course far exceed those which
can be seen by unaided vision--is a matter of no great difficulty. Those
visible in a small telescope of 2-3/4 inches aperture have all been
observed and catalogued; and even those shown on photographs taken with
large telescopes can be easily counted. The present writer has made an
attempt in this direction, and taking an average of a large number of
counts in various parts of the sky, as shown on stellar photographs, he
finds a total of about 64 millions for the whole sky in both
hemispheres.[270] Probably the total number will not exceed 100 millions.
But this is a comparatively small number, even when compared with the
human population of our little globe.
With reference to the charts made by photography in the International
scheme commenced some years ago, it has now been estimated that the charts
will probably contain a total of about 9,854,000 stars down to about the
14th magnitude (13·7). The "catalogue plates" (taken with a shorter
exposure) will, it is expected, include about 2,676,500 stars down to
11-1/2 magnitude. These numbers may, however, be somewhat increased when
the work has been completed.[271] If this estimate proves to be correct,
the number of stars visible down to the 14th magnitude will be
considerably less than former estimates have made it.
Prof. E. C. Pickering estimates that the total number of stars visible on
photographs down to the 16th magnitude (about the faintest visible in the
great Lick telescope) will be about 50 millions.[272] In the present
writer's enumeration, above referred to, many stars fainter than the 16th
magnitude were included.
Admiral Smyth says, with reference to Sir William Herschel--perhaps the
greatest observer that ever lived--"As to Sir William himself, he could
unhesitatingly call every star down to the 6th magnitude, by its name,
letter, or number."[273] This shows great powers of observation, and a
wonderful memory.
On a photographic plate of the Pleiades taken with the Bruce telescope and
an exposure of 6 hours, Prof. Bailey of Harvard has counted "3972 stars
within an area 2° square, having Alcyone at its centre."[274] This would
give a total of about 41 millions for the whole sky, if of the same
richness.
With an exposure of 16 hours, Prof. H. C. Wilson finds on an area of less
that 110' square a total of 4621 stars. He thinks, "That all of these
stars belong to the Pleiades group is not at all probable. The great
majority of them probably lie at immense distances beyond the group, and
simply appear in it by projection."[274] He adds, "It has been found,
however, by very careful measurements made during the last 75 years at the
Königsbergh and Yale Observatories, that of the sixty-nine brighter stars,
including those down to the 9th magnitude, only eight show any certain
movement with reference to Alcyone. Since Alcyone has a proper motion or
drift of 6" per century, this means that all the brightest stars except
the eight mentioned are drifting with Alcyone and so form a true cluster,
at approximately the same distance from the earth. Six of the eight stars
which show relative drift are moving in the opposite direction to the
movement of Alcyone, and at nearly the same rate, so that their motion is
only apparent. They are really stationary, while Alcyone and the rest of
the cluster are moving past them."[275] This tends to show that the faint
stars are really _behind_ the cluster, and are unconnected with it.
It is a popular idea with some people that the Pole Star is the nearest of
all the stars to the celestial pole. But photographs show that there are
many faint stars nearer to the pole than the Pole Star. The Pole Star is
at present at a distance of 1° 13' from the real pole of the heavens, but
it is slowly approaching it. The minimum distance will be reached in the
year 2104. From photographs taken by M. Flammarion at the Juvisy
Observatory, he finds that there are at least 128 stars nearer to the pole
than the Pole Star! The nearest star to the pole was, in the year 1902, a
small star of about 12-1/2 magnitude, which was distant about 4 minutes of
arc from the pole.[276] The estimated magnitude shows that the Pole Star
is nearly 10,000 times brighter than this faint star!
It has been found that Sirius is bright enough to cast a shadow under
favourable conditions. On March 22, 1903, the distinguished French
astronomer Touchet succeeded in photographing the shadow of a brooch cast
by this brilliant star. The exposure was 1{h} 5{m}.
Martinus Hortensius seems to have been the first to see stars in daylight,
perhaps early in the seventeenth century. He mentions the fact in a letter
to Gassendi dated October 12, 1636, but does not give the date of his
observation. Schickard saw Arcturus in broad daylight early in 1632. Morin
saw the same bright star half an hour after sunset in March, 1635.
Some interesting observations were made by Professors Payne and H. C.
Wilson, in the summer of 1904, at Midvale, Montana (U.S.A.), at a height
of 4790 feet above sea-level. At this height they found the air very clear
and transparent. "Many more stars were visible at a glance, and the
familiar stars appeared more brilliant.... In the great bright cloud of
the Milky Way, between [Greek: b] and [Greek: g] Cygni, one could count
easily sixteen or seventeen stars, besides the bright ones [Greek: ê] and
[Greek: ch],[277] while at Northfield it is difficult to distinctly see
eight or nine with the naked eye." Some nebulæ and star fields were
photographed with good results by the aid of a 2-1/2-inch Darlot lens and
3 hours' exposure.[278]
Prof. Barnard has taken some good stellar photographs with a lens of only
1-1/2 inches in diameter, and 4 or 5 inches focus belonging to an
ordinary "magic lantern"! He says that these "photographs with the small
lens show us in the most striking manner how the most valuable and
important information may be obtained with the simplest means."[279]
With reference to the rising and setting of the stars due to the earth's
rotation on its axis, the late Sir George B. Airy, Astronomer Royal of
England, once said to a schoolmaster, "I should like to know how far your
pupils go into the first practical points for which reading is scarcely
necessary. Do they know that the stars rise and set? Very few people in
England know it. I once had a correspondence with a literary man of the
highest rank on a point of Greek astronomy, and found that he did not know
it!"[280]
Admiral Smyth says, "I have been struck with the beautiful blue tint of
the smallest stars visible in my telescope. This, however, may be
attributed to some optical peculiarity." This bluish colour of small stars
agrees with the conclusion arrived at by Prof. Pickering in recent years,
that the majority of faint stars in the Milky Way have spectra of the
Sirian type and, like that brilliant star, are of a bluish white colour.
Sir William Herschel saw many stars of a redder tinge than other observers
have noticed. Admiral Smyth says, "This may be owing to the effect of his
metallic mirror or to some peculiarity of vision, or perhaps both."[281]
The ancient astronomers do not mention any coloured stars except white and
red. Among the latter they only speak of Arcturus, Aldebaran, Pollux,
Antares, and Betelgeuse as of a striking red colour. To these Al-Sufi adds
Alphard ([Greek: a] Hydræ).
Sir William Herschel remarked that no decidedly green or blue star "has
ever been noticed unassociated with a companion brighter than itself." An
exception to Herschel's rule seems to be found in the case of the star
[Greek: b] Libræ, which Admiral Smyth called "pale emerald." Mr. George
Knott observed it on May 19, 1852, as "beautiful pale green" (3·7 inches
achromatic, power 80), and on May 9, 1872, as "fine pale green" (5·5
inches achromatic, power 65).
The motion of stars in the line of sight, as shown by the
spectroscope--should theoretically alter their brightness in the course of
time; those approaching the earth becoming gradually brighter, while those
receding should become fainter. But the distance of the stars is so
enormous that even with very high velocities the change would not become
perceptible for ages. Prof. Oudemans found that to change the brightness
of a star by only one-tenth of a magnitude--a quantity barely perceptible
to the eye-a number of years would be necessary, which is represented by
the formula
5916 years
-----------------
parallax × motion
for a star approaching the earth, and for a receding star
6195 years
----------
p × m
This is in geographical miles, 1 geographical mile being equal to 4·61
English miles.
Reducing the above to English miles, and taking an average for both
approaching and receding stars, we have
27,660 years
------------
p × m
where p = parallax in seconds of arc, and m = radial velocity in English
miles per second.
Prof. Oudemans found that the only star which could have changed in
brightness by one-tenth of a magnitude since the time of Hipparchus is
Aldebaran. This is taking its parallax as 0"·52. But assuming the more
reliable parallax 0"·12 found by Dr. Elkin, this period is 4-1/3 times
longer. For Procyon, the period would be 5500 years.[282] The above
calculation shows how absurd it is to suppose that any star could have
gained or lost in brightness by motion in the line of sight during
historical times. The "secular variation" of stars is quite another
thing. This is due to physical changes in the stars themselves.
The famous astronomer Halley, the second Astronomer Royal at Greenwich,
says (_Phil. Trans._, 1796), "Supposing the number of 1st magnitude stars
to be 13, at twice the distance from the sun there may be placed four
times as many, or 52; which with the same allowance would nearly represent
the star we find to be of the 2nd magnitude. So 9 × 13, or 117, for those
at three times the distance; and at ten times the distance 100 × 13, or
1300 stars; of which distance may probably diminish the light of any of
the stars of the 1st magnitude to that of the 6th, it being but the
hundredth part of what, at their present distance, they appear with." This
agrees with the now generally accepted "light ratio" of 2·512 for each
magnitude, which makes a first magnitude star 100 times the light of a 6th
magnitude.
On the 4th of March, 1796,[283] the famous French astronomer Lalande
observed on the meridian a star of small 6th magnitude, the exact position
of which he determined. On the 15th of the same month he again observed
the star, and the places found for 1800 refer to numbers 16292-3 of the
reduced catalogue. In the observation of March 4 he attached the curious
remark, "Étoile singulière" (the observation of March 15 is without
note). This remark of Lalande has puzzled observers who failed to find any
peculiarity about the star. Indeed, "the remark is a strange one for the
observer of so many thousands of stars to attach unless there was really
something singular in the star's aspect at the time." On the evening of
April 18, 1887, the star was examined by the present writer, and the
following is the record in his observing book, "Lalande's étoile
singulière (16292-3) about half a magnitude less than [Greek: ê] Cancri.
With the binocular I see two streams of small stars branching out from it,
north preceding like the tails of comet." This may perhaps have something
to do with Lalande's curious remark.
The star numbered 1647 in Baily's _Flamsteed Catalogue_ is now known to
have been an observation of the planet Uranus.[284]
Prof. Pickering states that the fainter stars photographed with the 8-inch
telescope at Cambridge (U.S.A.) are invisible to the eye in the 15-inch
telescope.[285]
Sir Norman Lockyer finds that the lines of sulphur are present in the
spectrum of the bright star Rigel ([Greek: b] Orionis).[286]
About 8-1/2° south of the bright star Regulus ([Greek: a] Leonis) is a
faint nebula (H I, 4 Sextantis). On or near this spot the Capuchin monk De
Rheita fancied he saw, in the year 1643, a group of stars representing
the napkin of S. Veronica--"sudarium Veronicæ sive faciem Domini maxima
similitudina in astris expressum." And he gave a picture of the napkin and
star group. But all subsequent observers have failed to find any trace of
the star group referred to by De Rheita![287]
The Bible story of the star of the Magi is also told in connection with
the birth of the sun-gods Osiris, Horus, Mithra, Serapis, etc.[288] The
present writer has also heard it suggested that the phenomenon may have
been an apparition of Halley's comet! But as this famous comet is known to
have appeared in the year B.C. 11, and as the date of the Nativity was
probably not earlier than B.C. 5, the hypothesis seems for this (and other
reasons) to be inadmissible. It has also been suggested that the
phenomenon might have been an appearance of Tycho Brahé's temporary star
of 1572, known as the "Pilgrim star"; but there seems to be no real
foundation for such an hypothesis. There is no reason to think that
"temporary" or new stars ever appear a second time.
Admiral Smyth has well said, "It checks one's pride to recollect that if
our sun with the whole system of planets, asteroids, and moons, and comets
were to be removed from the spectator to the distance of the nearest
fixed star, not one of them would be visible, except the sun, which would
then appear but as a star of perhaps the 2nd magnitude. Nay, more, were
the whole system of which our globe forms an insignificant member, with
its central luminary, suddenly annihilated, no effect would be produced on
those unconnected and remote bodies; and the only annunciation of such a
catastrophe in the Sidereal "Times" would be that a small star once seen
in a distant quarter of the sky had ceased to shine."[289]
Prof. George C. Comstock finds that the average parallax of 67 selected
stars ranging in brightness between the 9th and the 12th magnitude, is of
the value of 0"·0051.[290] This gives a distance representing a journey
for light of about 639 years!
Mr. Henry Norris Russell thinks that nearly all the bright stars in the
constellation of Orion are practically at the same distance from the
earth. His reasons for this opinion are: (1) the stars are similar in
their spectra and proper motions, (2) their proper motions are small,
which suggests a small parallax, and therefore a great distance from the
earth. Mr. Russell thinks that the average parallax of these stars may
perhaps be 0"·005, which gives a distance of about 650 "light
years."[291]
According to Sir Norman Lockyer's classification of the stars, the order
of _increasing_ temperature is represented by the following, beginning
with those in the earliest stage of stellar evolution:--Nebulæ, Antares,
Aldebaran, Polaris, [Greek: a] Cygni, Rigel, [Greek: e] Tauri, [Greek: b]
Crucis. Then we have the hottest stars represented by [Greek: e] Puppis,
[Greek: g] Argus, and Alnitam ([Greek: e] Orionis). _Decreasing_
temperature is represented by (in order), Achernar, Algol, Markab, Sirius,
Procyon, Arcturus, 19 Piscium, and the "Dark Stars."[292] But other
astronomers do not agree with this classification. Antares and Aldebaran
are by some authorities considered to be _cooling_ suns.
According to Ritter's views of the Constitution of the Celestial Bodies,
if we "divide the stars into three classes according to age corresponding
to these three stages of development, we shall assign to the first class,
A, those stars still in the nebular phase of development; to the second
class, B, those in the transient stage of greatest brilliancy; and to the
class C, those stars which have already entered into the long period of
slow extinction. It should be noted in this classification that we refer
to relative and not absolute age, since a star of slight mass passes
through the successive phases of its development more rapidly than the
star of greater mass."[293] Ritter comes to the conclusion that "the
duration of the period in which the sun as a star had a greater brightness
than at present was very short in comparison with the period in which it
had and will continue to have a brightness differing only slightly from
its present value."[294]
In a valuable and interesting paper on "The Evolution of Solar
Stars,"[295] Prof. Schuster says that "measurements by E. F. Nichols on
the heat of Vega and Arcturus indicated a lower temperature for Arcturus,
and confirms the conclusion arrived at on other grounds, that the hydrogen
stars have a higher temperature than the solar stars." "An inspection of
the ultraviolet region of the spectrum gives the same result. These
different lines of argument, all leading to the same result, justify us in
saying that the surface temperature of the hydrogen stars is higher than
that of the solar stars. An extension of the same reasoning leads to the
belief that the helium stars have a temperature which is higher still."
Hence we have Schuster, Hale, and Sir William Huggins in agreement that
the Sirian stars are hotter than the solar stars; and personally I agree
with these high authorities. The late Dr. W. E. Wilson, however, held the
opinion that the sun is hotter that Sirius!
Schuster thinks that Lane's law does not apply to the temperature of the
photosphere and the absorbing layers of the sun and stars, but only to the
portions between the photosphere and the centre, which probably act like a
perfect gas. On this view he says the interior might become "hotter and
hotter until the condensation had reached a point at which the laws of
gaseous condensation no longer hold."
With reference to the stars having spectra of the 3rd and 4th type
(usually orange and red in colour), Schuster says--
"The remaining types of spectra belong to lower temperature still, as
in place of metallic lines, or in addition to them, certain bands
appear which experiments show us invariably belong to lower
temperature than the lines of the same element.
"If an evolutionary process has been going on, which is similar for
all stars, there is little doubt that from the bright-line stars down
to the solar stars the order has been (1) helium or _Orion_ stars, (2)
hydrogen or Sirian stars, (3) calcium or Procyon stars, (4) solar or
Capellan stars."
My investigations on "The Secular Variation of Starlight" (_Studies in
Astronomy_, chap. 17, and _Astronomical Essays_, chap. 12) based on a
comparison of Al-Sufi's star magnitudes (tenth century) with modern
estimates and measures, tend strongly to confirm the above views.
With regard to the 3rd-type stars, such as Betelgeuse and Mira Ceti,
Schuster says, "It has been already mentioned that observers differ as to
whether their position is anterior to the hydrogen or posterior to the
solar stars, and there are valid arguments on both sides."
Scheiner, however, shows, from the behaviour of the lines of magnesium,
that stars of type I. (Sirian) are the hottest, and type III. the coolest,
and he says, we have "for the first time a direct proof of the correctness
of the physical interpretation of Vogel's spectral classes, according to
which class II. is developed by cooling from I., and III. by a further
process of cooling from II."[296]
Prof. Hale says that "the resemblance between the spectra of sun-spots and
of 3rd-type stars is so close as to indicate that the same cause is
controlling the relative intensities of many lines in both instances. This
cause, as the laboratory work indicates, is to be regarded as reduced
temperature."[297]
According to Prof. Schuster, "a spectrum of bright lines may be given by a
mass of luminous gas, even if the gas is of great thickness. There is,
therefore, no difficulty in explaining the existence of stars giving
bright lines." He thinks that the difference between "bright line" stars
and those showing dark lines depends upon the rate of increase of the
temperature from the surface towards the centre. If this rate is slow,
bright lines will be seen. If the rate of increase is rapid, the
dark-line spectrum shown by the majority of the stars will appear. This
rate, he thinks, is regulated by the gravitational force. So that in the
early stages of condensation bright lines are more likely to occur. "If
the light is not fully absorbed," both bright and dark lines of the same
element may be visible in the same star. Schuster considers it quite
possible that if we could remove the outer layers of the Sun's atmosphere,
we should obtain a spectrum of bright lines.[298]
M. Stratonoff finds that stars having spectra of the Orion and Sirian
types--supposed to represent an early stage in stellar evolution--tend to
congregate in or near the Milky Way. Star clusters in general show a
similar tendency, "but to this law the globular clusters form an
exception."[299] We may add that the spiral nebulæ--which seem to be
scattered indifferently over all parts of the sky--also seem to form an
exception; for the spectra of these wonderful objects seem to show that
they are really star clusters, in which the components are probably
relatively small; that is, small in comparison with our sun.
If we accept the hypothesis that suns and systems were evolved from
nebulæ, and if we consider the comparatively small number of nebulæ
hitherto discovered in the largest telescopes--about half a million; and
if we further consider the very small number of red stars, or those having
spectra of the third and fourth types--usually considered to be dying-out
suns--we seem led to the conclusion that our sidereal system is now at
about the zenith of its life-history; comparatively few nebulæ being left
to consolidate into stars, and comparatively few stars having gone far on
the road to the final extinction of their light.
Prof. Boss of Albany (U.S.A.) finds that about forty stars of magnitudes
from 3-1/2 to 7 in the constellation Taurus are apparently drifting
together towards one point. These stars are included between about R.A.
3{h} 47{m} to 5{h} 4{m}, and Declination + 5° to + 23° (that is, in the
region surrounding the Hyades). These motions apparently converge to a
point near R.A. 6{h}, Declination + 7° (near Betelgeuse). Prof. Boss has
computed the velocity of the stars in this group to be 45·6 kilometres
(about 28 miles) a second towards the "vanishing point," and he estimated
the average parallax of the group to be 0"·025--about 130 years' journey
for light. Although the motions are apparently converging to a point, it
does not follow that the stars in question will, in the course of ages,
meet at the "vanishing point." On the contrary, the observed motions show
that the stars are moving in parallel lines through space. About 15
kilometres of the observed speed is due to the sun's motion through space
in the opposite direction. Prof. Campbell finds from spectroscopic
measures that of these forty stars, nine are receding from the earth with
velocities varying from 12 to 60 kilometres a second, and twenty-three
others with less velocities than 38 kilometres.[300] It will be obvious
that, as there is a "vanishing point," the motion in the line of sight
must be one of _recession_ from the earth.
It has been found that on an average the parallax of a star is about
one-seventh of its "proper motion."[301]
Adopting Prof. Newcomb's parallax of 0"·14 for the famous star 1830
Groombridge, the velocity perpendicular to the line of sight is about 150
miles a second. The velocity _in_ the line of sight--as shown by the
spectroscope--is 59 miles a second approaching the earth. Compounding
these two velocities we find a velocity through space of about 161 miles a
second!
An eminent American writer puts into the mouth of one of his characters, a
young astronomer, the following:--
"I read the page
Where every letter is a glittering sun."
From an examination of the heat radiated by some bright stars, made by
Dr. E. F. Nicholls in America with a very sensitive radiometer of his own
construction, he finds that "we do not receive from Arcturus more heat
than we should from a candle at a distance of 5 or 6 miles."
With reference to the progressive motion of light, and the different times
taken by light to reach the earth from different stars, Humboldt says,
"The aspect of the starry heavens presents to us objects of _unequal
date_. Much has long ceased to exist before the knowledge of its presence
reaches us; much has been otherwise arranged."[302]
The photographic method of charting the stars, although a great
improvement on the old system, seems to have its disadvantages. One of
these is that the star images are liable to disappear from the plates in
the course of time. The reduction of stellar photograph plates should,
therefore, be carried out as soon as possible after they are taken. The
late Dr. Roberts found that on a plate originally containing 364 stars, no
less than 130 had completely disappeared in 9-1/4 years!
It has been assumed by some writers on astronomy that the faint stars
visible on photographs of the Pleiades are at practically the same
distance from the earth as the brighter stars of the cluster, and that
consequently there must be an enormous difference in actual size between
the brighter and fainter stars. But there is really no warrant for any
such assumption. Photographs of the vicinity show that the sky all round
the Pleiades is equally rich in faint stars. It seems, therefore, more
reasonable to suppose that most of the faint stars visible in the Pleiades
are really far behind the cluster in space. For if _all_ the faint stars
visible on photographs belonged to the cluster, then if we imagine the
cluster removed, a "hole" would be left in the sky, which is of course
utterly improbable, and indeed absurd. An examination of the proper
motions tends to confirm this view of the matter, and indicates that the
Pleiades cluster is a comparatively small one and simply projected on a
background of fainter stars.
It has long been suspected that the famous star 61 Cygni, which is a
double star, forms a binary system--that is, that the two stars composing
it revolve round their common centre of gravity and move together through
space. But measures of parallax made by Herman S. Davis and Wilsing seem
to show a difference of parallax between the two components of about 0·08
of a second of arc. This difference of parallax implies a distance of
about 2-1/4 "light years" between the two stars, and "if this is correct,
the stars are too remote to form a binary system. The proper motions of
5"·21 and 5"·15 seem to show that they are moving in nearly parallel
directions; but are probably slowly separating." Mr. Lewis, however,
thinks that a physical connection probably exists.[303]
Dante speaks of the four bright stars of the Southern Cross as
emblematical of the four cardinal virtues, Justice, Temperance, Fortitude,
and Prudence; and he seems to refer to the stars Canopus, Achernar, and
Foomalhaut under the symbols of Faith, Hope, and Charity. The so-called
"False Cross" is said to be formed by the stars [Greek: k], [Greek: d],
[Greek: e], and [Greek: i] of the constellation Argo Navis. But it seems
to me that a better (although larger) cross is formed by the stars [Greek:
a] Centauri and [Greek: a], [Greek: b], and [Greek: g] of Triangulum
Australis.
Mr. Monck has pointed out that the names of the brightest stars seem to be
arranged alphabetically in order of colour, beginning with red and ending
with blue. Thus we have Aldebaran, Arcturus, Betelgeuse, Capella, Procyon,
Regulus, Rigel, Sirius, Spica and Vega. But as the origin of these names
is different, this must be merely a curious coincidence.[304] And, to my
eye at least, Betelgeuse is redder than Arcturus.
The poet Longfellow speaks of the--
"Stars, the thoughts of God in the heavens,"[305]
and Drayton says--
"The stars to me an everlasting book
In that eternal register, the sky."[306]
Observing at a height of 12,540 feet on the Andes, the late Dr. Copeland
saw Sirius with the naked eye less than 10 minutes before sunset.[307] He
also saw Jupiter 3{m} 47{s} before sunset; and the following bright
stars--Canopus, 0{m} 52{s} before sunset; Rigel ([Greek: b] Orionis) 16{m}
32{s} after sunset; and Procyon 11{m} 28{s} after sunset. From a height of
12,050 feet at La Paz, Bolivia, he saw with the naked eye in February,
1883, ten stars in the Pleiades in full moonlight, and seventeen stars in
the Hyades. He also saw [Greek: s] Tauri double.[308]
Humboldt says, "In whatever point the vault of heaven has been pierced by
powerful and far-penetrating telescopic instruments, stars or luminous
nebulæ are everywhere discoverable, the former in some cases not exceeding
the 20th or 24th degree of telescopic magnitude."[309] But this is a
mistake. No star of even the 20th magnitude has ever been seen by any
telescope. Even on the best photographic plates it is doubtful that any
stars much below the 18th magnitude are visible. To show a star of the
20th magnitude--if such stars exist--would require a telescope of 144
inches or 12 feet in aperture. To show a star of the 24th magnitude--if
such there be--an aperture of 33 feet would be necessary![310]
It is a popular idea that stars may be seen in the daytime from the bottom
of a deep pit or high chimney. But this has often been denied. Humboldt
says, "While practically engaged in mining operations, I was in the habit,
during many years, of passing a great portion of the day in mines where I
could see the sky through deep shafts, yet I never was able to observe a
star."[311]
Stars may, however, be seen in the daytime with even small telescopes. It
is said that a telescope of 1 inch aperture will show stars of the 2nd
magnitude; 2 inches, stars of the 3rd magnitude; and 4 inches, stars of
the 4th magnitude. But I cannot confirm this from personal observation. It
may be so, but I have not tried the experiment.
Sir George Darwin says--
"Human life is too short to permit us to watch the leisurely procedure
of cosmical evolution, but the celestial museum contains so many
exhibits that it may become possible, by the aid of theory, to piece
together, bit by bit, the processes through which stars pass in the
course of their evolutions."[312]
The so-called "telluric lines" seen in the solar spectrum, are due to
water vapour in the earth's atmosphere. As the light of the stars also
passes through the atmosphere, it is evident that these lines should also
be visible in the spectra of the stars. This is found to be the case by
Prof. Campbell, Director of the Lick Observatory, who has observed all the
principal bands in the spectrum of every star he has examined.[313]
The largest "proper motion" now known is that of a star of the 8-1/2
magnitude in the southern hemisphere, known as Cordoba Zone V. No. 243.
Its proper motion is 8·07 seconds of arc per annum, thus exceeding that of
the famous "runaway star," 1830 Groombridge, which has a proper motion of
7·05 seconds per annum. This greater motion is, however, only apparent.
Measures of parallax show that the southern "runaway" is much nearer to us
than its northern rival, its parallax being 0"·32, while that of
Groombridge 1830 is only 0"·14. With these data the actual velocity across
the line of sight can be easily computed. That of the southern star comes
out 80 miles a second, while that of Groombridge 1830 is 148 miles a
second. The actual velocity of Arcturus is probably still greater.
The poet Barton has well said--
"The stars! the stars! go forth at night,
Lift up thine eyes on high,
And view the countless orbs of light,
Which gem the midnight sky.
Go forth in silence and alone,
This glorious sight to scan,
And bid the humbled spirit own
The littleness of man."
CHAPTER XV
Double and Binary Stars
Prof. R. G. Aitken, the eminent American observer of double stars, finds
that of all the stars down to the 9th magnitude--about the faintest
visible in a powerful binocular field-glass--1 in 18, or 1 in 20, on the
average, are double, with the component stars less than 5 seconds of arc
apart. This proportion of double stars is not, however, the same for all
parts of the sky; while in some reg
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