Astronomy: The Science of the Heavenly Bodies
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Title: Astronomy: The Science of the Heavenly Bodies
Author: David P. Todd
Release date: March 15, 2012 [eBook #39142]
Language: English
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[Illustration: Photo, Mt. Wilson Solar Observatory
_An Active Prominence of the Sun, 140,000 Miles High,
photographed July 9, 1917._]
ASTRONOMY
THE SCIENCE OF THE HEAVENLY BODIES
BY
DAVID TODD
DIRECTOR EMERITUS, AMHERST COLLEGE OBSERVATORY
[Illustration: Harper & Brothers logo]
NEW YORK AND LONDON
HARPER & BROTHERS
PUBLISHERS MCMXXII
Copyright 1922
BY P. F. COLLIER & SON COMPANY
Sir William Rowan Hamilton, the eminent mathematician of Dublin, has, of
all writers ancient and modern, most fittingly characterized the ideal
science of astronomy as man's golden chain connecting the heavens to the
earth, by which we "learn the language and interpret the oracles of the
universe."
The oldest of the sciences, astronomy is also the broadest in its
relations to human knowledge and the interests of mankind. Many are the
cognate sciences upon which the noble structure of astronomy has been
erected: foremost of all, geometry and the higher mathematics, which
tell us of motions, magnitudes and distances; physics and chemistry, of
the origin, nature, and destinies of planets, sun, and star;
meteorology, of the circulation of their atmospheres; geology, of the
structure of the moon's surface; mineralogy, of the constitution of
meteorites; while, if we attack, even elementally, the fascinating,
though perhaps forever unsolvable, problem of life in other worlds, the
astronomer must invoke all the resources that his fellow biologists and
their many-sided science can afford him.
The progress of astronomy from age to age has been far from
uniform--rather by leaps and bounds: from the earliest epoch when man's
planet earth was the center about which the stupendous cosmos wheeled,
for whom it was created, and for whose edification it was
maintained--down to the modern age whose discoveries have ascertained
that even our stellar universe, the vast region of the solar domain, is
but one of the thousands of island universes that tenant the
inconceivable immensities of space.
Such results have been attainable only through the successful
construction and operation of monster telescopes that bring to the eye
and visualize on photographic plates the faintest of celestial objects
which were the despair of astronomers only a few years ago.
But the end is not yet; astronomy to-day is but passing from infancy to
youth. And with new and greater telescopes, with new photographic
processes of higher sensitivity, with the help of modern invention in
overcoming the obstacle of the air--that constant foe of the
astronomer--who will presume to set down any limit to the leaps and
bounds of astronomy in the future?
So rapid, indeed, has been the progress of astronomy in very recent
years that the present is especially favorable for setting forth its
salient features; and this book is an attempt to present the wide range
of astronomy in readable fashion, as if a story with a definite plot,
from its origin with the shepherds of ancient Chaldea down to
present-day ascertainment of the actual scale of the universe, and
definite measures of the huge volume of supersolar giants among the
stars.
DAVID TODD
AMHERST COLLEGE OBSERVATORY
November, 1921
CONTENTS
CHAPTER PAGE
I. ASTRONOMY A LIVING SCIENCE 9
II. THE FIRST ASTRONOMERS 19
III. PYRAMID, TOMB, AND TEMPLE 23
IV. ORIGIN OF GREEK ASTRONOMY 27
V. MEASURING THE EARTH--ERATOSTHENES 30
VI. PTOLEMY AND HIS GREAT BOOK 33
VII. ASTRONOMY OF THE MIDDLE AGES 37
VIII. COPERNICUS AND THE NEW ERA 42
IX. TYCHO, THE GREAT OBSERVER 45
X. KEPLER, THE GREAT CALCULATOR 49
XI. GALILEO, THE GREAT EXPERIMENTER 53
XII. AFTER THE GREAT MASTERS 57
XIII. NEWTON AND MOTION 62
XIV. NEWTON AND GRAVITATION 66
XV. AFTER NEWTON 73
XVI. HALLEY AND HIS COMET 83
XVII. BRADLEY AND ABERRATION 90
XVIII. THE TELESCOPE 93
XIX. REFLECTORS--MIRROR TELESCOPES 102
XX. THE STORY OF THE SPECTROSCOPE 111
XXI. THE STORY OF ASTRONOMICAL PHOTOGRAPHY 125
XXII. MOUNTAIN OBSERVATORIES 139
XXIII. THE PROGRAM OF A GREAT OBSERVATORY 152
XXIV. OUR SOLAR SYSTEM 162
XXV. THE SUN AND OBSERVING IT 165
XXVI. SUN SPOTS AND PROMINENCES 174
XXVII. THE INNER PLANETS 189
XXVIII. THE MOON AND HER SURFACE 193
XXIX. ECLIPSES OF THE MOON 206
XXX. TOTAL ECLIPSES OF THE SUN 209
XXXI. THE SOLAR CORONA 219
XXXII. THE RUDDY PLANET 227
XXXIII. THE CANALS OF MARS 235
XXXIV. LIFE IN OTHER WORLDS 242
XXXV. THE LITTLE PLANETS 254
XXXVI. THE GIANT PLANET 260
XXXVII. THE RINGED PLANET 264
XXXVIII. THE FARTHEST PLANETS 267
XXXIX. THE TRANS-NEPTUNIAN PLANET 270
XL. COMETS--THE HAIRY STARS 273
XLI. WHERE DO COMETS COME FROM? 279
XLII. METEORS AND SHOOTING STARS 283
XLIII. METEORITES 290
XLIV. THE UNIVERSE OF STARS 294
XLV. STAR CHARTS AND CATALOGUES 300
XLVI. THE SUN'S MOTION TOWARD LYRA 304
XLVII. STARS AND THEIR SPECTRAL TYPE 307
XLVIII. STAR DISTANCES 311
XLIX. THE NEAREST STARS 319
L. ACTUAL DIMENSIONS OF THE STARS 321
LI. THE VARIABLE STARS 324
LII. THE NOVAE, OR NEW STARS 331
LIII. THE DOUBLE STARS 334
LIV. THE STAR CLUSTERS 336
LV. MOVING CLUSTERS 341
LVI. THE TWO STAR STREAMS 345
LVII. THE GALAXY OR MILKY WAY 350
LVIII. STAR CLOUDS AND NEBULAE 357
LIX. THE SPIRAL NEBULAE 361
LX. COSMOGONY 366
LXI. COSMOGONY IN TRANSITION 380
LIST OF ILLUSTRATIONS
ACTIVE PROMINENCE OF THE SUN, 140,000 MILES HIGH _Frontispiece_
FACING PAGE
NICHOLAS COPERNICUS 64
GALILEO GALILEI 64
JOHANN KEPLER 65
SIR ISAAC NEWTON 65
THE HUNDRED-INCH REFLECTING TELESCOPE AT MOUNT WILSON 96
THE FORTY-INCH REFRACTING TELESCOPE, YERKES OBSERVATORY 96
150-FOOT TOWER, MOUNT WILSON, A DIAGRAM OF TOWER AND PIT 97
150-FOOT TOWER--EXTERIOR VIEW 97
VIEW LOOKING DOWN INTO THE PIT BENEATH 150-FOOT TOWER 97
MOUNT WILSON SOLAR OBSERVATORY--THE 100-FOOT DOME 128
MOUNT CHIMBORAZO, THE BEST SITE IN THE WORLD FOR
AN OBSERVATORY 128
LICK OBSERVATORY, MOUNT HAMILTON, CALIFORNIA 129
PHOTOGRAPHING WITH THE 40-INCH REFRACTOR 129
GREAT SUNSPOT GROUP OF AUGUST 8, 1917 160
CALCIUM FLOCCULI ON THE SUN 161
ECLIPSE OF THE MOON, WITH THE LUNAR SURFACE VISIBLE 161
MOON'S SURFACE IN THE REGION OF COPERNICUS 192
SOUTH CENTRAL PORTION OF THE MOON, AT LAST QUARTER 193
CORONA OF THE SUN DURING AN ECLIPSE 224
VENUS, IN THE CRESCENT PHASE 225
MARS, SHOWING BRIGHT POLAR CAP 225
JUPITER, THE GIANT PLANET 256
NEPTUNE AND ITS SATELLITES 256
SATURN, WITH EDGE OF RINGS ONLY IN VIEW 257
SATURN, WITH RINGS DISPLAYED TO FULLEST EXTENT 257
TWO VIEWS OF HALLEY'S COMET 288
SWIFT'S COMET, WHICH SHOWED REMARKABLE TRANSFORMATIONS 288
METEOR TRAIL IN FIELD WITH FINE NEBULAE 289
RING NEBULA IN LYRA 320
DUMB-BELL NEBULA 321
STAR CLOUDS AND BLACK HOLES IN SAGITTARIUS 352
GREAT NEBULA IN ANDROMEDA 353
CHAPTER I
ASTRONOMY A LIVING SCIENCE
Like life itself we do not know when astronomy began; we cannot conceive
a time when it was not. Man of the early stone age must have begun to
observe sun, moon, and stars, because all the bodies of the cosmos were
there, then as now. With his intellectual birth astronomy was born.
Onward through the childhood of the race he began to think on the things
he observed, to make crude records of times and seasons; the Chaldeans
and Chinese began each their own system of astronomy, the causes of
things and the reasons underlying phenomena began to attract attention,
and astronomy was cultivated not for its own sake, but because of its
practical utility in supplying the data necessary to accurate
astrological prediction. Belief in astrology was universal.
The earth set in the midst of the wonders of the sky was the reason for
it all. Clearly the earth was created for humanity; so, too, the heavens
were created for the edification of the race. All was subservient to
man; naturally all was geocentric, or earth-centered. From the savage
who could count only to five, the digits of one hand, civilized man very
slowly began to evolve; he noted the progress of the seasons; the old
records of eclipses showed Thales, an early Greek, how to predict their
happenings, and true science had its birth when man acquired the power
to make forecasts that always came true.
Few ancient philosophers were greater than Pythagoras, and his
conceptions of the order of the heavens and the shape and motion of the
earth were so near the truth that we sometimes wonder how they could
have been rejected for twenty centuries. We must remember, however, that
man had not yet learned the art of measuring things, and the world could
not be brought into subjection to him until he had. To measure he must
have tools--instruments; to have instruments he must learn the art of
working in metals, and all this took time; it was a slow and in large
part imperceptible process; it is not yet finished.
The earliest really sturdy manifestation of astronomical life came with
the birth of Greek science, culminating with Aristarchus, Hipparchus and
Ptolemy. The last of these great philosophers, realizing that only the
art of writing prevents man's knowledge from perishing with him, set
down all the astronomical knowledge of that day in one of the three
greatest books on astronomy ever written, the Almagest, a name for it
derived through the Arabic, and really meaning "the greatest."
The system of earth and heaven seemed as if finished, and the authority
of Ptolemy and his Almagest were as Holy Writ for the unfortunate
centuries that followed him. With fatal persistence the fundamental
error of his system delayed the evolutionary life of the science through
all that period.
But man had begun to measure. Geometry had been born and Eratosthenes
had indeed measured the size of the earth. Tools in bronze and iron
were fashioned closely after the models of tools of stone; astrolabes
and armillary spheres were first built on geometric spheres and circles;
and science was then laid away for the slumber of the Dark Ages.
Nevertheless, through all this dreary period the life of the youthful
astronomical giant was maintained. Time went on, the heavens revolved;
sun, moon, and stars kept their appointed places, and Arab and Moor and
the savage monarchs of the East were there to observe and record, even
if the world-mind was lying fallow, and no genius had been born to
inspire anew that direction of human intellect on which the later growth
of science and civilization depends. With the growth of the collective
mind of mankind, from generation to generation, we note that ordered
sequence of events which characterizes the development of astronomy from
earliest peoples down to the age of Newton, Herschel, and the present.
It is the unfolding of a story as if with a definite plot from the
beginning.
Leaving to philosophical writers the great fundamental reason underlying
the intellectual lethargy of the Dark Ages, we only note that astronomy
and its development suffered with every other department of human
activity that concerned the intellectual progress of the race. To
knowledge of every sort the medieval spirit was hostile. But with the
founding and growth of universities, a new era began. The time was ripe
for Copernicus and a new system of the heavens. The discovery of the New
World and the revival of learning through the universities added that
stimulus and inspiration which marked the transition from the Middle
Ages to our modern era, and the life of astronomy, long dormant, was
quickened to an extraordinary development.
It fell to the lot of Copernicus to write the second great book on
astronomy, "De Revolutionibus Orbium Coelestium." But the new
heliocentric or sun-centered system of Copernicus, while it was the true
system bidding fair to replace the false, could not be firmly
established except on the basis of accurate observation.
How fortunate was the occurrence of the new star of 1572, that turned
the keen intellect of Tycho Brahe toward the heavens! Without the
observational labors of Tycho's lifetime, what would the mathematical
genius of Kepler have availed in discovery of his laws of motion of the
planets?
Historians dwell on the destruction and violent conflicts of certain
centuries of the Middle Ages, quite overlooking the constructive work in
progress through the entire era. Much of this was of a nature absolutely
essential to the new life that was to manifest itself in astronomy. The
Arabs had made important improvements in mathematical processes,
European artisans had made great advances in the manufacture of glass
and in the tools for working in metals.
Then came Galileo with his telescope revealing anew the universe to
mankind. It was the north of Italy where the Renaissance was most
potent, recalling the vigorous life of ancient Greece. Copernicus had
studied here; it was the home of Galileo. Columbus was a Genoese, and
the compass which guided him to the Western World was a product of deft
Italian artisans whose skill with that of their successors was now
available to construct the instruments necessary for further progress in
the accurate science of astronomical observation. Even before
Copernicus, Johann Mueller, better known as Regiomontanus, had imbibed
the learning of the Greeks while studying in Italy, and founded an
observatory and issued nautical almanacs from Nuremberg, the basis of
those by which Columbus was guided over untraversed seas.
About this time, too, the art of printing was invented, and the
interrelation of all the movements then in progress led up to a general
awakening of the mind of man, and eventually an outburst in science and
learning, which has continued to the present day. Naturally it put new
life into astronomy, and led directly up from Galileo and his
experimental philosophy to Newton and the _Principia_, the third in the
trinity of great astronomical books of all time.
To get to the bottom of things, one must study intimately the history of
the intellectual development of Europe through the fifteenth and
sixteenth centuries. Many of the western countries were ruled by
sovereigns of extraordinary vigor and force of character, and their
activities tended strongly toward that firm basis on which the
foundations of modern civilization were securely laid.
Contemporaneously with this era, and following on through the
seventeenth century, came the measurements of the earth by French
geodesists, the construction of greater and greater telescopes and the
wonderful discoveries with them by Huygens, Cassini, and many others.
Most important of all was the application of telescopes to the
instruments with which angles are measured. Then for the first time man
had begun to find out that by accurate measures of the heavenly bodies,
their places among the stars, their sizes and distances, he could attain
to complete knowledge of them and so conquer the universe.
But he soon realized the insufficiency of the mathematical tools with
which he worked--how unsuited they were to the solution of the problem
of three bodies (sun, earth, and moon) under the Newtonian law of
gravitation, let alone the problem of n-bodies, mutually attracting each
the other; and every one perturbing the motion of every other one. So
the invention of new mathematical tools was prosecuted by Newton and his
rival Leibnitz, who, by the way, showed himself as great a man as
mathematician: "taking mathematics," wrote Leibnitz, "from the beginning
of the world to the times when Newton lived, what he had done was much
the better half." Newton was the greatest of astronomers who, since the
revival of learning, had observed the motions of the heavenly bodies and
sought to find out why they moved.
Copernicus, Tycho Brahe, Galileo, Kepler, Newton, all are bound together
as in a plot. Not one of them can be dissociated from the greatest of
all discoveries. But Newton, the greatest of them all, revealed his
greatness even more by saying: "If I have seen further than other men,
it is because I have been standing on the shoulders of giants."
Elsewhere he says: "All this was in the two plague years of 1665 and
1666 [he was then but twenty-four], for in those days I was in the prime
of my age for invention, and minded mathematics and philosophy more than
at any time since." All school children know these as the years of the
plague and the fire; but very few, in school or out, connect these years
with two other far-reaching events in the world's history, the
invention of the infinitesimal calculus and the discovery of the law of
gravitation.
We have passed over the name of Descartes, almost contemporary with
Galileo, the founder of modern dynamics, but his initiation of one of
the greatest improvements of mathematical method cannot be overlooked.
This era was the beginning of the Golden Age of Mathematics that
embraced the lives of the versatile Euler, equally at home in dynamics
and optics and the lunar theory; of La Grange, author of the elegant
"Mecanique Analytique"; and La Place, of the unparalleled "Mecanique
Celeste." With them and a fully elaborated calculus Newton's universal
law had been extended to all the motions of the cosmos. Even the tides
and precession of the equinoxes and Bradley's nutation were accounted
for and explained. Mathematical or gravitational astronomy had attained
its pinnacle--it seemed to be a finished science: all who were to come
after must be but followers.
The culmination of one great period, however, proved to be but the
inception of another epoch in the development of the living science.
The greatest observer of all time, with a telescope built by his own
hands, had discovered a great planet far beyond the then confines of the
solar system. Mathematicians would take care of Uranus, and Herschel was
left free to build bigger telescopes still, and study the construction
of the stellar universe. Down to his day astronomy had dealt almost
wholly with the positions and motions of the celestial bodies--astronomy
was a science of _where_. To inquire _what_ the heavenly bodies _are_,
seemed to Herschel worthy of his keenest attention also. While "a
knowledge of the construction of the heavens has always been the
ultimate object of my observations," as he said, and his ingenious
method of star-gauging was the first practicable attempt to investigate
the construction of the sidereal universe, he nevertheless devoted much
time to the description of nebulae and their nature, as well as their
distribution in space. He was the founder of double-star astronomy, and
his researches on the light of the stars by the simple method of
sequences were the inception of the vast fields of stellar photometry
and variable stars. The physics of the sun, also, was by no means
neglected; and his lifework earned for him the title of father of
descriptive astronomy.
While progress and discovery in the earlier fields of astronomy were
going on, the initial discoveries in the vast group of small planets
were made at the beginning of the nineteenth century. The great Bessel
added new life to the science by revolutionizing the methods and
instruments of accurate observation, his work culminating in the measure
of the distance of 61 Cygni, first of all the stars whose distance from
the sun became known.
Wonderful as was this achievement, however, a greater marvel still was
announced just before the middle of the century--a new planet far beyond
Uranus, whose discovery was made as a direct result of mathematical
researches by Adams and Le Verrier, and affording an extraordinary
verification of the great Newtonian law. These were the days of great
discoveries, and about this time the giant of all the astronomical tools
of the century was erected by Lord Rosse, the "Leviathan" reflector with
a speculum six feet in diameter, which remained for more than half a
century the greatest telescope in the world, and whose epochal
discovery of spiral nebulae has greater significance than we yet know or
perhaps even surmise.
The living science was now at the height of a vigorous development, when
a revolutionary discovery was announced by Kirchhoff which had been
hanging fire nearly half a century--the half century, too, which had
witnessed the invention of photography, the steam engine, the railroad,
and the telegraph: three simple laws by which the dark absorption lines
of a spectrum are interpreted, and the physical and chemical
constitution of sun and stars ascertained, no matter what their distance
from us.
Huggins in England and Secchi in Italy were quick to apply the discovery
to the stars, and Draper and Pickering by masterly organization have
photographed and classified the spectra of many hundred thousand stars
of both hemispheres, a research of the highest importance which has
proved of unique service in studies of stellar movements and the
structure of the universe by Eddington and Shapley, Campbell and
Kapteyn, with many others who are still engaged in pushing our knowledge
far beyond the former confines of the universe.
Few are the branches of astronomy that have not been modified by
photography and the spectroscope. It has become a measuring tool of the
first order of accuracy; measuring the speed of stars and nebulae toward
and from us; measuring the rotational speed of sun and planets, corona
and Saturnian ring; measuring the distances of whole classes of stars
from the solar system; measuring afresh even the distance of the
sun--the yardstick of our immediate universe; measuring the drift of the
sun with his entire family of planets twelve miles every second in the
direction of Alpha Lyrae; and discovering and measuring the speed of
binary suns too close together for our telescopes, and so making real
the astronomy of the invisible.
Impatient of the handicap of a turbulent atmosphere, the living science
has sought out mountain tops and there erected telescopes vastly greater
than the "Leviathan" of a past century. There the sun in every detail of
disk and spectrum is photographed by day, and stars with their spectra
and the nebulae by night. Great streams of stars are discovered and the
speed and direction of their drift ascertained. The marvels of the
spiral nebulae are unfolded, their multitudinous forms portrayed and
deciphered.
And their distances? And the distances of the still more wonderful
clusters? Far, inconceivably far beyond the Milky Way. And are they
"island universes"? And can man, the measurer, measure the distance of
the "mainland" beyond?
CHAPTER II
THE FIRST ASTRONOMERS
Who were the first astronomers? And who wrote the first treatise on
astronomy, oldest of the sciences?
Questions not easy to answer in our day. With the progress of
archaeological research, or inquiry into the civilization and monuments
of early peoples, it becomes certain that man has lived on this planet
earth for tens of thousands of years in the past as an intelligent,
observing, intellectual being; and it is impossible to assign any time
so remote that he did not observe and philosophize upon the firmament
above.
We can hardly imagine a people so primitive that they would fail to
regard the sun as "Lord of the Day," and therefore all important in the
scheme of things terrestrial. Says Anne Bradstreet of the sun in her
"Contemplations":
What glory's like to thee?
Soul of this world, this universe's eye,
No wonder some made thee deity.
To the Babylonians belongs the credit of the oldest known work on
astronomy. It was written nearly six thousand years ago, about B. C.
3800, by their monarch Sargon the First, King of Agade. Only the merest
fragments of this historic treatise have survived, and they indicate the
reverence of the Babylonians for the sun. Another work by Sargon is
entitled "Omens," which shows the intimate relationship of astronomy to
mysticism and superstitious worship at this early date, and which
persists even at the present day.
As remotely as B. C. 3000, the sun-god Shamash and his wife Aya are
carved upon the historic cylinders of hematite and lapis lazuli, and one
of the oldest designs on these cylinders represents the sun-god coming
out of the Door of Sunrise, while a porter is opening the Gate of the
East. The Semitic religion had as its basis a reverence for the bodies
of the sky; and Samson, Hebrew for sun, was probably the sun-god of the
Hebrews. The Phoenician deity, Baal, was a sun-god under differing
designations; and at the epoch of the Shepherd Kings, about B. C. 1500,
during the Hyksos dynasty, the sun-god was represented by a circle or
disk with extended rays ending in hands, possibly the precursor of the
frequently recurring Egyptian design of the winged disk or winged solar
globe. Hittites, Persians, and Assyrians, as well as the Phoenicians,
frequently represented the sun-god in similar fashion in their sacred
glyphs or carvings.
For a long period in early human history, astronomy and astrology were
pretty much the same. We can trace the history of astrology back as far
as B. C. 3000 in ancient Babylonia. The motions of the sun, moon, and
the five lucid planets of that time indicated the activity of the
various gods who influenced human affairs. So the Babylonian priests
devised an elaborate system of interpreting the phenomena of the
heavens; and attaching the proper significance in human terms to
everything that took place in the sky. In Babylonia and Assyria it was
the king and his people for whom the prognostications were made out. It
was the same in Egypt. Later, about the fifth century B. C., astrology
spread through Greece, where astrologers developed the idea of the
influence of planets upon individual concerns. Astrology persisted
through the Dark Ages, and the great astronomers Copernicus, Tycho,
Kepler, Gassendi, and Huygens were all astrologers as well. Milton makes
many references to planetary influence, our language has many words with
a direct origin in astrology, and in our great cities to-day are many
astrologers who prepare individual horoscopes of more than ordinary
interest.
It is difficult to assign the antiquity of the Chinese astronomy with
any approach to definiteness. Their earliest records appear to have been
total eclipses of the sun, going back nearly 2,200 years before the
Christian era; and nearly a thousand years earlier the Hindu astronomy
sets down a conjunction of all the planets, concerning which, however,
there is doubt whether it was actually observed or merely calculated
backward. Owing to a colossal misfortune, the burning of all native
scientific books by order of the Emperor Tsin-Chi-Hwang-Ti, in B. C.
221, excepting only the volumes relating to agriculture, medicine, and
astrology, the Chinese lost a precious mass of astronomical learning,
accumulated through the ages. No less an authority than Wells Williams
credits them with observing 600 solar eclipses between B. C. 2159 and A.
D. 1223, and there must have been some centuries of eclipses observed
and recorded anterior to B. C. 2159, as this is the date assigned to the
eclipse which came unheralded by the astronomers royal, Hi and Ho, who
had become intoxicated and forgot to warn the Court, in accord with
their duty. China was thereby exposed to the anger of the gods, and Hi
and Ho were executed by his Majesty's command. It is doubtful if there
is an earlier record of any celestial phenomenon.
CHAPTER III
PYRAMID, TOMB, AND TEMPLE
Inquiry into the beginnings of astronomy in ancient Egypt reveals most
interesting relations of the origins of the science to the life and work
and worship of the people. Their astronomers were called the "mystery
teachers of heaven"; their monuments indicate a civilization more or
less advanced; and their temples were built on astronomical principles
and dedicated to purpose of worship. The Egyptian records carry us back
many thousands of years, and we find that in Egypt, as in other early
civilizations, observation of the heavenly bodies may be embraced in
three pretty distinct stages. Awe, fear, wonder and worship were the
first. Then came utility: a calendar was necessary to tell men when "to
plow and sow, to reap and mow," and a calendar necessitated astronomical
observations of some sort. Following this, the third direction required
observations of celestial positions and phenomena also, because
astrology, in which the potentates of every ancient realm believed,
could only thrive as it was based on astronomy.
Sun worship was preeminent in early Egypt as in India, where the primal
antithesis between night and day struck terror in the unformed mind of
man. In one of the Vedas occurs this significant song to the god of day:
"Will the Sun rise again? Will our old friend the Dawn come back again?
Will the power of Darkness be conquered by the God of Light?"
Quite different from India, however, is Egypt in matters of record: in
India, records in papyrus, but no monuments of very great antiquity; in
Egypt, no papyrus, but monuments of exceeding antiquity in abundance.
Herodotus and Pliny have told us of the great antiquity of these
monuments, even in their own day, and research by archaeologist and
astronomer has made it certain that the pyramids were built by a race
possessing great knowledge of astronomy. Their temples, too, were
constructed in strict relation to stars. Not only are the temples, as
Edfu and Denderah, of exceeding interest in themselves, but associated
with them are often huge monoliths of syenite, obelisks of many hundred
tons in weight, which the astronomer recognizes as having served as
observation pillars or gnomons. Specimens of these have wandered as far
from home as Central Park and the bank of the Thames. But there is an
even more remarkable wealth of temple inscriptions, zodiacs especially.
Next to the sun himself was the worship of the Dawn and Sunrise, the
great revelations of nature. There were numerous hymns to the still more
numerous sun-gods and the powers of sunlight. Ra was the sun-god in his
noontide strength; Osiris, the dying sun of sunset. Only two gods were
associated with the moon, and for the stars a special goddess, Sesheta.
Sacrifices were made at day-break; and the stars that heralded the dawn
were the subjects of careful observation by the sacrificial priests, who
must therefore have possessed a good knowledge of star places and names,
doubtless in belts of stars extending clear around the heavens. These
decans, as they were called, are the exact counterparts of the moon
stations devised by the Arabians, Indians, and other peoples for a like
purpose.
The plane or circle of observation, both in Egypt and India, was always
the horizon, whether the sun was observed or moon or stars. So the sun
was often worshiped by the ancient Egyptians as the "Lord of the Two
Horizons." It is sometimes difficult to keep in mind the fact, in regard
to all temples of the ancients, whether in Egypt or elsewhere, that in
studying them we must deal with the risings or settings of the heavenly
bodies in quite different fashion from that of the astronomer of to-day,
who is mainly concerned only with observing them on the meridian. The
axis of the temple shows by its direction the place of rising or
setting: if the temple faces directly east or west, its amplitude is 0.
Now the sun, moon, and planets are, as everyone knows, very erratic as
to their amplitudes (i. e., horizon points) of rising and setting; so it
must have been the stars that engrossed the attention of the earliest
builders of temples. After that, temples were directed to the rising
sun, at the equinox or solstices. Then came the necessity of finding out
about the inclination or obliquity of the ecliptic, and this is where
the gnomon was employed.
At Karnak are many temples of the solstitial order: the wonderful temple
of Amen-Ra is so oriented that its axis stands in amplitude 26 degrees
north of west, which is the exact amplitude of the sun at Thebes at
sunset of the summer solstice. The axis of a lesser temple adjacent
points to 26 degrees south of east, which is the exact amplitude of
sunrise at the winter solstice. At Gizeh we find the temples oriented,
not solstitially, but by the equinoxes, that is, they face due east and
west. Peoples who worshiped the sun at the solstice must have begun
their year at the solstice; and Sir Norman Lockyer shows how the rise of
the Nile, which took place at the summer solstice, dominated not only
the industry but the astronomy and religion of Egypt.
Looking into the question of temple orientation in other countries, as
China, for example, Lockyer finds that the most important temple of that
country, the Temple of the Sun at Peking, is oriented to the winter
solstice; and Stonehenge, as has long been known, is oriented to sunrise
at the summer solstice.
In like fashion the rising and setting of many stars were utilized by
the Egyptians, in both temple and pyramid; and no astronomer who has
ever seen these ancient structures and studied their orientations can
doubt that they were built by astronomers for use by astronomers of that
day. The priests were the astronomers, and the temples had a deep
religious significance, with a ceremony of exceeding magnificence
wherever observations of heavenly bodies were undertaken, whether of sun
or stars.
Hindu and Persian astronomy must be passed over very briefly.
Interesting as their systems are historically, there were few, if any,
original contributions of importance, and the Indian treatises bear
strong evidence of Greek origin.
CHAPTER IV
ORIGIN OF GREEK ASTRONOMY
While the Greeks laid the foundations of modern scientific astronomy,
they were not as a whole observers: rather philosophers, we should say.
The later representatives of the Greek School, however, saw the
necessity of observation as a basis of true induction; and they
discovered that real progress was not possible unless their speculative
ideas were sufficiently developed and made definite by the aid of
geometry, so that they became capable of detailed comparison with
observation. This was the necessary and ultimate test with them, and the
same is true to-day. The early Greek philosophers were, however, mainly
interested, not in observations, but in guessing the causes of
phenomena.
Thales of Miletus, founder of the Ionian School, introduced the system
of Egyptian astronomy into Greece, about the end of the seventh century
B. C. He is universally known as the first astronomer who ever predicted
a total eclipse of the sun that happened when he said it would: the
eclipse of B. C. 585. This he did by means of the Chaldean eclipse cycle
of 18 years known as the Saros.
Aristarchus of Samos was the first and most eminent of the Alexandrian
astronomers, and his treatise "On the Magnitudes and Distances of the
Sun and Moon" is still extant. This method of ascertaining how many
times farther the sun is than the moon is very simple, and
geometrically exact. Unfortunately it is impossible, even to-day, to
observe with accuracy the precise time when the moon "quarters," (an
observation essential to his method), because the moon's terminal, or
line between day and night, is not a straight line as required by
theory, but a jagged one. By his observation, the sun was only twenty
times farther away than the moon, a distance which we know to be nearly
twenty times too small.
His views regarding other astronomical questions were right, although
they found little favor among contemporaries. Not only was the earth
spherical, he said, but it rotated on its axis and also traveled round
the sun. Aristarchus was, indeed, the true originator of the modern
doctrine of motions in the solar system, and not Copernicus, seventeen
centuries later; but Seleucus appears to have been his only follower in
these very advanced conceptions. Aristarchus made out the apparent
diameters of sun and moon as practically equal to one another, and
inferred correctly that their real diameters are in proportion to their
distances from the earth. Also he estimated, from observations during an
eclipse of the moon, that the moon's diameter is about one-third that of
the earth. Aristarchus appears to have been one of the clearest and most
accurate thinkers among the ancient astronomers; even his views
concerning the distances of the stars were in accord with the fact that
they are immeasurably distant as compared with the distances of the sun,
moon, and planets.
Practically contemporary with Aristarchus were Timocharis and
Aristillus, who were excellent observers, and left records of position
of sun and planets which were exceedingly useful to their successors,
Hipparchus and Ptolemy in particular. Indeed their observations of star
positions were such that, in a way, they deserve the fame of having made
the first catalogue, rather than Hipparchus, to whom is universally
accorded that honor.
Spherical astronomy had its origin with the Alexandrian school, many
famous geometers, and in particular Euclid, pointing the way. Spherics,
or the doctrine of the sphere, was the subject of numerous treatises,
and the foundations were securely laid for that department of
astronomical research which was absolutely essential to farther advance.
The artisans of that day began to build rude mechanical adaptations of
the geometric conceptions as concrete constructions in wood and metal,
and it became the epoch of the origin of astrolabes and armillary
spheres.
CHAPTER V
MEASURING THE EARTH--ERATOSTHENES
All told, the Greek philosophers were probably the keenest minds that
ever inhabited the planet, and we cannot suppose them so stupid as to
reject the doctrine of a spherical earth. In fact so certain were they
that the earth's true figure is a sphere that Eratosthenes in the third
century B. C. made the first measure of the dimensions of the
terrestrial sphere by a method geometrically exact.
At Syene in Upper Egypt the sun at the summer solstice was known to pass
through the zenith at noon, whereas at Alexandria Eratosthenes estimated
its distance as seven degrees from the zenith at the same time. This
difference being about one-fiftieth of the entire circumference of a
meridian, Eratosthenes correctly inferred that the distance between
Alexandria and Syene must be one-fiftieth of the earth's circumference.
So he measured the distance between the two and found it 5,000 stadia.
This figured out the size of the earth with a percentage of error
surprisingly small when we consider the rough means with which
Eratosthenes measured the sun's zenith distance and the distance between
the two stations.
Greatest of all the Greek astronomers and one of the greatest in the
history of the science was Hipparchus who had an observatory at Rhodes
in the middle of the second century B. C. His activities covered every
department of astronomy; he made extensive series of observations which
he diligently compared with those handed down to him by the earlier
astronomers, especially Aristillus and Timocharis. This enabled him to
ascertain the motion of the equinoxial points, and his value of the
constant of precession of the equinoxes is exceedingly accurate for a
first determination.
In 134 B. C. a new star blazed out in the constellation Scorpio, and
this set Hipparchus at work on a catalogue of the brighter stars of the
firmament, a monumental work of true scientific conception, because it
would enable the astronomers of future generations to ascertain what
changes, if any, were taking place in the stellar universe. There were
1,080 stars in his catalogue, and he referred their positions to the
ecliptic and the equinoxes. Also he originated the present system of
stellar magnitudes or orders of brightness, and his catalogue was in use
as a standard for many centuries.
Hipparchus was a great mathematician as well, and he devoted himself to
the improvement of the method of applying numerical calculations to
geometrical figures: trigonometry, both plane and spherical, that is;
and by some authorities he is regarded as the inventor of original
methods in trigonometry. The system of spheres of Eudoxus did not
satisfy him, so he devised a method of representing the paths of the
heavenly bodies by perfectly uniform motion in circles. There is slight
evidence that Apollonius of Perga may have been the originator of the
system, but it was reserved for Hipparchus to work it out in final form.
This enabled him to ascertain the varying length of the seasons, and he
fixed the true length of the year as 365-1/4 days. He had almost equal
success in dealing with the irregularities of the moon's motion,
although the problem is much more complicated. The distance and size of
the moon, by the method of Aristarchus, were improved by him, and he
worked out, for the distance of the sun, 1,200 radii of the earth--a
classic for many centuries.
Hipparchus devoted much attention to eclipses of both sun and moon, and
we owe to him the first elucidation of the subject of parallax, or the
effect of difference of position of an observer on the earth's surface
as affecting the apparent projection of the moon against the sun when a
solar eclipse takes place; whereas an eclipse of the moon is unaffected
by parallax and can be seen at the same time by observers everywhere, no
matter what their location on the earth. Indeed, with all that
Hipparchus achieved, we need not be surprised that astronomy was
regarded as a finished science, and made practically no progress
whatever for centuries after his time.
Then came Claudius Ptolemaeus, generally known as Ptolemy, the last great
name in Greek astronomy. He lived in Alexandria about the middle of the
second century A. D. and wrote many minor astronomical and astrological
treatises, also works on geography and optics, in the last of which the
atmospheric refraction of rays of light from the heavenly bodies,
apparently elevating them toward the zenith, is first dealt with in true
form.
CHAPTER VI
PTOLEMY AND HIS GREAT BOOK
Ptolemy was an observer of the heavens, though not of the highest order;
but he had all the work of his predecessors, best of all Hipparchus, to
build upon. Ptolemy's greatest work was the "Megale Syntaxis," generally
known as the Almagest. It forms a nearly complete compendium of the
ancient astronomy, and although it embodies much error, because built on
a wrong theory, the Almagest nevertheless is competent to follow the
motions of all the bodies in the sky with a close approach to accuracy,
even at the present day. This marvelous work written at this critical
epoch became as authoritative as the philosophy of Aristotle, and for
many centuries it was the last word in the science. The old astrology
held full sway, and the Ptolemaic theory of the universe supplied
everything necessary: further progress, indeed, was deemed impossible.
The Almagest comprises in all thirteen books, the first two of which
deal with the simpler observations of the celestial sphere, its own
motion and the apparent motions of sun, moon, and planets upon it. He
discusses, too, the postulates of his system and exhibits great skill as
an original geometer and mathematician. In the third book he takes up
the length of the year, and in the fourth book similarly the moon and
the length of the month. Here his mathematical powers are at their best,
and he made a discovery of an inequality in the moon's motion known as
the evection. Book five describes the construction and use of the
astrolabe, a combination of graduated circles with which Ptolemy made
most of his observations. In the sixth book he follows mainly Hipparchus
in dealing with eclipses of sun and moon. In the seventh and eighth
books he discusses the motion of the equinox, and embodies a catalogue
of 1,028 stars, substantially as in Hipparchus. The five remaining books
of the Almagest deal with the planetary motions, and are the most
important of all of Ptolemy's original contributions to astronomy.
Ptolemy's fundamental doctrines were that the heavens are spherical in
form, all the heavenly motions being in circles. In his view, the earth
too is spherical, and it is located at the center of the universe, being
only a point, as it were, in comparison. All was founded on mere
appearance combined with the philosophical notion that the circle being
the only perfect curve, all motions of heavenly bodies must take place
in earth-centered circles. For fourteen or fifteen centuries this false
theory persisted, on the authority of Ptolemy and the Almagest,
rendering progress toward the development of the true theory impossible.
Ptolemy correctly argued that the earth itself is a sphere that is
curved from east to west, and from north to south as well, clinching his
argument, as we do to-day, by the visibility of objects at sea, the
lower portions of which are at first concealed from our view by the
curved surface of the water which intervenes. To Ptolemy also the earth
is at the center of the celestial sphere, and it has no motion of
translation from that point; but his argument fails to prove this. Truth
and error, indeed, are so deftly intermingled that one is led to wonder
why the keen intelligence of this great philosopher permitted him to
reject the simple doctrine of the earth's rotation on its axis. But if
we reflect that there was then no science of natural philosophy or
physics proper, and that the age was wholly undeveloped along the lines
of practical mechanics, we shall see why the astronomers of Ptolemy's
time and subsequent centuries were content to accept the doctrines of
the heavens as formulated by him.
When it came to explaining the movements of the "wandering stars," or
planets, as we term them, the Ptolemaic theory was very happy in so far
as accuracy was concerned, but very unhappy when it had to account for
the actual mechanics of the cosmos in space. Sun and moon were the only
bodies that went steadily onward, easterly: whereas all the others,
Mercury, Venus, Mars, Jupiter, Saturn, although they moved easterly most
of the time, nevertheless would at intervals slow down to stationary
points, where for a time they did not move at all, and then actually go
backward to the west, or retrograde, then become stationary again,
finally resuming their regular onward motion to the east.
To help out of this difficulty, the worst possible mechanical scheme was
invented, that known as the epicycle. Each of the five planets was
supposed to have a fictitious "double," which traveled eastward with
uniformity, attached to the end of a huge but mechanically impossible
bar. The earth-centered circle in which this traveled round was called
the "deferent." What this bar was made of, what stresses it would be
subjected to, or what its size would have to be in order to keep from
breaking--none of these questions seems to have agitated the ancient and
medieval astronomers, any more than the flat-earth astronomy of the
Hindu is troubled by the necessity of something to hold up the tortoise
that holds up the elephant that holds up the earth.
But at the end of this bar is jointed or swiveled another shorter bar,
to the revolving end of which is attached the actual planet itself; and
the second bar, by swinging once round the end of the primary advancing
bar, would account for the backward or retrograde motion of the planet
as seen in the sky. For every new irregularity that was found, in the
motion of Mars, for instance, a new and additional bar was
requisitioned, until interplanetary space was hopelessly filled with
revolving bars, each producing one of the epicycles, some large, some
small, that were needed to take up the vagaries of the several planets.
The Arabic astronomers who kept the science alive through the Middle
Ages added epicycle to epicycle, until there was every justification for
Milton's verses descriptive of the sphere:
With Centric and Eccentric scribbled o'er,
Cycle and Epicycle, Orb in Orb.
CHAPTER VII
ASTRONOMY OF THE MIDDLE AGES
With the fall of Alexandria and the victory of Mohammed throughout the
West, and a consequent decline in learning, supremacy in science passed
to the East and centered round the caliphs of Bagdad in the seventh and
eighth centuries. They were interested in astronomy only as a practical,
and to them useful, science, in adjusting the complicated lunar calendar
of the Mohammedans, in ascertaining the true direction of Mecca which
every Mohammedan must know, and in the revival of astrology, to which
the Greeks had not attached any particular significance.
Harun al-Rashid ordered the Almagest and many other Greek works
translated, of which the modern world would otherwise no doubt never
have heard, as the Greek originals are not extant.
Splendid observatories were built at Damascus and Bagdad, and fine
instruments patterned after Greek models were continuously used in
observing. The Arab astronomers, although they had no clocks, were
nevertheless so fully impressed with the importance of time that they
added extreme value to their observations of eclipses, for example, by
setting down the altitudes of sun or stars at the same time. On very
important occasions the records were certified on oath by a body of
barristers and astronomers conjointly--a precedent which fortunately has
never been followed.
About the middle of the ninth century, the Caliph Al-Mamun directed his
astronomers to revise the Greek measures of the earth's dimensions, and
they had less reverence for the Almagest than existed in later
centuries: indeed, Tabit ben Korra invented and applied to the tables of
the Almagest a theoretical fluctuation in the position of the ecliptic
which he called "trepidation," which brought sad confusion into
astronomical tables for many succeeding centuries.
Albategnius was another Arab prince whose record in astronomy in the
ninth and tenth centuries was perhaps the best: the Ptolemaic values of
the precession of the equinoxes and of the obliquity of the ecliptic
were improved by new observations, and his excellence as mathematician
enabled him to make permanent improvements in the astronomical
application of trigonometry.
Abul Wefa was the last of the Bagdad astronomers in the latter half of
the tenth century, and his great treatise on astronomy known as the
Almagest is sometimes confused with Ptolemy's work. Following him was
Ibn Yunos of Cairo, whose labors culminated in the famous Hakemite
Tables, which became the standard in mathematical and astronomical
computations for several centuries.
Mohammedan astronomy thrived, too, in Spain and northern Africa.
Arzachel of Toledo published the Toledan Tables, and his pupils made
improvements in instruments and the methods of calculation. The Giralda
was built by the Moors in Seville in 1196, the first astronomical
observatory on the continent of Europe; but within the next half century
both Seville and Cordova became Christian again, and Arab astronomy was
at an end.
Through many centuries, however, the science had been kept alive, even
if no great original advances had been achieved; and Arab activities
have modified our language very materially, adding many such words as
almanac, zenith, and radii, and a wealth of star names, as Aldebaran,
Rigel, Betelgeuse, Vega, and so on.
Meanwhile, other schools of astronomy had developed in the East, one at
Meraga near the modern Persia, where Nassir Eddin, the astronomer of
Hulagu Khan, grandson of the Mongol emperor Genghis Khan, built and used
large and carefully constructed instruments, translated all the Greek
treatises on astronomy, and published a laborious work known as the
Ilkhanic Tables, based on the Hakemite Tables of Ibn Yunos.
More important still was the Tartar school of astronomy under Ulugh Beg,
a grandson of Tamerlane, who built an observatory at Samarcand in 1420,
published new tables of the planets, and made with his excellent
instruments the observations for a new catalogue of stars, the first
since Hipparchus, the star places being recorded with great precision.
The European astronomy of the Middle Ages amounted to very little
besides translation from the Arabic authors into Latin, with
commentaries. Astronomers under the patronage of Alfonso X of Leon and
Castile published in 1252 the Alfonsine Tables, which superseded the
Toledan tables and were accepted everywhere throughout Europe. Alfonso
published also the "Libros del Saber," perhaps the first of all
astronomical cyclopedias, in which is said to occur the earliest diagram
representing a planetary orbit as an ellipse: Mercury's supposed path
round the earth as a center.
Purbach of Vienna about the middle of the 15th century began his
"Epitome of Astronomy" based on the "Almagest" of Ptolemy, which was
finished by his collaborator Regiomontanus, who was an expert in
mathematics and published a treatise on trigonometry with the first
table of sines calculated for every minute from 0 deg. to 90 deg., a most
helpful contribution to theoretical astronomy.
Regiomontanus had a very picturesque career, finally taking up his
residence in Nuremberg, where a wealthy citizen named Walther became his
patron, pupil, and collaborator. The artisans of the city were set at
work on astronomical instruments of the greatest accuracy, and the comet
of 1472 was the first to be observed and studied in true scientific
fashion. Regiomontanus was very progressive and the invention of the new
art of printing gave him an opportunity to publish Purbach's treatise,
which went through several editions and doubtless had much to do in
promoting dissatisfaction with the ancient Ptolemaic system, and was
thus most significant in preparing a background for the coming of the
new Copernican order.
The Nuremberg presses popularized astronomy in other important ways,
issuing almanacs, the first precursors of our astronomical Ephemerides.
Regiomontanus was practical as well, and invented a new method of
getting a ship's position at sea, with tables so accurate that they
superseded all others in the great voyages of discovery, and it is
probable that they were employed by Columbus in his discovery of the
American continent. Regiomontanus had died several years earlier, in
1475 at Rome, where he had gone by invitation of the Pope to effect a
reformation in the calendar. He was only forty, and his patron Walther
kept on with excellent observations, the first probably to be corrected
for the effect of atmospheric refraction, although its influence had
been known since Ptolemy. The Nuremberg School lasted for nearly two
centuries.
Nearly contemporary with Regiomontanus were Fracastoro and Peter Apian,
whose original observations on comets are worthy of mention because they
first noticed that the tails of these bodies always point away from the
sun. Leonardo da Vinci was the first to give the true explanation of
earth-shine on the moon, and similarly the moon-illumination of the
earth; and this no doubt had great weight in disposing of the popular
notion of an essential difference of nature between the earth and
celestial bodies--all of which helped to prepare the way for Copernicus
and the great revolution in astronomical thought.
CHAPTER VIII
COPERNICUS AND THE NEW ERA
Throughout the Middle Ages the progress of astronomy was held back by a
combination of untoward circumstances. A prolonged reaction from the
heights attained by the Greek philosophers was to be expected. The
uprising of the Mohammedan world, and the savage conquerors in the East
did not produce conditions favorable to the origin and development of
great ideas.
At the birth of Copernicus, however, in 1473, the time was ripening for
fundamental changes from the ancient system, the error of which had
helped to hold back the development of the science for centuries. The
fifteenth century was most fruitful in a general quickening of
intelligence, the invention of printing had much to do with this, as it
spread a knowledge of the Greek writers, and led to conflict of
authorities. Even Aristotle and Ptolemy were not entirely in harmony,
yet each was held inviolate. It was the age of the Reformation, too, and
near the end of the century the discovery of America exerted a powerful
stimulus in the advance of thought.
Copernicus searched the works of the ancient writers and philosophers,
and embodied in this new order such of their ideas as commended
themselves in the elaboration of his own system.
Pythagoras alone and his philosophy looked in the true direction. Many
believe that he taught that the sun, not the earth, is at the center of
our solar system; but his views were mingled with the speculative
philosophy of the Greeks, and none of his writings, barring a few meager
fragments, have come down to our modern age.
To many philosophers, through all these long centuries, the true theory
of the celestial motions must have been obvious, but their views were
not formulated, nor have they been preserved in writing. So the fact
remains that Copernicus alone first proved the truth of the system which
is recognized to-day. This he did in his great treatise entitled "De
Revolutionibus Orbium Coelestium," the first printed copy of which was
dramatically delivered to him on his deathbed, in May, 1543. The seventy
years of his life were largely devoted to the preparation of this work,
which necessitated many observations as well as intricate calculations
based upon them. Being a canon in the church, he naturally hesitated
about publishing his revolutionary views, his friend Rheticus first
doing this for him in outline in 1540.
So simple are the great principles that they may be embodied in very few
words; what appears to us as the daily revolution of the heavens is not
a real motion, but only an apparent one; that is, the heavens are at
rest, while the earth itself is in motion, turning round an axis which
passes through its center. And the second proposition is that the earth
is simply one of the six known planets; and they all revolve round the
sun as the true center. The solar system, therefore, is "heliocentric,"
or sun-centered, not "geocentric" or earth-centered, as taught by the
Ptolemaic theory.
Copernicus demonstrates clearly how his system explains the retrograde
motion of the planets and their stationary points, no matter whether
they are within the orbit of the earth, as Mercury and Venus, or outside
of it, as Mars, Jupiter, and Saturn. His system provides also the means
of ascertaining with accuracy the proportions of the solar system, or
the relative distances of the planets from the sun and from each other.
In this respect also his system possessed a vast advantage over that of
Ptolemy, and the planetary distances which Copernicus computed are very
close approximations to the measures of the present day.
Reinhold revised the calculations of Copernicus and prepared the "Tabulae
Prutenicae," based on the "De Revolutionibus," which proved far superior
to the Alfonsine Tables, and were only supplanted by the Rudolphine
Tables of Kepler. On the whole we may regard the lifework of Copernicus
as fundamentally the most significant in the history and progress of
astronomy.
CHAPTER IX
TYCHO, THE GREAT OBSERVER
Clear as Copernicus had made the demonstration of the truth of his new
system, it nevertheless failed of immediate and universal acceptance.
The Ptolemaic system was too strongly intrenched, and the motions of all
the bodies in the sky were too well represented by it. Accurate
observations were greatly needed, and the Landgrave William IV. of Hesse
built the Cassel Observatory, which made a new catalogue of stars, and
introduced the use of clocks to carry on the time as measured by the
uniform motion of the celestial sphere. Three years after the death of
Copernicus, Tycho Brahe was born, and when he was 30 the King of Denmark
built for him the famous observatory of Uraniborg, where the great
astronomer passed nearly a quarter of a century in critically observing
the positions of the stars and planets. Tycho was celebrated as a
designer and constructor of new types of astronomical instruments, and
he printed a large volume of these designs, which form the basis of many
in use at the present day. Unfortunately for the genius of Tycho and the
significance of his work, the invention of the telescope had not yet
been made, so that his observations had not the modern degree of
accuracy. Nevertheless, they were destined to play a most important part
in the progress of astronomy.
Tycho was sadly in error in his rejection of the Copernican system,
although his reasons, in his day, seemed unanswerable. If the outer
planets were displaced among the stars by the annual motion of the earth
round the sun, he argued, then the fixed stars must be similarly
displaced--unless indeed they be at such vast distances that their
motions would be too slight to be visible. Of course we know now that
this is really true, and that no instruments that Tycho was able to
build could possibly have detected the motions, the effects of which we
now recognize in the case of the nearer fixed stars in their annual, or
parallactic, orbits.
The remarkably accurate instruments devised by Tycho Brahe and employed
by him in improving the observations of the positions of the heavenly
bodies were no doubt built after descriptions of astrolabes such as
Hipparchus used, as described by Ptolemy. In his "Astronomiae Instauratae
Mechanica" we find illustrations and descriptions of many of them.
One is a polar astrolabe, mounted somewhat as a modern equatorial
telescope is, and the meridian circle is adjustable so that it can be
used in any place, no matter what its latitude might be. There is a
graduated equatorial ring at right angles to the polar axis, so that the
astrolabe could be used for making observations outside the meridian as
well as on it. This equatorial circle slides through grooves, and is
furnished with movable sights, and a plumb line from the zenith or
highest point of the meridian circle makes it possible to give the
necessary adjustment in the vertical. Screws for adjustment at the
bottom are provided, just as in our modern instruments, and two
observers were necessary, taking their sights simultaneously; unless,
as in one type of the instrument, a clock, or some sort of measure of
time, was employed.
Another early type of instrument is called by Tycho the ecliptic
astrolabe (_Armillae Zodiacales_, or the Zodiacal Rings). It resembles
the equatorial astrolabe somewhat, but has a second ring inclined to the
equatorial one at an angle equal to the obliquity of the ecliptic. In
observing, the equatorial ring was revolved round till the ecliptic ring
came into coincidence with the plane of the ecliptic in the sky. Then
the observation of a star's longitude and latitude, as referred to the
ecliptic plane, could be made, quite as well as that of right ascension
and declination on the equatorial plane. But it was necessary to work
quickly, as the adjustment on the ecliptic would soon disappear and have
to be renewed.
Tycho is often called the father of the science of astronomical
observation, because of the improvements in design and construction of
the instruments he used. His largest instrument was a mural quadrant, a
quarter-circle of copper, turning parallel to the north-and-south face
of a wall, its axis turning on a bearing fixed in the wall. The radius
of this quadrant was nine feet, and it was graduated or divided so as to
read the very small angle of ten seconds of arc--an extraordinary degree
of precision for his day.
Tycho built also a very large alt-azimuth quadrant, of six feet radius.
Its operation was very much as if his mural quadrant could be swung
round in azimuth. At several of the great observatories of the present
day, as Greenwich and Washington, there are instruments of a similar
type, but much more accurate, because the mechanical work in brass and
steel is executed by tools that are essentially perfect, and besides
this the power of the telescope is superadded to give absolute
direction, or pointing on the object under observation.
Excellent clocks are necessary for precise observation with such an
instrument; but neither Tycho Brahe, nor Hevelius was provided with such
accessories. Hevelius did not avail himself of the telescope as an aid
to precision of observation, claiming that pinhole sights gave him more
accurate results. It was a dispute concerning this question that Halley
was sent over from London to Danzig to arbitrate.
There could be but one way to decide; the telescope with its added power
magnifies any displacement of the instrument, and thereby enables the
observer to point his instrument more exactly. So he can detect smaller
errors and differences of direction than he can without it. And what is
of great importance in more modern astronomy, the telescope makes it
possible to observe accurately the position of objects so faint that
they are wholly invisible to the naked eye.
CHAPTER X
KEPLER, THE GREAT CALCULATOR
Most fortunate it was for the later development of astronomical theory
that Tycho Brahe not only was a practical or observational astronomer of
the highest order, but that he confined himself studiously for years to
observations of the places of the planets. Of Mars he accumulated an
especially long and accurate series, and among those who assisted him in
his work was a young and brilliant pupil named Johann Kepler.
Strongly impressed with the truth of the Copernican System, Kepler was
free to reject the erroneous compromise system devised by Tycho Brahe,
and soon after Tycho's death Kepler addressed himself seriously to the
great problem that no one had ever attempted to solve, viz: to find out
what the laws of motion of the planets round the sun really are. Of
course he took the fullest advantage of all that Ptolemy and Copernicus
had done before him, and he had in addition the splendid observations of
Tycho Brahe as a basis to work upon.
Copernicus, while he had effected the tremendous advance of substituting
the sun for the earth as the center of motion, nevertheless clung to the
erroneous notion of Ptolemy that all the bodies of the sky must perforce
move at uniform speeds, and in circular curves, the circle being the
only "perfect curve." Kepler was not long in finding out that this
could not be so, and he found it out because Tycho Brahe's observations
were much more accurate than any that Copernicus had employed.
Naturally he attempted the nearest planet first, and that was Mars--the
planet that Tycho had assigned to him for research. How fortunate that
the orbit of Mars was the one, of all the planets, to show practically
the greatest divergence from the ancient conditions of uniform motion in
a perfectly circular orbit! Had the orbit of Mars chanced to be as
nearly circular as is that of Venus, Kepler might well have been driven
to abandon his search for the true curve of planetary motion.
However, the facts of the cosmos were on his side, but the calculations
essential in testing his various hypotheses were of the most tedious
nature, because logarithms were not yet known in his day. His first
discovery was that the orbit of Mars is certainly not a circle, but oval
or elliptic in figure. And the sun, he soon found, could not be in the
center of the ellipse, so he made a series of trial calculations with
the sun located in one of the foci of the ellipse instead.
Then he found he could make his calculated places of Mars agree quite
perfectly with Tycho Brahe's observed positions, if only he gave up the
other ancient requisite of perfectly uniform motion. On doing this, it
soon appeared that Mars, when in perihelion, or nearest the sun, always
moved swiftest, while at its greatest distance from the sun, or
aphelion, its orbital velocity was slowest.
Kepler did not busy himself to inquire why these revolutionary
discoveries of his were as they were; he simply went on making enough
trials on Mars, and then on the other planets in turn, to satisfy
himself that all the planetary orbits are elliptical, not circular in
form, and are so located in space that the center of the sun is at one
of the two foci of each orbit. This is known as Kepler's first law of
planetary motion.
The second one did not come quite so easy; it concerned the variable
speed with which the planet moves at every point of the orbit. We must
remember how handicapped he was in solving this problem: only the
geometry of Euclid to work with, and none of the refinements of the
higher mathematics of a later day. But he finally found a very simple
relation which represented the velocity of the planet everywhere in its
orbit. It was this: if we calculate the area swept, or passed over, by
the planet's radius vector (that is, the line joining its center to the
sun's center) during a week's time near perihelion, and then calculate
the similar area for a week near aphelion, or indeed for a week when
Mars is in any intermediate part of its orbit, we shall find that these
areas are all equal to each other. So Kepler formulated his second great
law of planetary motion very simply: the radius vector of any planet
describes, or sweeps over, equal areas in equal times. And he found this
was true for all the planets.
But the real genius of the great mathematician was shown in the
discovery of his third law, which is more complex and even more
significant than the other two--a law connecting the distances of the
planets from the sun with their periods of revolution about the sun.
This cost Kepler many additional years of close calculation, and the
resulting law, his third law of planetary motion is this: The cubes of
the mean or average distances of the planets from the sun are
proportional to the squares of their times of revolution around him.
So Kepler had not only disposed of the sacred theories of motion of the
planets held by the ancients as inviolable, but he had demonstrated the
truth of a great law which bound all the bodies of the solar system
together. So accurately and completely did these three laws account for
all the motions, that the science of astronomy seemed as if finished;
and no matter how far in the future a time might be assigned, Kepler's
laws provided the means of calculating the planet's position for that
epoch as accurately as it would be possible to observe it. Kepler paused
here, and he died in 1630.
CHAPTER XI
GALILEO, THE GREAT EXPERIMENTER
The fifteenth and sixteenth centuries, containing the lives and work of
Copernicus, Tycho, Galileo, Kepler, Huygens, Halley, and Newton, were a
veritable Golden Age of astronomy. All these men were truly great and
original investigators.
None had a career more picturesque and popular than did Galileo. Born a
few years earlier and dying a few years later than Kepler, the work of
each of these two great astronomers was wholly independent of the other
and in entirely different fields. Kepler was discovering the laws of
planetary motion, while Galileo was laying the secure foundations of the
new science of dynamics, in particular the laws of falling bodies, that
was necessary before Kepler's laws could be fully understood. When only
eighteen Galileo's keen power of observation led to his discovery of the
laws of pendulum motion, suggested by the oscillation to and fro of a
lamp in the cathedral of Pisa.
The world-famous leaning tower of this place, where he was born, served
as a physical laboratory from the top of which he dropped various
objects, and thus was led to formulate the laws of falling bodies. He
proved that Aristotle was all wrong in saying that a heavy body must
fall swifter in proportion to its weight than a lighter one. These and
other discoveries rendered him unpopular with his associates, who
christened him the "Wrangler."
The new system of Copernicus appealed to him; and when he, first of all
men, turned a telescope on the heavenly bodies, there was Venus with
phases like those of the moon, and Jupiter with satellites traveling
about it--a Copernican system in miniature. Nothing could have happened
that would have provided a better demonstration of the truth of the new
system and the falsity of the old. His marvelous discoveries caused the
greatest excitement--consternation even, among the anti-Copernicans.
Galileo published the "Sidereus Nuncius," with many observations and
drawings of the moon, which he showed to be a body not wholly dissimilar
to the earth: this, too, was obviously of great moment in corroboration
of the Copernican order and in contradiction to the Ptolemaic, which
maintained sharp lines of demarcation between things terrestrial and
things celestial.
His telescopes, small as they were, revealed to him anomalous
appearances on both sides of the planet Saturn which he called _ansae_,
or handles. But their subsequent disappearance was unaccountable to him,
and later observers, who kept on guessing ineffectively till Huygens,
nearly a half century after, showed that the true nature of the
appendage was a ring. Spots on the sun were frequently observed by
Galileo and led to bitter controversies. He proved, however, that they
were objects on the sun itself, not outside it, and by noticing their
repeated transits across the sun's disk, he showed that the sun turned
round on his axis in a little less than a month--another analogy to the
like motion of the earth on the Copernican plan.
Galileo's appointment in 1610 as "First Philosopher and Mathematician"
to the Grand Duke of Tuscany gave him abundant time for the pursuit of
original investigations and the preparation of books and pamphlets. His
first visit to Rome the year following was the occasion of a reception
with great honor by many cardinals and others of high rank. His lack of
sympathy with others whose views differed from his, and his naturally
controversial spirit, had begun to lead him headlong into controversies
with the Jesuits and the church, which culminated in his censure by the
authorities of the church and persecution by the Inquisition.
In 1618 three comets appeared, and Galileo was again in controversial
hot water with the Jesuits. But it led to the publication five years
later of "Il Saggiatore" (The Assayer), of no great scientific value,
but only a brilliant bit of controversial literature dedicated to the
newly elevated Pope, Urban VIII. Later he wrote through several years a
great treatise, more or less controversial in character, entitled a
"Dialogue on the Two Chief Systems of the World" between three speakers,
and extending through four successive days. Simplicio argues for the
Aristotelians, Salviati for the Copernicans, while Sagredo does his best
to be neutral. It will always be a very readable book, and we are
fortunate to have a recent translation by Professor Crew of Evanston.
Here we find the first suggestion of the modern method of getting
stellar parallaxes, the relative parallax, that is, of two stars in the
same field--a method not put into service till Bessel's time, two
centuries later. But the most important chapters of the "Dialogue" deal
with Galileo's investigations of the laws of motion of bodies in
general, which he applied to the problem of the earth's motion. In this
he really anticipated Newton in the first of his three laws of motion,
and in a subsequent work, dealing with the theory of projectiles, he
reaches substantially the results of Newton's second law of motion,
although he gave no general statement of the principle. Nevertheless, in
the epoch where his life was lived and his work done, his telescopic
discoveries, combined with his dynamic researches in untrodden fields,
resulted in the complete and final overthrow of the ancient system of
error, and the secure establishment of the Copernican system beyond
further question and discussion. Only then could the science of
astronomy proceed unhampered to the fullest development by the master
minds of succeeding centuries.
CHAPTER XII
AFTER THE GREAT MASTERS
Following Kepler and Galileo was a half century of great astronomical
progress along many lines laid out by the work of the great masters. The
telescope seemed only a toy, but its improvement in size and quality
showed almost inconceivable possibilities of celestial discoveries.
Hevelius of Danzig took up the study of the moon, and his
"Selenographia" was finely illustrated by plates which he not only drew
but engraved himself. Lunar names of mountains, plains, and craters we
owe very largely to him. Also he published among other works two on
comets, the second of which was published in 1668 and called the
"Cometographia," the first detailed account of all the comets observed
and recorded to date.
Many were the telescopes turned on the planet Saturn, and every variety
of guess was made as to the actual shape and physical nature of the
weird appendages discovered by Galileo. The true solution was finally
reached by Huygens, whose mechanical genius had enabled him to grind and
polish larger and better lenses than his contemporaries; in 1659 he
published the "Systema Saturnium" interpreting the ring and the cause of
its various configurations, and the first discovery of a Saturnian
satellite is due to him.
Gascoigne in England about 1640 was the first to make the important
application of the micrometer to enhance the accuracy of measurement of
small angles in the telescopic field; an invention made and applied
independently many years later by Huygens in Holland and Auzout and
Picard in France, where the instrument was first regularly employed as
an accessory in the work of an observatory.
Another Englishman, Jeremiah Horrocks, was the first observer of a
transit of Venus over the disk of the sun, in 1639. Horrocks was
possessed of great ability in calculational astronomy also. This was
about the time of the invention of the pendulum clock by Huygens, which
in conjunction with the later invention of the transit instrument by
Roemer wrought a revolution in the exacting art of practical astronomy.
This was because it enabled the time to be carried along continuously,
and the revolution of the earth could be utilized in making precise
measures of the position of sun, moon, and stars. Louis XIV had just
founded the new Observatory at Paris in 1668, and Picard was the first
to establish regular time-observations there.
Huygens followed up the motion of the pendulum in theory as well as
practice in his "Horologium Oscillatorium" (1673), showing the way to
measure the force of gravity, and his study of circular motion showed
the fundamental necessity of some force directed toward the center in
planetary motions.
The doctrine of the sphericity of the earth being no longer in doubt,
the great advance in accuracy of astronomical observation indicated to
Willebrord Snell in Holland the best way to measure an arc of meridian
by triangulation. Picard repeated the measurements near Paris with even
greater accuracy, and his results were of the utmost significance to
Newton in establishing his law of gravitation.
Domenico Cassini, an industrious observer, voluminous writer, and a
strong personality, devised telescopes of great size, discovered four
Saturnian satellites and the main division in the ring of Saturn,
determined the rotation periods of Mars and Jupiter, and prepared tables
of the eclipses of Jupiter's satellites. At his suggestion Richer
undertook an expedition to Cayenne in latitude 5 degrees north, where it
was found that the intensity of gravity was less than at Paris, and his
clock therefore lost time, thus indicating that the earth was not a
perfect sphere as had been thought, but a spheroid instead.
The planet Mars passed a near opposition, and Richer's observations of
it from Cayenne, when combined with those of Cassini and others in
France, gave a new value of the sun's parallax and distance, really the
first actual measurement worth the name in the history of astronomy.
To close this era of signal advance in astronomy we may cite a discovery
by Roemer of the first order: no less than that of the velocity of
transmission of light through space. At the instigation of Picard,
Roemer in studying the motions of Jupiter's satellites found that the
intervals between eclipses grew less and less as Jupiter and the earth
approached each other, and greater and greater than the average as the
two planets separated farther and farther. Roemer correctly attributed
this difference to the progressive motion of light and a rough value of
its velocity was calculated, though not accepted by astronomers
generally for more than a century.
Why the laws of Kepler should be true, Kepler himself was unable to say.
Nor could anyone else in that day answer these questions: (1) The
planets move in orbits that are elliptical not circular--why should they
move in an imperfect curve, rather than the perfect one in which it had
always been taught that they moved? (2) Why should our planet vary its
velocity at all, and travel now fast, now slow; especially why should
the speed so vary that the line of varying length, joining the planet to
the sun, always passes over areas proportional to the time of describing
them? And (3) Why should there be any definite relation of the distances
of planets from the sun to their times of revolution about him? Why
should it be exactly as the cube of one to the square of the other?
We must remember that the Copernican system itself was not yet, in the
beginning of the seventeenth century, accepted universally; and the
great minds of that period were most concerned in overturning the
erroneous theory of Ptolemy.
The next step in logical order was to find a basic explanation of the
planetary motions, and Descartes and his theory of vortices are worthy
of mention, among many unsuccessful attempts in this direction.
Descartes was a brilliant French philosopher and mathematician, but his
hypothesis of a multitude of whirlpools in the ether, while ingenious in
theory, was too vague and indefinite to account for the planetary
motions with any approach to the precision with which the laws of Kepler
represented them.
Another great astronomer whose labors helped immensely in preparing the
way for the signal discoveries that were soon to come was Huygens, a man
of versatility as natural philosopher, mechanician, and astronomical
observer. Huygens was born thirteen years before the death of Galileo,
and to the discovery of the laws of motion by the latter Huygens added
researches on the laws of action of centrifugal forces. Neither of them,
however, appeared to see the immediate bearing on the great general
problem of celestial motions in its true light, and it was reserved for
another generation, and an astronomer of another country, to make the
one fundamental discovery that should explain the whole by a single
simple law.
CHAPTER XIII
NEWTON AND MOTION
"How is it that you are able to make these great discoveries?" was once
asked of Sir Isaac Newton, _facile princeps_ of all philosophers, and
the discoverer of the great law of universal gravitation.
"By perpetually thinking about them," was Newton's terse and
illuminating reply. He had set for himself the definite problem of
Kepler's laws: why is it that they are true, and is there not some
single, general law that will embody all the circumstances of the
planetary motions?
Newton was born in 1643, the year after the death of Galileo. He had a
thorough training in the mathematics of his day, and addressed himself
first to an investigation and definite formulation of the general laws
of motion, which he found to be three in number, and which he was able
to put in very simple terms. The first one is: Any body, once it is set
in motion, will continue to move forward in a straight line with a
uniform velocity forever, provided it is acted upon by no force
whatever. In other words, a state of motion is as natural as a state of
rest (rest in relation to things everywhere adjacent) in which we find
all things in general.
Here on earth where gravity itself pulls all objects downward toward the
earth, and where resistance of the air tends to hold a moving body back
and bring it to rest, and where friction from contact with whatever
material substance may be in its path is perpetually tending to
neutralize all motion--with all three of these forces always at work to
stop a moving body, the truth of this first and fundamental law of
motion was not apparent on the surface.
Till Galileo's time everyone had made the mistake of supposing that some
force or other must be acting continually on every moving body to keep
it in motion. Ptolemy, Copernicus, Kepler, Leonardo da Vinci--all failed
to see the truth of this law which Newton developed in the immortal
_Principia_. And at the present day it is not always easy to accept at
first, although the progress of mechanical science, by reducing friction
and resistance, has produced machines in which motion of large masses
may be kept up indefinitely with the application of only the merest
minimum of force.
Once a planet is set in motion round the sun, it would go on forever
through frictionless, non-resistant space; but there must be a central
force, as Huygens saw clearly, to hold it in its orbit. Otherwise it
would at any moment take the direction of a tangent to the orbit. Here
is where Newton's second law of motion comes in, and he formulated it
with great definiteness. When any force acts on a moving body, its
deviation from a straight line will be in the direction of the force
applied and proportional to that force.
In accord with this law, Newton first began to inquire whether the force
of attraction here on earth, which everyone commonly recognizes as
gravity, drawing all things down toward the center of the earth, might
not extend upward indefinitely. It is found in operation on the summits
of mountain peaks, and the clouds above them and the rain falling from
them are obviously drawn downward by the same force. May it not extend
outward into space, even as far as the moon?
This was an audacious question, but Newton not only asked, but tried to
answer it in the year 1665, when he was only twenty-three. On the
surface of the earth this attraction is strong enough to draw a falling
body downward through a vertical space of sixteen feet in a second of
time. What ought it to be at the distance of the moon. The distance of
the moon in Newton's time was better known in terms of the earth's size
than was the size of the earth itself: the earth's radius was known to
be one-sixtieth of the moon's distance, but the earth's diameter was
thought to be something under 7,000 miles, so that Newton's first
calculations were most disappointing, and he laid them aside for nearly
twenty years.
Meanwhile the French astronomers led by Picard had measured the earth
anew, and showed it to be nearly 8,000 miles in diameter. As soon as
Newton learned of this, he revised his calculations, and found that by
the law of the inverse square the moon, in one second, should fall away
from a tangent to its orbit one thirty-six hundredth of sixteen feet.
This accorded exactly with his original supposition that the earth's
attraction extended to the moon. So he concluded that the force which
makes a stone fall, or an apple, as the story goes, is the same force
that holds the moon in its orbit, and that this force diminishes in the
exact proportion that the square of the distance from the earth's center
increases. The moon, indeed, becomes a falling body; only, as Kingdon
Clifford puts it: "She is going so fast and is so far off that she falls
quite around to the other side of the earth, instead of hitting it; and
so goes on forever."
[Illustration: NICHOLAS COPERNICUS]
[Illustration: GALILEO GALILEI]
[Illustration: JOHANN KEPLER]
[Illustration: SIR ISAAC NEWTON]
Newton goes on in the _Principia_ to explain the extension of
gravitation to the other bodies of the solar system beyond the earth and
moon. Clearly the same gravitation that holds the moon in its orbit
round the earth, must extend outward from the sun also, and hold all the
planets in their orbits centered about him. Newton demonstrates by
calculation based on Kepler's third law that (1) the forces drawing the
planets toward the sun are inversely as the squares of their mean
distances from him; and (2) if the force be constantly directed toward
the sun, the radius vector in an elliptic orbit must pass over equal
areas in equal times.
CHAPTER XIV
NEWTON AND GRAVITATION
So all of Kepler's laws could be embodied in a single law of gravitation
toward a central body, whose force of attraction decreases outward in
exact proportion as the square of the distance increases.
Only one farther step had to be taken, and this the most complicated of
all: he must make all the bodies of the sky conform to his third law of
motion. This is: Action and reaction are equal, or the mutual actions of
any two bodies are always equal and oppositely directed. There must be
mutual attractions everywhere: earth for sun as well as sun for earth,
moon for sun and sun for moon, earth for Venus and Venus for earth,
Jupiter for Saturn and Saturn for Jupiter, and so on.
The motions of the planets in the undisturbed ellipses of Kepler must be
impossible. As observations of the planets became more accurate, it was
found that they really did fail to move in exact accord with Kepler's
laws unmodified. Newton was unable, with the imperfect processes of the
mathematics of his day to ascertain whether the deviations then known
could be accounted for by his law of gravitation; but he nevertheless
formulated the law with entire precision, as follows:
Every particle of matter in the universe attracts every other particle
with a force exactly proportioned to the product of their masses, and
inversely as the square of the distance between their centers.
The centuries of astronomical research since Newton's day, however, have
verified the great law with the utmost exactness. Practically every
irregularity of lunar and planetary motion is accounted for; indeed, the
intricacies of the problems involved, and the nicety of their solution,
have led to the invention of new mathematical processes adequate to the
difficulties encountered.
And about the middle of the last century, when Uranus departed from the
path laid out for it by the mathematical astronomers, its orbital
deviations were made the basis of an investigation which soon led to the
assignment of the position where a great planet could be found that
would account for the unexplained irregularities of the motion of
Uranus. And the immediate discovery of this planet, Neptune, became the
most striking verification of the Newtonian law that the solar system
could possibly afford.
The astronomers of still later days investigating the statelier motions
of stellar systems find the Newtonian law regnant everywhere among the
stars where our most powerful telescopes have as yet reached. So that
Newton's law is known as the law of Universal Gravitation, and its
author is everywhere held as the greatest scientist of the ages.
Newton's _Principia_ may be regarded as the culminating research of the
inductive method, and further outline of its contents is desirable. It
is divided into three books following certain introductory sections. The
first book treats of the problems of moving bodies, the solutions being
worked out generally and not with special reference to astronomy. The
second book deals with the motion of bodies through resistant media, as
fluids, and has very little significance in astronomy. The third book is
the all important one, and applies his general principles to the case of
the actual solar system, providing a full explanation of the motions of
all the bodies of the system known in his day. Anyone who critically
reads the _Principia_ of Newton will be forced to conclude that its
author was a genius in the highest sense of the word. The elegance and
thoroughness of the demonstrations, and the completeness of application
of the law of gravitation are especially impressive.
The universality of his new law was the feature to which he gave
particular attention. It was clear to him that the gravitation of a
planet, although it acted as if wholly concentrated at the center, was
nevertheless resident in every one of the particles of which the planet
is composed. Indeed, his universal law was so formulated as to make
every particle attract every other particle; and an investigation known
as the Cavendish experiment--a research of great delicacy of
manipulation--not only proves this, but leads also to a measurement of
the earth's mean density, from which we can calculate approximately how
much the earth actually weighs.
Another way to attack the same problem is by measuring the attraction of
mountains, as Maskelyne, Astronomer Royal of Scotland did on Mount
Schehallien in Scotland, which was selected because of its sheer
isolation. The attraction of the mountain deflected the plumb-lines by
measurable amounts, the volume of the mountain was carefully
ascertained by surveys, and geologists found out what rocks composed it.
So the weight of the entire mountain became pretty well known, and
combining this with the observed deflection, an independent value of the
earth's weight was found.
Still other methods have been applied to this question, and as an
average it is found that the materials composing the earth are about
five and a half times as heavy as water, and the total weight of the
earth is something like six sextillions of tons.
What is the true shape of the earth? And does the earth's turning round
on its axis affect this shape? Newton saw the answer to these questions
in his law of gravitation. A spherical figure followed as a matter of
course from the mutual attraction of all materials composing the earth,
providing it was at rest, or did not turn round on its axis. But
rotation bulges it at the equator and draws it in at the poles, by an
amount which calculation shows to be in exact agreement with the amount
ascertained by actual measurement of the earth itself.
Another curious effect, not at first apparent, was that all bodies
carried from high latitudes toward the equator would get lighter and
lighter, in consequence of the centrifugal force of rotation. This was
unexpectedly demonstrated by Richer when the French Academy sent him
south to observe Mars in 1672. His clock had been regulated exactly in
Paris, and he soon found that it lost time when set up at Cayenne. The
amount of loss was found by observation, and it was exactly equal to the
calculated effect that the reduction of gravity by centrifugal action
should produce.
Also Newton saw that his law of gravitation would afford an explanation
of the rise and fall of the tides. The water on the side of the earth
toward the moon, being nearer to the moon, would be more strongly
attracted toward it, and therefore raised in a tide. And the water on
the farther side of the earth away from the moon, being at a greater
distance than the earth itself, the moon would attract the earth more
strongly than this mass of water, tending therefore to draw the earth
away from the water, and so raising at the same time a high tide on the
side of the earth away from the moon. As the earth turns round on its
axis, therefore, two tidal waves continually follow each other at
intervals of about twelve hours.
The sun, too, joins its gravitating force with that of the moon, raising
tides nearly half as high as those which the moon produces, because the
sun's vaster mass makes up in large part for its much greater distance.
At first and third quarters of the moon, the sun acts against the moon,
and the difference of their tide-producing forces gives us "neap tides";
while at new moon and full, sun and moon act together, and produce the
maximum effect known as "spring tides."
Newton passed on to explain, by the action of gravitation also, the
precession of the equinoxes, a phenomenon of the sky discovered by
Hipparchus, who pretty well ascertained its amount, although no reason
for it had ever been assigned. The plane of the earth's equator extended
to the celestial sphere marks out the celestial equator, and the two
opposite points where it intersects the plane of the ecliptic, or the
earth's path round the sun, are called the equinoctial points, or simply
the equinoxes. And precession of the equinoxes is the motion of these
points westward or backward, about 50 seconds each year, so that a
complete revolution round the ecliptic would take place in about 26,000
years.
Newton saw clearly how to explain this: it is simply due to the
attraction of the sun's gravitation upon the protuberant bulge around
the earth's equator, acting in conjunction with the earth's rotation on
its axis, the effect being very similar to that often seen in a spinning
top, or in a gyroscope. The moon moving near the ecliptic produces a
precessional effect, as also do the planets to a very slight degree; and
the observed value of precession is the same as that calculated from
gravitation, to a high degree of precision.
Newton died in 1727, too early to have witnessed that complete and
triumphant verification of his law which ultimately has accounted for
practically every inequality in the planetary motions caused by their
mutual attractions. The problems involved are far beyond the complexity
of those which the mathematical astronomer has to deal with, and the
mathematicians of France deserve the highest credit for improving the
processes of their science so that obstacles which appeared insuperable
were one after another overcome.
Newton's method of dealing with these problems was mainly geometric, and
the insufficiency of this method was apparent. Only when the French
mathematicians began to apply the higher methods of algebra was progress
toward the ultimate goal assured. D'Alembert and Clairaut for a time
were foremost in these researches, but their places were soon taken by
Lagrange, who wrote the "Mecanique Analytique," and Laplace, whose
"Mecanique Celeste" is the most celebrated work of all. In large part
these works are the basis of the researches of subsequent mathematical
astronomers who, strictly speaking, cannot as yet be said to have
arrived at a complete and rigorous solution of all the problems which
the mutual attractions of all the bodies of the solar system have
originated.
It may well be that even the mathematics of the present day are
incompetent to this purpose. When the brilliant genius of Sir William
Hamilton invented quaternion analysis and showed the marvelous facility
with which it solved the intricate problems of physics, there was the
expectation that its application to the higher problems of mathematical
astronomy might effect still greater advances; but nothing in that
direction has so far eventuated. Some astronomers look for the invention
of new functions with numerical tables bearing perhaps somewhat the
relation to present tables of logarithms, sines, tangents, and so on,
that these tables do to the simple multiplication table of Pythagoras.
CHAPTER XV
AFTER NEWTON
We have said that practically all the motions in the solar system have
been accounted for by the Newtonian law of gravitation. It will be of
interest to inquire into the instances that lead to qualification of
this absolute statement.
One relates to the planet Mercury, whose orbit or path round the sun is
the most elliptical of all the planetary orbits. This will be explained
a little later.
The moon has given the mathematical astronomers more trouble than any
other of the celestial bodies, for one reason because it is nearest to
us and very minute deviations in its motion are therefore detectible.
Halley it was who ascertained two centuries ago that the moon's motion
round the earth was not uniform, but subject to a slight acceleration
which greatly puzzled Lagrange and Laplace, because they had proved
exactly this sort of thing to be impossible, unless indeed the body in
question should be acted on by some other force than gravitation. But
Laplace finally traced the cause to the secular or very slow reduction
in the eccentricity of the earth's own orbit. The sun's action on the
moon was indeed progressively changing from century to century in such
manner as to accelerate the moon's own motion in its orbit round the
earth.
Adams, the eminent English astronomer, revised the calculations of
Laplace, and found the effect in question only half as great as Laplace
had done; and for years a great mathematical battle was on between the
greatest of astronomical experts in this field of research. Adams, in
conjunction with Delaunay, the greatest of the French mathematicians a
half century ago, won the battle in so far as the mathematical
calculations were concerned; but the moon continues to the present day
her slight and perplexing deviation, as if perhaps our standard
time-keeper, the earth, by its rotation round its axis, were itself
subject to variation. Although many investigations have been made of the
uniformity of the earth's rotation, no such irregularity has been
detected, and this unexplained variation of the moon's motion is one of
the unsolved problems of the gravitational astronomer of to-day.
But we are passing over the most impressive of all the earlier
researches of Lagrange and Laplace, which concerned the exceedingly slow
changes, technically called the secular variations of the elements of
the planetary orbits. These elements are geometrical relations which
indicate the form of the orbit, the size of the orbit, and its position
in space; and it was found that none of these relations or quantities
are constant in amount or direction, but that all, with but one
exception, are subject to very slow, or secular, change, or oscillation.
This question assumed an alarming significance at an early day,
particularly as it affected the eccentricity of the earth's orbit round
the sun. Should it be possible for this element to go on increasing for
indefinite ages, clearly the earth's orbit would become more and more
elliptical, and the sun would come nearer and nearer at perihelion, and
the earth would drift farther and farther from the sun at aphelion,
until the extremes of temperature would bring all forms of life on the
earth to an end. The refined and powerful analysis of Lagrange, however,
soon allayed the fears of humanity by accounting for these slow
progressive changes as merely part of the regular system of mere
oscillations, in entire accord with the operation of the law of
gravitation; and extending throughout the entire planetary system.
Indeed, the periods of these oscillations were so vast that none of them
were shorter than 50,000 years, while they ranged up to two million
years in length--"great clocks of eternity which beat ages as ours beat
seconds."
About a century ago, an eminent lecturer on astronomy told his audience
that the problem of weighing the planets might readily be one that would
seem wholly impossible to solve. To measure their sizes and distances
might well be done, but actually to ascertain how many tons they
weigh--never!
Yet if a planet is fortunate enough to have one satellite or more, the
astronomer's method of weighing the planet is exceedingly simple; and
all the major planets have satellites except the two interior ones,
Mercury and Venus. As the satellite travels round its primary, just as
the moon does round the earth, two elements of its orbit need to be
ascertained, and only two. First, the mean distance of the satellite
from its primary, and second the time of revolution round it.
Now it is simply a case of applying Kepler's third law. First take the
cube of the satellite's distance and divide it by the square of the
time of revolution. Similarly take the cube of the planet's distance
from the sun and divide by the square of the planet's time of revolution
round him. The proportion, then, of the first quotient to the second
shows the relation of the mass (that is the weight) of the planet to
that of the sun. In the case of Jupiter, we should find it to be 1,050,
in that of Saturn 3,500, and so on.
The range of planetary masses, in fact, is very curious, and is
doubtless of much significance in the cosmogony, with which we deal
later. If we consider the sun and his eight planets, the mass or weight
of each of the nine bodies far exceeds the combined mass of all the
others which are lighter than itself.
To illustrate: suppose we take as our unit of weight the one-billionth
part of the sun's weight; then the planets in the order of their masses
will be Mercury, Mars, Venus, Earth, Uranus, Neptune, Saturn, and
Jupiter. According to their relative masses, then, Mercury being a
five-millionth part the weight of the sun will be represented by 200;
similarly Venus, a four hundred and twenty-five thousandth part by
2,350, and so on. Then we have
Mercury 200
Mars 340
------
Sum of weights of Mercury and Mars 540
Venus 2,350
------
Sum of weights of Mercury, Mars, and Venus 2,890
The Earth 3,060
------
Sum of weights of four inner planets 5,950
Uranus 44,250
------
Sum of weights of five planets 50,200
Neptune 51,600
-------
Sum of weights of six planets 101,800
Saturn 285,580
---------
Sum of weights of seven planets 387,380
Jupiter 954,300
---------
Sum of weights of all the planets 1,341,680
Mass or weight of the sun 1,000,000,000
Curious and interesting it is that Saturn is nearly three times as heavy
as the six lighter planets taken together, Jupiter between two and three
times heavier than all the other planets combined, while the sun's mass
is 750 times that of all the great planets of his system rolled into
one.
All the foregoing masses, except those of Mercury and Venus, are pretty
accurately known because they were found by the satellite method just
indicated. Mercury's mass is found by its disturbing effects on Encke's
comet whenever it approaches very near. The mass of Venus is ascertained
by the perturbations in the orbital motion of the earth. In such cases
the Newtonian law of gravitation forms the basis of the intricate and
tedious calculations necessary to find out the mass by this indirect
method.
Its inferiority to the satellite method was strikingly shown at the
Observatory in Washington soon after the satellites of Mars were
discovered in 1877. The inaccurate mass of that planet, as previously
known by months of computation based upon years and years of
observation, was immediately discarded in favor of the new mass derived
from the distance and period of the outer satellite by only a few
minutes' calculation.
In weighing the planets, astronomers always use the sun as the unit.
What then is the sun's own weight? Obviously the law of gravitation
answers this question, if we compare the sun's attraction with the
earth's at equal distances. First we conceive of the sun's mass as if
all compressed into a globe the size of the earth, and calculate how far
a body at the surface of this globe would fall in one second. The
relation of this number to 16.1 feet, the distance a body falls in one
second on the actual earth, is about 330,000, which is therefore the
number of times the sun's weight exceeds that of the earth.
A word may be added regarding the force of gravitation and what it
really is. As a matter of fact Newton did not concern himself in the
least with this inquiry, and says so very definitely. What he did was to
discover the law according to which gravitation acts everywhere
throughout the solar system. And although many physicists have
endeavored to find out what gravitation really is, its cause is not yet
known. In some manner as yet mysterious it acts instantaneously over
distances great and small alike, and no substance has been found which,
if we interpose it between two bodies, has in any degree the effect of
interrupting their gravitational tendency toward each other.
While the Newtonian law of gravitation has been accepted as true because
it explained and accounted for all the motions of the heavenly bodies,
even including such motions of the stars as have been subjected to
observation, astronomers have for a long time recognized that quite
possibly the law might not be absolutely exact in a mathematical sense,
and that deviations from it would surely make their appearance in time.
A crude instance of this was suggested about a century ago, when the
planet Uranus was found to be deviating from the path marked out for it
by Bouvard's tables based on the Newtonian law; and the theory was
advocated by many astronomers that this law, while operant at the medium
distances from the sun where the planets within Jupiter and Saturn
travel, could not be expected to hold absolutely true at the vast
distance of Uranus and beyond. The discovery of Neptune in 1846,
however, put an end to all such speculation, and has universally been
regarded as an extraordinary verification of the law, as indeed it is.
When, however, Le Verrier investigated the orbit of Mercury he found an
excess of motion in the perihelion point of the planet's orbit which
neither he nor subsequent investigators have been able to account for by
Newtonian gravitation, pure and simple. If Newton's theory is absolutely
true, the excess motion of Mercury's perihelion remains a mystery.
Only one theory has been advanced to account for this discrepancy, and
that is the Einstein theory of gravitation. This ingenious speculation
was first propounded in comprehensive form nearly fifteen years ago, and
its author has developed from it mathematical formulae which appear to
yield results even more precise than those based on the Newtonian
theory.
In expressing the difference between the law of gravitation and his own
conception, Einstein says: "Imagine the earth removed, and in its place
suspended a box as big as a moon or a whole house and inside a man
naturally floating in the center, there being no force whatever pulling
him. Imagine, further, this box being, by a rope or other contrivance,
suddenly jerked to one side, which is scientifically termed 'difform
motion,' as opposed to 'uniform motion.' The person would then naturally
reach bottom on the opposite side. The result would consequently be the
same as if he obeyed Newton's law of gravitation, while, in fact, there
is no gravitation exerted whatever, which proves that difform motion
will in every case produce the same effects as gravitation.... The term
relativity refers to time and space. According to Galileo and Newton,
time and space were absolute entities, and the moving systems of the
universe were dependent on this absolute time and space. On this
conception was built the science of mechanics. The resulting formulas
sufficed for all motions of a slow nature; it was found, however, that
they would not conform to the rapid motions apparent in
electrodynamics.... Briefly the theory of special relativity discards
absolute time and space, and makes them in every instance relative to
moving systems. By this theory all phenomena in electrodynamics, as well
as mechanics, hitherto irreducible by the old formulae, were
satisfactorily explained."
Natural phenomena, then, involving gravitation and inertia, as in the
planetary motions, and electro-magnetic phenomena, including the motion
of light, are to be regarded as interrelated, and not independent of one
another. And the Einstein theory would appear to have received a
striking verification in both these fields. On this theory the
Newtonian dynamics fails when the velocities concerned are a near
approach to that of light. The Newtonian theory, then, is not to be
considered as wrong, but in the light of a first approximation. Applying
the new theory to the case of the motion of Mercury's perihelion, it is
found to account for the excess quite exactly.
On the electro-magnetic side, including also the motion of light, a
total eclipse of the sun affords an especially favorable occasion for
applying the critical test, whether a huge mass like the sun would or
would not deflect toward itself the rays of light from stars passing
close to the edge of its disk, or limb. A total eclipse of exceptional
duration occurred on May 29, 1919, and the two eclipse parties sent out
by the Royal Society of London and the Royal Astronomical Society were
equipped especially with apparatus for making this test. Their stations
were one on the east coast of Brazil and the other on the west coast of
Africa.
Accurate calculation beforehand showed just where the sun would be among
the stars at the time of the eclipse; so that star plates of this region
were taken in England before the expeditions went out. Then, during the
total eclipse, the same regions were photographed with the eclipsed sun
and the corona projected against them. To make doubly sure, the stars
were a third time photographed some weeks after the eclipse, when the
sun had moved away from that particular region.
Measuring up the three sets of plates, it was found that an appreciable
deflection of the light of the stars nearest alongside the sun actually
exists; and the amount of it is such as to afford a fair though not
absolutely exact verification of the theory. The observed deflection
may of course be due to other causes, but the English astronomers
generally regard the near verification as a triumph for the Einstein
theory. Astronomers are already beginning preparations for a repetition
of the eclipse programme with all possible refinement of observation,
when the next total eclipse of the sun occurs, September 20, 1922,
visible in Australia and the islands of the Indian Ocean.
A third test of the theory is perhaps more critical than either of the
others, and this necessitates a displacement of spectral lines in a
gravitational field toward the red end of the spectrum; but the experts
who have so far made measures for detecting such displacement disagree
as to its actual existence. The work of St. John at Mt. Wilson is
unfavorable to the theory, as is that of Evershed of Kodiakanal, who has
made repeated tests on the spectrum of Venus, as well as in the cyanogen
bands of the sun.
The enthusiastic advocates of the Einstein theory hold that, as Newton
proved the three laws of Kepler to be special cases of his general law,
so the "universal relativity theory" will enable eventually the
Newtonian law to be deduced from the Einstein theory. "This is the way
we go on in science, as in everything else," wrote Sir George Airy,
Astronomer Royal; "we have to make out that something is true; then we
find out under certain circumstances that it is not quite true; and then
we have to consider and find out how the departure can be explained."
Meanwhile, the prudent person keeps the open mind.
CHAPTER XVI
HALLEY AND HIS COMET
Halley is one of the most picturesque characters in all astronomical
history. Next to Newton himself he was most intimately concerned in
giving the Newtonian law to the world.
Edmund Halley was born (1656) in stirring times. Charles I. had just
been executed, and it was the era of Cromwell's Lord Protectorate and
the wars with Spain and Holland. Then followed (1660) the promising but
profligate Charles II. (who nevertheless founded at Greenwich the
greatest of all observatories when Halley was nineteen), the frightful
ravages of the Black Plague, the tyrannies of James II., and the
Revolution of 1688--all in the early manhood of Halley, whose scientific
life and works marched with much of the vigor of the contending
personalities of state.
The telescope had been invented a half century earlier, and Galileo's
discoveries of Jupiter's moons and the phases of Venus had firmly
established the sun-centered theory of Copernicus.
The sun's distance, though, was known but crudely; and why the stars
seemed to have no yearly orbits of their own corresponding to that of
the earth was a puzzle. Newton was well advanced toward his supreme
discovery of the law of universal gravitation; and the authority of
Kepler taught that comets travel helter-skelter through space in
straight lines past the earth, a perpetual menace to humanity.
"Ugly monsters," that comets always were to the ancient world, the
medieval church perpetuated this misconception so vigorously that even
now these harmless, gauzy visitors from interstellar space possess a
certain "wizard hold upon our imagination." This entertaining phase of
the subject is excellently treated in President Andrew D. White's
"History of the Doctrine of Comets," in the Papers of the American
Historical Association. Halley's brilliant comet at its earlier
apparitions had been no exception.
Halley's father was a wealthy London soap maker, who took great pride in
the growing intellectuality of his son. Graduating at Queen's College,
Oxford, the latter began his astronomical labors at twenty by publishing
a work on planetary orbits; and the next year he voyaged to St. Helena
to catalogue the stars of the southern firmament, to measure the force
of terrestrial gravity, and observe a transit of Mercury over the disk
of the sun.
While clouds seriously interfered with his observations on that lonely
isle, what he saw of the transit led to his invention of "Halley's
method," which, as applied to the transit of Venus, though not till long
after his death, helped greatly in the accurate determination of the
sun's distance from the earth. Halley's researches on the proper motions
of the stars of both hemispheres soon made him famous, and it was said
of him, "If any star gets displaced on the globe, Halley will presently
find it out."
His return to London and election to the Royal Society (of which he was
many years secretary) added much to his fame, and he was commissioned
by the society to visit Danzig and arbitrate an astronomical controversy
between Hooke and Hevelius, both his seniors by a generation.
On the continent he associated with other great astronomers, especially
Cassini, who had already found three Saturnian moons; and it was then he
observed the great comet of 1680, which led up to the most famous event
of Halley's life.
The seerlike Seneca may almost be said to have predicted the advent of
Halley, when he wrote ("Quaestiones Naturales," vii): "Some day there
will arise a man who will demonstrate in what region of the heavens
comets pursue their way; why they travel apart from the planets; and
what their sizes and constitution are. Then posterity will be amazed
that simple things of this sort were not explained before."
To Newton it appeared probable that cometary voyagers through space
might have orbits of their own; and he proved that the comet of 1680
never swerved from such a path. As it could nowhere approach within the
moon's orbit, clearly threats of its wrecking the earth and punishing
its inhabitants ought to frighten no more.
Halley then became intensely interested in comets, and gathered whatever
data concerning the paths of all these bodies he could find. His first
great discovery was that the comets seen in 1531 by Apian, and in 1607
by Kepler, traveled round the sun in identical paths with one he had
himself observed in 1682. A still earlier appearance of Halley's comet
(1456) seems to have given rise to a popular and long-reiterated myth of
a papal bull excommunicating "the Devil, the Turk, and the Comet."
No longer room for doubt: so certain was Halley that all three were one
and the same comet, completing the round of its orbit in about
seventy-six years, that he fearlessly predicted that it would be seen
again in 1758 or 1759. And with equal confidence he might have foretold
its return in 1835 and 1910; for all three predictions have come true to
the letter.
Halley's span of existence did not permit his living to see even the
first of these now historic verifications. But we in our day may
emphatically term the epoch of the third verified return _Annus
Halleianus_.
Says Turner, Halley's successor in the Savilian chair at Oxford to-day:
"There can be no more complete or more sensational proof of a scientific
law, than to predict events by means of it. Halley was deservedly the
first to perform this great service for Newton's Law of Gravitation, and
he would have rejoiced to think how conspicuous a part England was to
play in the subsequent prediction of the existence of Neptune."
Halley rose rapidly among the chief astronomical figures of his day. But
he had little veneration for mere authority, and the significant veering
of his religious views toward heterodoxy was for years an obstacle to
his advance.
Still Halley the astronomer was great enough to question any
contemporary dicta that seemed to rest on authority alone. Everyone
called the stars "fixed" stars; but Halley doubting this, made the first
discovery of a star's individual motion--proper motion, as astronomers
say. To-day, two hundred years after, every star is considered to be in
motion, and astronomers are ascertaining their real motions in the
celestial spaces to a nicety undreamed of by even the exacting Halley.
The moon, of priceless service to the early navigator, was regarded by
all astronomers as endowed with an average rate of motion round the
earth that did not vary from age to age. But Halley questioned this too;
and on comparing with the ancient value from Chaldean eclipses, he made
another discovery--the secular acceleration of the moon's mean motion,
as it is technically termed. This was a colossal discovery in celestial
dynamics; and the reason underlying it lay hidden in Newton's law for
yet another century, till the keener mathematics of Laplace detected its
true origin.
* * * * *
With Newton, Halley laid down the firm foundations of celestial
mechanics, and they pushed the science as far as the mathematics of
their day would permit. Halley, however, was not content with
elucidating the motion of bodies nearest the earth, and pressed to the
utmost confines of the solar system known to him. Here, too, he made a
signal discovery of that mutual disturbance of the planets in their
motion round the sun, called the great inequality of Jupiter and Saturn.
Halley's versatile genius attacked all the great problems of the day.
His observation of the sun's total eclipse in 1715 is the earliest
reliable account of such a phenomenon by a trained astronomer. He
described the corona minutely and was the first to see that other
interesting phenomenon which only an alert observer can detect, which a
great astronomer of a later day compared to the "ignition of a fine
train of gunpowder," and which has ever since borne the name of "Baily's
beads."
Besides being a great astronomer, Halley was a man of affairs as well,
which Newton, although the greater mathematician, was not. Without
Halley, Newton's superb discovery might easily have been lost to the age
and nation, for the latter was bent merely on making discoveries, and on
speculative contemplation of them, with never a thought of publishing to
the world.
Halley, more practical and businesslike, insisted on careful writing out
and publication. Newton was then only forty-two, and Halley fully
fourteen years his junior. But the philosophers of that day were keenly
alive to the mystery of Kepler's laws, and Halley was fully conscious of
the grandeur and far-reaching significance of Newton's great
generalization which embodied all three of Kepler's laws in one.
Newton at last yielded, though reluctantly, and the "Principia" was
given to the world, though wholly at Halley's private charges.
But Halley was far from being completely engrossed with the absorbing
problems of the sky; things terrestrial held for years his undivided
attention. Imagine present-day Lords Commissioners of the Admiralty
intrusting a ship of the British navy to civilian command. Yet such was
their confidence in Halley that he was commissioned as captain of H.
M.'s pink _Paramour_ in 1698, with instructions to proceed to southern
seas for geographical discoveries, and for improving knowledge of the
longitude problem, and of the variations of the compass. Trade winds and
monsoons, charts of magnetic variation, tides and surveys of the Channel
coast, and experiments with diving bells were practical activities that
occupied his attention.
Halley in 1720 became Astronomer Royal. He was the second incumbent of
this great office, but the first to supply the Royal Observatory with
instruments of its own, some of which adorn its walls even to-day. His
long series of lunar observations and his magnetic researches were of
immense practical value in navigation.
Halley lived to a ripe old age and left the world vastly better than he
found it. His rise from humblest obscurity was most remarkable, and he
lived to gratify all the ambitions of his early manhood. "Of attractive
appearance, pleasing manners, and ready wit," says one of his
biographers, "loyal, generous, and free from self-seeking, he was one of
the most personally engaging men who ever held the office of Astronomer
Royal."
He died in office at Greenwich in 1742.
"Halley was buried," says Chambers, "in the churchyard of St.
Margaret's, Lee, not far from Greenwich, and it has lately been
announced that the Admiralty have decided to repair his tomb at the
public expense, no descendants of his being known." There is no suitable
monument in England to the memory of one of her greatest scientific men.
In any event the collection and republication of his epoch-making papers
would be welcomed by astronomers of every nation.
CHAPTER XVII
BRADLEY AND ABERRATION
Living at Kew in London early in the 18th century was an enthusiastic
young astronomer, James Bradley. He is famous chiefly for his accurate
observations of star places which have been invaluable to astronomers of
later epochs in ascertaining the proper motions of stars.
The latitude of Bradley's house in Kew was very nearly the same as the
declination of the bright star Gamma Draconis, so that it passed through
his zenith once every day. Bradley had a zenith sector, and with this he
observed with the greatest care the zenith distance of Gamma Draconis at
every possible opportunity. This he did by pointing the telescope on the
star and then recording the small angle of its inclination to a fine
plumb line. So accurate were his measures that he was probably certain
of the star's position to the nearest second of arc.
What he hoped to find was the star's motion round a very slight orbit
once each year, and due to the earth's motion in its orbit round the
sun. In other words, he sought to find the star's parallax if it turned
out to be a measurable quantity.
It is just as well now that his method of observation proved
insufficiently delicate to reveal the parallax of Gamma Draconis; but
his assiduity in observation led him to an unexpected discovery of
greater moment at that time. What he really found was that the star had
a regular annual orbit; but wholly different from what he expected, and
very much larger in amount. This result was most puzzling to Bradley.
The law of relative motion would require that the star's motion in its
expected orbit should be opposite to that of the earth in its annual
orbit; instead of which the star was all the time at right angles to the
earth's motion.
Bradley was a frequent traveler by boat on the Thames, and the apparent
change in the direction of the wind when the boat was in motion is said
to have suggested to him what caused the displacement of Gamma Draconis.
The progressive motion of light had been roughly ascertained by Roemer:
let that be the velocity of the wind. And the earth's motion in its
orbit round the sun, let that be the speed of the boat. Then as the wind
(to an observer on the moving boat) always seems to come from a point in
advance of the point it actually proceeds from (to an observer at rest),
so the star should be constantly thrown forward by an angle given by the
relation of the velocity of light to the speed of the earth in orbital
revolution round the sun.
The apparent places of all stars are affected in this manner, and this
displacement is called the aberration of light. Astronomers since
Bradley's discovery of aberration in 1726 have devoted a great deal of
attention to this astronomical constant, as it is called, and the arc
value of it is very nearly 20".5. This means that light travels more
than ten thousand times as fast as the earth in its orbit (186,330 miles
per second as against the earth's 18.5). And we can ascertain the sun's
distance by aberration also because the exact values of the velocity of
light and of the constant of aberration when properly combined give the
exact orbital speed of the earth; and this furnishes directly by
geometry the radius of the earth's orbit, that is the distance of the
sun.
In fact, this is one of the more accurate modern methods of ascertaining
the distance of the sun. As early as 1880 it enabled the writer to
calculate the sun's parallax equal to 8".80, a value absolutely
identical with that adopted by the Paris Conference of 1896, and now
universally accepted as the standard.
In whatever part of the sky we observe, every star is affected by
aberration. At the poles of the ecliptic, 23-1/2 degrees from the
earth's poles, the annual aberration orbits of the stars are very small
circles, 41" in diameter. Toward the ecliptic the aberration orbits
become more and more oval, ellipses in fact of greater and greater
eccentricity, but with their major axes all of the same length, until we
reach the ecliptic itself; and then the ellipse is flattened into a
straight line 41" in length, in which the star travels forth and back
once a year. Exact correspondence of the aberration ellipses of the
stars with the annual motion of the earth round the sun affords
indisputable proof of this motion, and as every star partakes of the
movement, this proof of our motion round the sun becomes many
million-fold.
Indeed, if we were to push a little farther the refinement of our
analysis of the effect of aberration on stellar positions, we could
prove also the rotation of the earth on its axis, because that motion is
swift enough to bear an appreciable ratio to the velocity of light.
Diurnal aberration is the term applied to this slight effect, and as
every star partakes of it, demonstration of the earth's turning round on
its axis becomes many million-fold also.
CHAPTER XVIII
THE TELESCOPE
Had anyone told Ptolemy that his earth-centered system of sun, moon, and
stars would ultimately be overthrown, not by philosophy but by the
overwhelming evidence furnished by a little optical instrument which so
aided the human eye that it could actually see systems of bodies in
revolution round each other in the sky, he would no doubt have
vehemently denied that any such thing was possible. To be sure, it took
fourteen centuries to bring this about, and the discovery even then was
without much doubt due to accident.
Through all this long period when astronomy may be said to have merely
existed, practically without any forward step or development, its
devotees were unequipped with the sort of instruments which were
requisite to make the advance possible. There were astrolabes and
armillary spheres, with crudely divided circles, and the excellent work
done with them only shows the genius of many of the early astronomers
who had nothing better to work with. Regarding star-places made with
instruments fixed in the meridian, Bessel, often called the father of
practical astronomy, used to say that, even if you provided a bad
observer with the best of instruments, a genius could surpass him with a
gun barrel and a cart wheel.
Before the days of telescopes, that is, prior to the seventeenth
century, it was not known whether any of the planets except the earth
had a moon or not; consequently the masses of these planets were but
very imperfectly ascertained; the phases of Mercury and Venus were
merely conjectured; what were the actual dimensions of the planets could
only be guessed at; the approximate distances of sun, moon, and planets
were little better than guesses; the distances of the stars were wildly
inaccurate; and the positions of the stars on the celestial sphere, and
of sun, moon, and planets among them were far removed from modern
standards of precision--all because the telescope had not yet become
available as an optical adjunct to increase the power of the human eye
and enable it to see as if distances were in considerable measure
annihilated.
Galileo almost universally is said to have been the inventor of the
telescope, but intimate research into the question would appear to give
the honor of that original invention to another, in another country.
What Galileo deserves the highest praise for, however, is the
reinvention independently of an "optick tube" by which he could bring
distant objects apparently much nearer to him; and being an astronomer,
he was by universal acknowledgment first of all men to turn a telescope
on the heavenly bodies. This was in the year 1609, and his first
discovery was the phase of Venus, his second the four Medicean moons or
satellites of Jupiter, discoveries which at that epoch were of the
highest significance in establishing the truth of the Copernican system
beyond the shadow of doubt.
But the first telescopes of which we have record were made, so far as
can now be ascertained, in Holland very early in the 17th century.
Metius, a professor of mathematics, and Jansen and Lipperhey, who were
opticians in Middelburg--all three are entitled to consideration as
claimants of the original invention of the telescope. But that such an
instrument was pretty well known would appear to be shown by his
government's refusal of a patent to Lipperhey in 1608; while the
officials recognizing the value of such an instrument for purposes of
war, got him to construct several telescopes and ordered him to keep the
invention a secret.
Within a year Galileo heard that an instrument was in use in Holland by
which it was possible to see distant objects as if near at hand. Skilled
in optics as he was, the reinvention was a task neither long nor
difficult for him. One of his first instruments magnified but three
times; still it made a great sensation in Venice where he exhibited the
little tube to the authorities of that city, in which he first invented
it.
Galileo's telescope was of the simplest type, with but two lenses; the
one a double convex lens with which an image of the distant object is
formed, the other a double concave lens, much smaller which was the
eye-lens for examining the image. It is this simple form of Galilean
telescope that is still used in opera glasses and field glasses, because
of the shorter tube necessary.
Galileo carried on the construction of telescopes, all the time
improving their quality and enlarging their power until he built one
that magnified thirty times. What the diameter of the object glass was
we do not know, perhaps two inches or possibly a little more. Glass of a
quality good enough to make a telescope of cannot have been abundant or
even obtainable except with great difficulty in those early days.
Other discoveries by this first of celestial observers were the spots on
the sun, the larger mountains of the moon, the separate stars of which
the Milky Way is composed, and, greatest wonder of all, the anomalous
"handles" (_ansae_, he called them) of Saturn, which we now know as the
planet's ring, the most wonderful of all the bodies in the sky.
Since Galileo's time, only three centuries past, the progress in size
and improvement in quality of the telescope have been marvelous. And
this advance would not have been possible except for, first, the
discoveries still kept in large part secret by the makers of optical
glass which have enabled them to make disks of the largest size; second,
the consummate skill of modern opticians in fashioning these disks into
perfect lenses; and third, the progress in the mechanical arts and
engineering, by which telescope tubes of many tons' weight are mounted
or poised so delicately that the thrust of a finger readily swerves them
from one point of the heavens to another.
As the telescope is the most important of all astronomical instruments,
it is necessary to understand its construction and adjustment and how
the astronomer uses it. Telescopes are optical instruments, and nothing
but optical parts would be requisite in making them, if only the optical
conditions of their perfect working could be obtained without other
mechanical accessories.
In original principle, all telescopes are as simple as Galileo's; first,
an object glass to form the image of the distant object; second the
eyepiece usually made of two lenses, but really a microscope, to magnify
that image, and working in the same way that any microscope magnifies an
object close at hand; and third, a tube to hold all the necessary
lenses in the true relative positions.
[Illustration: THE 100-INCH HOOKER TELESCOPE, LARGEST REFLECTOR
IN THE WORLD, ON MT. WILSON. (_Photo, Mt. Wilson Solar
Observatory._)]
[Illustration: THE LARGEST REFRACTOR, THE 40-INCH TELESCOPE AT
YERKES OBSERVATORY. DOME 90 FT. IN DIAMETER. (_Photo, Yerkes
Observatory._)]
[Illustration: THE 150-FT. TOWER AT THE MT. WILSON SOLAR
OBSERVATORY. At the left is a diagram of tower, telescope
and pit. At the upper right is an exterior view of the
tower; below a view looking down into the pit, 75 ft. deep.
(_Photo, Mt. Wilson Solar Observatory._)]
The focal lengths of object glass and eyepiece will determine just what
distance apart the lenses must be in order to give perfect vision. But
it is quite as important that the axes of all the lenses be adjusted
into one and the same straight line, and then held there rigidly and
permanently. Otherwise vision with the telescope will be very imperfect
and wholly unsatisfactory. The distance from the objective, or object
glass to its focal point is called its focal length; and if we divide
this by the focal length of the eyepiece, we shall have the magnifying
power of the telescope. The eyepiece will usually be made of two lenses,
or more, and we use its focal length considered as a single lens, in
getting the magnifying power. A telescope will generally have many
eyepieces of different focal lengths, so that it will have a
corresponding range of magnifying powers. The lowest magnifying power
will be not less than four or five diameters for each inch of aperture
of the objective; otherwise the eye will fail to receive all the light
which falls upon the glass. A 4-inch telescope will therefore have no
eyepiece with a lower magnifying power than about 20 diameters. The
highest magnifying power advantageous for a glass of this size will be
about 250 to 300, the working rule being about 70 diameters to each inch
of aperture, although the theoretical limit is regarded as 100.
The reason for a variety of eyepieces with different magnifying powers
soon becomes apparent on using the telescope. Comets and nebulae call for
very low powers, while double stars and the planetary surfaces require
the higher powers, provided the state of the atmosphere at the moment
will allow it. If there is much quivering and unsteadiness, nothing is
gained by trying the higher powers, because all the waves of
unsteadiness are magnified also in the same proportion, and sharpness of
vision, or fine definition, or "good seeing," as it is called, becomes
impossible. The vibrations and tremors of the atmosphere are the
greatest of all obstacles to astronomical observation, and the search is
always in order for regions of the world, in deserts or on high
mountains, where the quietest atmosphere is to be found.
Quite another power of the telescope is dependent on its objective
solely: its light-gathering power. Light by which we see a star or
planet is admitted to the retina of the eye through an adjustable
aperture called the pupil. In the dark or at night, the pupil expands to
an average diameter of one-fourth of an inch. But the object-glass of a
telescope, by focusing the rays from a star, pours into the eye, almost
as a funnel acts with water, all the light which falls on its larger
surface. And as geometry has settled it for us that areas of surfaces
are proportioned to the squares of their diameters, a two-inch object
glass focuses upon the retina of the eye 64 times as much light as the
unassisted eye would receive. And the great 40-inch objective of the
Yerkes telescope would, theoretically, yield 25,600 times as much light
as the eye alone. But there would be a noticeable percentage of this
lost through absorption by the glasses of the telescope and scattering
by their surfaces.
The first makers of telescopes soon encountered a most discouraging
difficulty, because it seemed to them absolutely insuperable. This is
known as chromatic aberration, or the scattering of light in a
telescope due simply to its color or wave length. When light passes
through a prism, red is refracted the least and violet the most. Through
a lens it is the same, because a lens may be regarded as an indefinite
system of prisms. The image of a star or planet, then, formed by a
single lens cannot be optically perfect; instead it will be a confused
intermingling of images of various colors. With low powers this will not
be very troublesome, but great indistinctness results from the use of
high magnifying powers.
The early makers and users of telescopes in the latter part of the
seventeenth century found that the troublesome effects of chromatic
aberration could be much reduced by increasing the focal length of the
objective. This led to what we term engineering difficulties of a very
serious nature, because the tubes of great length were very awkward in
pointing toward celestial objects, especially near the zenith, where the
air is quietest. And it was next to impossible to hold an object
steadily in the field, even after all the troubles of getting it there
had been successfully overcome.
Bianchini and Cassini, Hevelius and Huygens were among the active
observers of that epoch who built telescopes of extraordinary length, a
hundred feet and upward. One tube is said to have been built 600 feet in
length, but quite certainly it could never have been used. So-called
aerial telescopes were also constructed, in which the objective was
mounted on top of a tower or a pole, and the eyepiece moved along near
the ground. But it is difficult to see how anything but fleeting
glimpses of the heavenly bodies could have been obtained with such
contrivances, even if the lenses had been perfect. Newton indeed, who
was expert in optics, gave up the problem of improving the refracting
telescope, and turned his energies toward the reflector.
In 1733, half a century after Newton and a century and a quarter after
Galileo, Chester More Hall, an Englishman, found by experiment that
chromatic aberration could be nearly eliminated by making the objective
of two lenses instead of one, and the same invention was made
independently by Dollond, an English optician, who took out letters
patent about 1760. So the size of telescopes seemed to be limited only
by the skill of the glassmaker and the size of disks that he might find
it practicable to produce.
What Hall and Dollond did was to make the outer or crown lens of the
objective as before, and place behind it a plano-concave lens of dense
flint glass. This had the effect of neutralizing the chromatic effect,
or color aberration, while at the same time only part of the refractive
effect of the crown lens was destroyed. This ingenious but costly
combination prepared the way for the great refracting telescopes of the
present day, because it solved, or seemed to solve, the important
problem of getting the necessary refraction of light rays without
harmful dispersion or decomposition of them.
Through the 18th century and the first years of the 19th many telescopes
of a size very great for that day were built, and their success seemed
complete. With large increase in the size of the disks, however, a new
trouble arose, quite inherent in the glass itself. The two kinds of
glass, flint and crown, do not decompose white light with uniformity, so
that when the so-called achromatic objective was composed of flint and
crown, there was an effect known as irrationality of dispersion, or
secondary spectrum, which produced a very troublesome residuum of blue
light surrounding the images of bright objects. This is the most serious
defect of all the great refractors of the day, and effectively it limits
their size to about 60 inches of aperture, with present types of flint
and crown. It is expected by present experimenters, however, that
further improvements in optical glass will do much to extend this limit;
so that a refracting telescope of much greater size than any now in
existence will be practicable.
Improvements in mounting telescopes, too, are still possible. Within
recent years, Hartness, of Springfield, Vermont, has erected a new and
ingenious type of turret telescope which protects the observer from wind
and cold while his instrument is outside. It affords exceptional
facilities for rapid and convenient observing, as for variable stars,
and is adaptable to both refractors and reflectors.
The captivating study of the heavens can of course be begun with the
naked eye alone, but very moderate optical assistance is a great help
and stimulates. An opera-glass affords such assistance; a field-glass
does still better, and best of all, for certain purposes, is a modern
prism-binocular.
CHAPTER XIX
REFLECTORS--MIRROR TELESCOPES
Cherished with the utmost care in the rooms of the Royal Society of
London is a world-famous telescope, a diminutive reflector made by the
hands of Sir Isaac Newton. We have already mentioned his connection with
the refractor; and how he abandoned that type of telescope in favor of
the reflecting mirror, or reflector in which the obstacles to great size
appeared to be purely mechanical. By many, indeed, Newton is regarded as
the inventor of the reflector.
By the principles of optics, all the rays from a star that strike a
concave mirror will be reflected to the geometric focal point, provided
a section of that mirror is a parabola. Such a mirror is called a
speculum, and is an alloy of tin, copper, and bismuth. Its surface takes
a very high polish, reflecting when newly polished nearly 90 per cent of
the light that falls upon it.
But the focus where the eyepiece must be used is in front of the mirror,
and if the eye were placed there, the observer's head would intercept
all or much of the light that would otherwise reach the mirror. Gregory,
probably the real inventor of the reflector, was the first to dodge this
difficulty by perforating the mirror at the center and applying the
eyepiece there, at the back of the speculum; but it was necessary to
first send the rays to that point by reflection from a second or
smaller mirror, in the optical axis of the speculum. This reflects the
rays backward down the tube to the eyepiece, or spectroscope, or camera.
Another English optician, Cassegrain, improved on this design somewhat
by placing the secondary mirror inside the focus of the speculum, or
nearer to it, so that the tube is shorter. This form is preferable for
many kinds of astronomical work, especially photography. Herschel sought
to do away with the secondary reflector entirely and save the loss of
light by tilting the speculum slightly, so as to throw the image at one
side of the tube; but this modification introduces bad definition of the
image and has never been much used.
A better plan is that of Newton, who placed a small plane speculum at an
angle of 45 degrees in the optical axis where the secondary mirror of
the Gregory-Cassegrainian type is placed. The rays are then received by
the eyepiece at the side of the upper end of the tube, the observer
looking in at right angles to the axis. And a modern improvement first
used by Draper is a small rectangular prism in place of the little plane
speculum, effecting a saving of five to ten per cent of the light.
It is not easy to say which type of telescope, the refractor or the
reflector, is the more famous. Nor which is the better or more useful,
or the more likely to lead in the astronomy of the future. When the
successors of Dollond had carried the achromatic refractor to the limit
enforced by the size of the glass disks they were able to secure, they
found these instruments not so great an improvement after all. The
single-lens telescopes of great focal length were nearly as good
optically, though much more awkward to handle. But the quality of the
glass obtainable in that day appeared to set an arbitrary limit to that
great amplification of size and power which progress in observational
astronomy demanded.
Then came the elder Herschel, best known and perhaps the greatest of all
astronomers. At Bath, England, music was his profession, especially the
organ. But he was dissatisfied with his little Gregorian reflector, and
being a very clever mechanician he set out to build a reflector for
himself. It is said that he cast and polished nearly 200 mirrors, in the
course of experiments on the most highly reflective type of alloys, and
the sort of mechanism that would enable him to give them the highest
polish. In all his work he was ably and enthusiastically aided by his
sister, Caroline Herschel, most famous of all women astronomers.
Upward in size of his mirrors he advanced, till he had a speculum of two
feet diameter with a tube 20 feet long. Twelve to fifteen years had
elapsed when in 1781, while testing one of these reflectors on stars in
the constellation Gemini, he made the first discovery of a planet since
the invention of the telescope--the great planet now known as Uranus.
Under the patronage of King George, he advanced to telescopes of still
greater size, his largest being no less than forty feet in length, with
a speculum of four feet in diameter. Two new satellites of Saturn were
discovered with this giant reflector, which was dismantled by Sir John
Herschel with appropriate ceremonies, including the singing of an ode by
the Herschel family assembled inside of the tube, on New Year's Eve,
1839-40.
We have record of but few attempts to improve the size and definition of
great reflectors by the continental astronomers during this era. In
England and Ireland, however, great progress was made. About 1860
Lassell built a two-foot reflector, with which he discovered two new
satellites of Uranus, and which he subsequently set up in the island of
Malta. Ten years later Thomas Grubb and Son of Dublin constructed a
four-foot reflector, now at the Observatory in Melbourne, Australia.
Calver in conjunction with Common of Ealing, London, about 1880-95 built
several large reflectors, the largest of five feet diameter, now owned
by Harvard College Observatory; and, rather earlier, Martin of Paris
completed a four-foot reflector.
The mirrors of these latter instruments were not made of speculum metal,
but of solid glass, which must be very thick (one-seventh their
diameter) in order to prevent flexure or bending by their own weight. So
sensitive is the optical surface to distortion that unless a complicated
series of levers and counterpoises is supplied, to support the under
surface of the mirror, the perfection of its optical figure disappears
when the telescope is directed to objects at different altitudes in the
sky. The upper or outer surface of the glass is the one which receives
the optical polish on a heavy coat of silver chemically deposited on the
polished glass after its figure has been tested and found satisfactory.
But far and away the most famous reflecting telescope of all is the
"Leviathan" of Lord Rosse, built at Birr Castle, Parsonstown, Ireland,
about the middle of the last century. His Lordship made many ingenious
improvements in grinding the mirror, which was of speculum metal, six
feet in diameter and weighed seven tons. It was ground to a focal
length of fifty-four feet and mounted between heavy walls of masonry, so
that the motion of the great tube was restricted to a few degrees on
both sides of the meridian. The huge mechanism was very cumbersome in
operation, and photography was not available in those days; nevertheless
Lord Rosse's telescope made the epochal discovery of the spiral nebulae,
which no other telescope of that day could have done.
In America the reflector has always kept at least even pace with the
refractor. As early as 1830, Mason and Smith, two students at Yale
College, enthused by Denison Olmsted, built a 12-inch speculum with
which they made unsurpassed observations of the nebulae. Dr. Henry
Draper, returning from a visit to Lord Rosse, began about 1865 the
construction of two silver-on-glass reflectors, one of 15 inches
diameter, the other of 28 inches, with which he did important work for
many years in photography and spectroscopy, and his mirrors are now the
property of Harvard College Observatory. Alvan Clark and Sons have in
later years built a 40-inch mirror for the Lowell Observatory in
Arizona, and very recently a 6-foot silver-on-glass mirror has been set
up in the Dominion of Canada Astrophysical Observatory at Victoria,
British Columbia, where it is doing excellent work in the hands of
Plaskett, its designer.
The huge glass disk for the reflector weighs two tons, and it must be
cast so that there are no internal strains; otherwise it is liable to
burst in fragments in the process of grinding. It should be free from
air-bubbles, too; so the glass is cast in one melting, if possible. This
disk was made by the St. Gobain Plate Glass Company, whose works have
been ruthlessly destroyed by the enemy during the war; but fortunately
the great disk had been shipped from Antwerp only a week before
declaration of hostilities.
Brashear of Allegheny was intrusted with the optical parts, which
occupied many months of critical work. The finished mirror is 73 inches
in diameter, its focal length is 30 feet, and its thickness 12 inches. A
central hole 10 inches in diameter makes possible its use as a Gregorian
or Cassegrainian type, as well as Newtonian. The mechanical parts of
this great telescope are by Warner and Swasey of Cleveland, after the
well-known equatorial mounting of the Melbourne reflector by Grubb of
Dublin. Friction of the polar and declination axes is reduced by ball
bearings. The 66-foot dome has an opening 15 feet wide and extending six
feet beyond the zenith. All motions of the telescope, dome shutters, and
observing platform are under complete control by electric motors.
Spectroscopic binaries form one of the special fields of research with
this powerful instrument, and many new binaries have already been
detected.
The great reflectors designed and constructed by Ritchey, formerly of
Chicago and now of Pasadena, deserve especial mention. While connected
with the Yerkes Observatory he constructed a two-foot reflector for that
institution, with which he had exceptional success in photography of the
stars and nebulae. Later he built a 5-foot reflector, now at the Carnegie
Observatory on Mount Wilson, California, with which the spiral nebulae
and many other celestial objects have been especially well photographed.
Ritchey's later years have been spent on the construction of an even
greater mirror, no less than 100 inches in diameter, which was completed
in 1919, and has already yielded photographic results dealt with farther
on, and far surpassing anything previously obtained. Theoretically this
huge mirror, if its surface were perfectly reflective so that it would
transmit all the rays falling upon it, would gather 160,000 times as
much light as the unaided eye alone.
Whether a 72-inch refractor, should it ever be constructed, would
surpass the 100-inch reflector as an all-round engine for astronomical
research, is a question that can only be fully answered by building it
and trying the two instruments alongside.
Probably three-quarters of all the really great astronomical work in the
past has been done by refractors. They are always ready and convenient
for use, and the optical surfaces rarely require cleaning and
readjustment. With increase of size, however, the secondary spectrum
becomes very bothersome in the great lenses; and the larger they are,
the more light is lost by absorption on account of the increasing
thickness of the lenses. With the reflector on the other hand, while
there is clearly a greater range of size, the reflective surface retains
its high polish only a brief period, so that mere tarnish effectively
reduces the aperture; and the great mirror is more or less ineffective
in consequence of flexure uncompensated by the lever system that
supports the back of the mirror.
Both types of telescope still have their enthusiastic devotees; and the
next great reflector would doubtless be a gratifying success, if mounted
in some elevated region of the world, like the Andes of northern Chile,
where the air is exceptionally steady and the sky very clear a large
part of the year. The highest magnifying powers suitable for work with
such a telescope could then be employed, and new discoveries added as
well as important work done in extension of lines already begun on the
universe of stars.
On the authority of Clark, even a six-foot objective would not
necessitate a combined thickness of its glasses in excess of six inches.
Present disks are vastly superior to the early ones in transparency, and
there is reason to expect still greater improvement. The engineering
troubles incident to execution of the mechanical side of the scheme need
not stand in the way; they never have, indeed the astronomer has but
just begun to invoke the fertile resources of the modern engineer. Not
long before his death the younger Clark who had just finished the great
lenses of the 40-inch Yerkes telescope, ventured this prevision, already
in part come true: "The new astronomy, as well as the old, demands more
power. Problems wait for their solution, and theories to be
substantiated or disproved. The horizon of science has been greatly
broadened within the last few years, but out upon the borderland I see
the glimmer of new lights that await for their interpretation, and the
great telescopes of the future must be their interpreters."
Practically all the great telescopes of the world have in turn
signalized the new accession of power by some significant astronomical
discovery: to specify, one of Herschel's reflectors first revealed the
planet Uranus; Lord Rosse's "Leviathan" the spiral nebulae; the 15-inch
Cambridge lens the crape, or dusky ring of Saturn; the 18-1/2-inch
Chicago refractor the companion of Sirius; the Washington 26-inch
telescope the satellites of Mars; the 30-inch Pulkowa glass the
nebulosities of the Pleiades; and the 36-inch Lick telescope brought to
light a fifth satellite of Jupiter. At the time these discoveries were
made, each of these great telescopes was the only instrument then in
existence with power enough to have made the discovery possible. So we
may advance to still farther accessions of power with the expectation
that greater discoveries will continue to gratify our confidence.
CHAPTER XX
THE STORY OF THE SPECTROSCOPE
Sir Isaac Newton ought really to have been the inventor of the
spectroscope, because he began by analyzing light in the rough with
prisms, was very expert in optics, and was certainly enough of a
philosopher to have laid the foundations of the science.
What Newton did was to admit sunlight into a darkened room through a
small round aperture, then pass the rays through a glass prism and
receive the band of color on a screen. He noticed the succession of
colors correctly--violet, indigo, blue, green, yellow, orange, red; also
that they were not pure colors, but overlapping bands of color.
Apparently neither he nor any other experimenter for more than a century
went any further, when the next essential step was taken by Wollaston
about 1802 in England. He saw that by receiving the light through a
narrow slit instead of a round hole, he got a purer spectrum, spectrum
being the name given to the succession of colors into which the prism
splits up or decomposes the original beam of white sunlight. This
seemingly insignificant change, a narrow slit replacing the round hole,
made Wollaston and not Newton the discoverer of the dark lines crossing
the spectrum at various irregular intervals, and these singularly
neglected lines meant the basis of a new and most important science.
Even Wollaston, however, passed them by, and it was Fraunhofer who in
1814-1815 first made a chart of them. Consequently they are known as
Fraunhofer lines, or dark absorption lines. Sending the beam of light
through a succession of prisms gives greater dispersion and increases
the power of the spectroscope. The greater the dispersion the greater
the number of absorption lines; and it is the number and intensity of
these lines, with their accurate position throughout the range of the
spectrum which becomes the basis of spectrum analysis.
The half century that saw the invention of the steam engine,
photography, the railroad and the telegraph elapsed without any farther
developments than mere mapping of the fundamental lines, A, B, C, D, E,
F, G, H of the solar spectrum. The moon, too, was examined and its
spectrum found the same, as was to be expected from sunlight simply
reflected.
Sir John Herschel and other experimenters came near guessing the
significance of the dark lines, but the problem of unraveling their
mystery was finally solved by Bunsen and Kirchhoff who ascertained that
an incandescent gas emits rays of exactly the same degree of
refrangibility which it absorbs when white light is passed through it.
This great discovery was at once received as the secure basis of
spectrum analysis, and Kirchhoff in 1858 put in compact and
comprehensive form the three following principles underlying the theory
of the science:
(1) Solid and liquid bodies, also gases under high pressure, give when
incandescent a continuous spectrum, that is one with a mere succession
of colors, and neither bright nor dark lines;
(2) Gases under low pressure give a discontinuous spectrum, crossed by
bright lines whose number and position in the spectrum differ according
to the substances vaporized;
(3) When white light passes through a gas, this medium absorbs or
quenches rays of identical wave-length with those composing its own
bright-line spectrum.
Clearly then it makes no difference where the light originates whether
it comes from sun or star. Only it must be bright enough so that we can
analyze it with the spectroscope. But our analysis of sun and star could
not proceed until the chemist had vaporized in the laboratory all the
elements, and charted their spectra with accuracy. When this had been
done, every substance became at once recognizable by the number and
position of its lines, with practical certainty.
How then can we be sure of the chemical and physical composition of sun
and stars? Only by detailed and critical comparison of their spectra
with the laboratory spectra of elements which chemical and physical
research have supplied. As in the sun, so in the stars, each of which is
encircled by a gaseous absorptive layer or atmosphere, the light rays
from the self-luminous inner sphere must pass through this reversing
layer, which absorbs light of exactly the same wave-length as the lines
that make up its own bright line spectrum. Whatever substances are here
found in gaseous condition, the same will be evident by dark lines in
the spectrum of sun or star, and the position of these dark lines will
show, by coincidence with the position of the laboratory bright lines,
all the substances that are vaporized in the atmospheres of the
self-luminous bodies of the sky.
Here then originated the science of the new astronomy: the old astronomy
had concerned itself mainly with positions of the heavenly bodies,
_where_ they are; the new astronomy deals with their chemical
composition and physical constitution, and _what_ they are. Between 1865
and 1875 the fundamental application of the basic principles was well
advanced by the researches of Sir William Huggins in England, of Father
Angelo Secchi in Rome, of Jules Janssen in Paris, and of Dr. Henry
Draper in New York.
In analyzing the spectrum of the sun, many thousands of dark absorption
lines are found, and their coincidences with the bright lines of
terrestrial elements show that iron, for instance, is most prominently
identified, with rather more than 2,000 coincidences of bright and dark
lines. Calcium, too, is indicated by peculiar intensity of its lines, as
well as their great number. Next in order are hydrogen, nickel and
sodium. By prolonged and minute comparison of the solar spectrum with
spectra of terrestrial elements, something like forty elemental
substances are now known to exist in the sun. Rowland's splendid
photographs of the solar spectrum have contributed most effectively.
About half of these elements, though not in order of certainty, are
aluminum, cadmium, calcium, carbon, chromium, cobalt, copper, hydrogen,
iron, magnesium, manganese, nickel, scandium, silicon, silver, sodium,
titanium, vanadium, yttrium, zinc, and zirconium. Oxygen, too, is pretty
surely indicated; but certain elements abundant on earth, as nitrogen
and chlorine, together with gold, mercury, phosphorus, and sulphur, are
not found in the sun.
The two brilliant red stars, Aldebaran in Taurus, and Betelgeuse in
Orion, were the first stars whose chemical constitution was revealed to
the eye of man, and Sir William Huggins of London was the astronomer
who achieved this epoch-making result. Father Secchi of the Vatican
Observatory proceeded at once with the visual examination of the spectra
of hundreds of the brighter stars, and he was the first to provide a
classification of stellar spectra. There were four types.
Secchi's type I is characterized chiefly by the breadth and intensity of
dark hydrogen lines, together with a faintness or entire absence of
metallic lines. These are bluish or white stars and they are very
abundant, nearly half of all the stars. Vega, Altair, and numerous other
bright stars belong to this type, and especially Sirius, which gives to
the type the name "Sirians."
Type II is characterized by a multitude of fine dark metallic lines,
closely resembling the lines of the solar spectrum. These stars are
somewhat yellowish in tinge like the sun, and from this similarity of
spectra they are called "solars." Arcturus and Capella are "solars," and
on the whole the solars are rather less numerous than the Sirians. Stars
nearest to the solar system are mostly of this type, and, according to
Kapteyn of Groningen, the absolute luminous power of first type stars
exceeds that of second type stars seven-fold.
Secchi's type III is characterized by many dark bands, well defined on
the side toward the blue end of the spectrum, but shading off toward the
red--a "colonnaded spectrum", as Miss Clerke aptly terms it. Alpha
Herculis, Antares, and Mira, together with orange and reddish stars and
most of the variable stars, belong in type III.
Type IV is also characterized by dark bands, often called "flutings,"
similar to those of type III, but reversed as to shading, that is, well
defined on the side toward the red, but fading out toward the blue.
Their atmospheres contain carbon; but they are not abundant, besides
being faint and nearly all blood-red in tint.
Following up the brilliant researches of Draper, who in 1872 obtained
the first successful photograph of a star's spectrum, that of Vega,
Pickering of Harvard supplemented Secchi's classification by Type V, a
spectrum characterized by bright lines. They, too, are not abundant and
are all found near the middle of the Galaxy. These are usually known as
Wolf-Rayet stars, from the two Paris astronomers who first investigated
their spectra. Type V stars are a class of objects seemingly apart from
the rest of the stellar universe, and many of the planetary nebulae yield
the same sort of a spectrum.
The late Mrs. Anna Palmer Draper, widow of Dr. Henry Draper, established
the Henry Draper Memorial at Harvard, and investigation of the
photographic spectra of all the brighter stars of the entire heavens has
been prosecuted on a comprehensive scale, those of the northern
hemisphere at Cambridge, and of the southern at Arequipa, Peru. These
researches have led to a broad reclassification of the stars into eight
distinct groups, a work of exceptional magnitude begun by the late Mrs.
Fleming and recently completed by Miss Annie Cannon, who classified the
photographic spectra of more than 230,000 stars on the new system, as
follows:--
The letters O, B, A, F, G, K, M, N represent a continuous gradation in
the supposed order of stellar evolution, and farther subdivision is
indicated by tenths, G5K meaning a type half way between G and K, and
usually written G5 simply. B2 would indicate a type between B and A, but
nearer to B than A, and so on. On this system, the spectrum of a star in
the earliest stages of its evolution is made up of diffuse bright bands
on a faint continuous background. As these bands become fewer and
narrower, very faint absorption lines begin to appear, first the helium
lines, followed by several series of hydrogen lines. On the
disappearance of the bright bands, the spectrum becomes wholly
absorptive bands and lines. Then comes a very great increase in
intensity of the true hydrogen spectrum, with wide and much diffused
lines, and few if any other lines. Then the H and K calcium lines and
other lines peculiar to the sun become more and more intense. Then the
hydrogen lines go through their long decline. The calcium spectrum
becomes intense, and later the spectrum becomes quite like that of the
sun with a great wealth of lines. Following this stage the spectrum
shortens from the ultra violet, the hydrogen lines fade out still
farther, and bands due to metallic compounds make their appearance, the
entire spectrum finally resembling that of sun spots. To designate these
types rather more categorically:--
Type O--bright bands on a faint continuous background, with five
subdivisions, Oa, Ob, Oc, Od, Oe, according to the varying width and
intensity of the bands.
Type B--the Orion type, or helium type, with additional lines of origin
unknown as yet, but without any of the bright bands of type O.
Type A--the Sirian type, the regular Balmer series of hydrogen lines
being very intense, with a few other lines not conspicuously marked.
Type F--the calcium type, hydrogen lines less strongly marked, but with
the narrow calcium lines H and K very intense.
Type G--the solar type, with multitudes of metallic lines.
Type K--in some respects similar to G, but with the hydrogen lines
fading out, and the metallic lines relatively more prominent.
Type M--spectrum with peculiar flutings due to titanium oxide, with
subdivisions Ma and Mb, and the variable stars of long period, with a
few bright hydrogen lines additional, in a separate class Md.
Type N--similar to M, in that both are pronouncedly reddish, but with
characteristic flutings probably indicating carbon compounds.
The Draper classification being based on photographic spectra, and the
original Secchi classification being visual, the relation of the two
systems is approximately as follows:
Secchi Type I includes Draper B & A
II includes Draper F, G & K
III includes Draper M
IV includes Draper N
Pickering's marked success in organization and execution of this great
programme was due to his adoption of the "slitless spectroscope," which
made it possible to photograph stellar spectra in vast numbers on a
single plate. The first observers of stellar spectra placed the
spectroscope beyond the focus of the telescope with which it was used,
thereby limiting the examination to but one star at a time. In the
slitless spectroscope, a large prism is mounted in front of the
objective (of short focus), so that the star's rays pass through it
first, and then are brought to the same focus on the photographic
plate, for all the stars within the field of view, sometimes many
thousand in number. This arrangement provides great advantages in the
comparison and classification of stellar spectra.
When spectroscopic methods were first introduced into astronomy, there
was no expectation that the field of the old or so-called exact
astronomy would be invaded. Physicists were sometimes jocularly greeted
among astronomers as "ribbon men," and no one even dreamed that their
researches were one day to advance to equal recognition with results
derived from micrometer, meridian circle, and heliometer.
The first step in this direction was taken in 1868 by Sir William
Huggins of London, who noticed small displacements in the lines of
spectra of very bright stars. In fact the whole spectrum appeared to be
shifted; in the case of Sirius it was shifted toward the red, while the
whole spectrum of Arcturus was shifted by three times this amount toward
the violet end of the spectrum. The reason was not difficult to assign.
As early as 1842 Doppler had enunciated the principle that when we are
approaching or are approached by a body which is emitting regular
vibrations, then the number of waves we receive in a second is
increased, and their wave-length correspondingly diminished; and just
the reverse of this occurs when the distance of the vibrating body is
increasing. It is the same with light as with sound, and everyone has
noticed how the pitch of a locomotive whistle suddenly rises as it
passes, and falls as suddenly on retreating from us. So Huggins drew the
immediate inference that the distance between the earth and Sirius was
increasing at the rate of nearly twenty miles per second, while
Arcturus was nearing us with a velocity of sixty miles per second.
These pioneer observations of motions in the line of sight, or radial
velocities as they are now called, led directly to the acceptance of the
high value of spectroscopic work as an adjunct of exact astronomy in
stellar research. Nor has it been found wanting in application to a
great variety of exact problems in the solar system which would have
been wholly impossible to solve without it.
Foremost is the sun, of course, because of the overplus of light. Young
early measured the displacement of lines in the spectra of the
prominences, and found velocities sometimes exceeding 250 miles per
second. Many astronomers, Duner among them, investigated the rotation of
the sun by the spectroscopic method. The sun's east limb is coming
toward us, while the west is going from us; and by measuring the sum of
the displacements, the rate of rotation has been calculated, not only at
the sun's equator but at many solar latitudes also, both north and
south. As was to be expected, these results agree well with the sun's
rotation as found by the transits of sun spots in the lower latitudes
where they make their appearance.
Belopolsky has applied the same method to the rotation of the planet
Venus, and Keeler, by measuring the displacement of lines in the
spectrum of Saturn, on opposite sides of the ring, provided a brilliant
observational proof of the physical constitution of the rings; because
he showed that the inner ring traveled round more swiftly than the outer
one, thus demonstrating that the ring could not be solid, but must be
composed of multitudes of small particles traveling around the ball of
Saturn, much as if they were satellites. Indeed, Keeler ascertained the
velocity of their orbital motion and found that in each case it agreed
exactly with that required by the Keplerian law.
Even the filmy corona of the sun was investigated in similar fashion by
Deslandres at the total eclipse of 1893, and he found that it rotates
bodily with the sun. But the complete vindication of the spectroscopic
method as an adjunct of the old astronomy came with its application to
measurement of the distance of the sun. The method is very interesting
and was first suggested by Campbell in 1892. Spectrum-line measurements
have become very accurate with the introduction of dry-plate
photography, and ecliptic stars were spectrographed, toward and from
which the earth is traveling by its orbital motion round the sun. By
accurate measurement of these displacements, the orbital velocity of the
earth is calculated; and as we know the exact length of the year, or a
complete period, the length of the orbit itself in miles becomes known,
and thus, by simple mensuration, the length of the radius of the
orbit--which is the distance of the sun.
If we pass from sun to star, the triumph of the spectroscope has been
everywhere complete and significant. As the spectroscopic survey of the
stars grew toward completeness, it became evident that the swarming
hosts of the stellar universe are in constant motion through space, not
only athwart the line of vision as their proper motions had long
disclosed, but some stars are swiftly moving toward our solar system and
others as swiftly from it.
Fixed stars, strictly speaking--there are no such. All are in relative
motion. Exact astronomy by discussion of the proper motions had
assigned a region of the sky toward which the sun and planets are
moving. Spectrography soon verified this direction not only, but gave a
determination of the velocity of our motion of twelve miles per second
in a direction approximately that of the constellation Lyra. From
corresponding radial velocities, we draw the ready conclusion that
certain groups or clusters of stars are actually connected in space and
moving as related systems, as in the Pleiades and Ursa Major.
Rather more than a quarter century ago, the spectroscope came to the
assistance of the telescope in helping to solve the intricate problem of
stellar distribution. Kapteyn, by combining the proper motions of
certain stars with their classification in the Draper catalogue of
stellar spectra, drew the conclusion that, as stars having very small
proper motions show a condensation toward the Galaxy, the stars
composing this girdle are mostly of the Sirian type, and are at vast
distances from the solar system. The proper motion of a star near to us
will ordinarily be large, and, in the case of solar stars, the larger
their proper motion the greater their number. So it would appear that
the solar stars are aggregated round the sun himself, and this
conclusion is greatly strengthened by the fact that of stars whose
distances and spectral type are both ascertained, seven of the eight
nearest to us are solar stars.
In 1889 the spectroscope achieved an unexpected triumph by enabling the
late Professor Pickering to make the first discovery of a spectroscopic
double, or binary star, a type of object now quite abundant. Unlike the
visual binary systems whose periods are years in length, the
spectroscopic binaries have short periods, reckoned in some cases in
days, or hours even. If the orbit of a very close binary is seen edge
on, the light of the two stars will coalesce twice in every revolution.
Halfway between these points there are two times when the two stars will
be moving, one toward the earth and the other from it. At all times the
light of the star, in so far as the telescope shows it, proceeds from a
single object.
Now photograph the star's spectrum at each of the four critical points
above indicated: in the first pair the lines are sharply defined and
single, because at conjunction the stars are simply moving athwart the
line of sight, while at the intermediate points the lines are double.
Doppler's principle completely accounts for this: the light from the
receding companion is giving lines displaced toward the red, while the
approaching companion yields lines displaced toward the violet. Mizar,
the double star at the bend of the handle in the Great Dipper was the
first star to yield this peculiar type of spectrum, and the period of
its invisible companion is about 52 days. The relative velocity of the
components is 100 miles a second, and applying Newton's law we find its
mass exceeds that of the sun forty-fold. Capella has been found to be a
spectroscopic binary; also the pole star. Spectroscopic binaries have
relatively short periods, one of the shortest known being only 35 hours
in length. It is in the constellation Scorpio. Beta Aurigae is another
whose lines double on alternate nights, giving a period of four days;
and the combined mass of both stars is more than twice that of the sun.
The catalogue of spectroscopic binaries is constantly enlarging; but
thousands doubtless exist that can never be discovered by this method,
as is evident if their orbits are perpendicular to the line of sight or
nearly so. The history of the spectroscopic binaries is one of the most
interesting chapters in astronomy, and affords a marvelous confirmation
of the prediction of Bessel who first wrote of "the astronomy of the
invisible."
Find a star's distance by the spectroscope? Impossible, everyone would
have said, even a very few years ago. Now, however, the thing is done,
and with increasing accuracy.
Adams of Mount Wilson has found, after protracted investigation, that
the relative intensity of certain spectral lines varies according to the
absolute brightness of a star; indeed, so close is the correspondence
that the spectroscopic observations are employed to provide in certain
cases a good determination of the absolute magnitude, and therefore of
the distance. To test this relation, the spectroscopic parallaxes have
been compared with the measured parallaxes in numerous instances, and an
excellent agreement is shown. This new method is adding extensively to
our knowledge of stellar luminosities and distances, and even the vast
distances of globular clusters and spiral nebulae are becoming known.
In fact, but few departments of the old astronomy are left which the new
astronomy has not invaded, and this latest triumph of the spectroscope
in determining accurately the distances of even the remotest stars is
enthusiastically welcomed by advocates of the old and new astronomy
alike.
CHAPTER XXI
THE STORY OF ASTRONOMICAL PHOTOGRAPHY
The most powerful ally of both telescope and spectroscope is
photography. Without it the marvelous researches carried on with both
these types of instrument would have been essentially impossible. Even
the great telescopes of Herschel and Lord Rosse, notwithstanding their
splendid record as optical instruments, might have achieved vastly more
had photography been developed in their time to the point where the
astronomer could have employed its wonderful capabilities as he does
to-day. And, with the spectroscope, it is hardly too much to say that no
investigator ever observes visually with that instrument any more:
practically every spectrum is made a matter of photographic record
first. The observing, or nowadays the measuring, is all done afterward.
All telescopes and cameras are alike, in that each must form or have
formed within it an image by means of a lens or mirror. In the telescope
the eye sees the fleeting image, in the camera the process of
registering the image on a plate or film is known as photography.
Daguerre first invented the process (silver film on a copper plate) in
1839. The year following it was first employed on the moon, in 1850 the
first star was photographed, in 1851 the first total eclipse of the sun;
all by the primitive daguerreotype process, which, notwithstanding its
awkwardness and the great length of exposure required, was found to
possess many advantages for astronomical work.
About the middle of the last century the wet plate process, so called
because the sensitized collodion film must be kept moist during
exposure, came into general use, and the astronomers of that period were
not slow to avail themselves of the advantages of a more sensitive
process, which in 1872, in the skillful hands of Henry Draper, produced
the first spectrum of a star. In 1880 a nebula was first photographed,
and in 1881 a comet.
Before this time, however, the new dry-plate process had been developed
to the point where astronomers began to avail of its greater convenience
and increased sensitiveness, even in spite of the coarseness of grain of
the film. Forty years of dry-plate service have brought a wealth of
advantages scarcely dreamed of in the beginning, and nearly every
department of astronomical research has been enhanced thereby, while
many entirely new photographic methods of investigation have been worked
out.
Continued improvement in photographic processes has provided the
possibility of pictures of fainter and fainter celestial objects, and
all the larger telescopes have photographed stars and nebulae of such
exceeding faintness that the human eye, even if applied to the same
instrument, would never be able to see them. This is because the eye, in
ten or twelve seconds of keen watching, becomes fatigued and must be
rested, whereas the action of very faint light rays is cumulative on the
highly sensitive film; so that a continuous exposure of many hours'
duration becomes readily visible to the eye on development. So a
supersensitive dry plate will often record many thousand stars in a
region where the naked eye can see but one.
Perhaps the greatest amplification of photography has taken place at the
Harvard Observatory under Pickering, where a library of many hundred
thousand plates has accumulated; and at Groningen, Holland, where
Kapteyn has established an astronomical laboratory without instruments
except such as are necessary to measure photographic plates, whenever
and wherever taken. So it is possible to select the clearest of skies,
all over the world, for exposure of the plates, and bring back the
photographs for expert discussion.
Of course the sun was the celestial body first photographed, and its
surpassing brilliance necessitates reduction of exposure to a minimum.
In moments of exceptional steadiness of the atmosphere, a very high
degree of magnification of the solar surface on the photographic plate
is permitted, and the details in formation, development, and ending of
sun spots are faithfully registered. Nevertheless, it cannot be said
that photography has yet entirely replaced the eye in this work, and
careful drawings of sun spots at critical stages in their life are
capable of registering fine detail which the plate has so far been
unable to record. Janssen of Paris took photographs of the solar
photosphere so highly magnified that the granulation or willow-leaf
structure of the surface was clearly visible, and its variations
traceable from hour to hour.
The advantages of sun spot photography in ascertaining the sun's
rotation, keeping count of the spots, and in a permanent record for
measurement of position of the sun's axis and the spot zones, are
obvious. In direct portrayal of the sun's corona during total eclipses,
photography has offered superior advantages over visual sketching, in
the form and exact location of the coronal streamers; but the
extraordinary differences of intensity between the inner corona and its
outlying extensions are such that halation renders a complete picture on
a single plate practically impossible. The filamentous detail of the
inner corona, and the faintest outlying extensions or streamers, the eye
must still reveal directly.
In solar spectrum photography, research has been especially benefited;
indeed, exact registry of the multitudinous lines was quite impossible
without it. Photographic maps of the spectrum by Thollon, McClean and
Rowland are so complete and accurate that no visual charts can approach
them. Rowland's great photographic map of the solar spectrum spread out
into a band about forty feet in length; and in the infra-red, Langley's
spectrobolometer extended the invisible heat spectrum photographically
to many times that length. At the other end of the spectrum, special
photographic processes have extended the ultra-violet spectrum far
beyond the ocular limit, to a point where it is abruptly cut off by
absorption of the earth's atmosphere. On the same plate with certain
regions of the sun's spectrum, the spectra of terrestrial metals are
photographed side by side, and exact coincidences of lines show that
about forty elemental substances known to terrestrial chemistry are
vaporized in the sun.
[Illustration: A VIEW OF THE 100-FOOT DOME IN WHICH THE LARGEST
TELESCOPE IN THE WORLD IS HOUSED. (_Courtesy, Mt. Wilson Solar
Observatory._)]
[Illustration: MOUNT CHIMBORAZO, NEAR THE EQUATOR. An observatory
located on this mountain would make it possible to study the
phenomena of northern and southern skies from the same point.
(_Courtesy, Pan-American Union._)]
[Illustration: LICK OBSERVATORY, ON THE SUMMIT OF MT. HAMILTON,
ABOUT TWENTY-FIVE MILES S. W. OF SAN JOSE, CALIFORNIA. It
contains the famous Lick telescope, a 36-inch refractor.]
[Illustration: NEAR VIEW OF THE EYE-END OF THE YERKES TELESCOPE.
The eyepiece is removed and its place taken by a photographic
plate.]
Young was the first to photograph a solar prominence in 1870, and twenty
years later Deslandres of Paris and Hale of Chicago independently
invented the spectroheliograph, by which the chromosphere and
prominences of the sun, as well as the disk of the sun itself, are all
photographed by monochromatic light on a single plate. Hale has
developed this instrument almost to the limit, first at the Yerkes
Observatory of the University of Chicago, and more recently at the Mount
Wilson Observatory of the Carnegie Institution, where spectroheliograms
of marvelous perfection are daily taken. It was with this instrument
that Hale discovered the effect of an electro-magnetic field in sun
spots which has revolutionized solar theories, a research impossible to
conceive of without the aid of photography.
When we apply Doppler's principle, photography becomes doubly
advantageous, whether we determine, as Duner did and more recently
Adams, the sun's own rotation and find it to vary in different solar
latitudes, the equator going fastest; or apply the method to the sun's
corona at the east and west limbs of the sun, which Deslandres in 1893
proved to be rotating bodily with the sun, because of the measured
displacement of spectral lines of the corona in juxtaposition on the
photographic plate.
In the solar astronomy of measurement, too, photography has been
helpfully utilized, as in registering the transits of Mercury over the
sun's disk, for correcting the tables of the planet's orbital motion;
and most prominently in the action taken by the principal governments of
the world in sending out expeditions to observe the transits of Venus in
1874 and 1882, for the purpose of determining the parallax of Venus and
so the distance of the earth from the sun.
In our studies of the moon, photography has almost completely superseded
ocular work during the past sixty years. Rutherfurd and Draper of New
York about 1865 obtained very excellent lunar photographs with wet
plates, which were unexcelled for nearly half a century. The Harvard,
Lick, and Paris Observatories have published pretty complete
photographic atlases of the moon, and the best negatives of these series
show nearly everything that the eye can discern, except under unusual
circumstances. Later lunar photography was taken up at the Yerkes
Observatory, and exceptionally fine photographs on a large scale were
obtained with the 40-inch refractor, using a color screen. More recently
the 60-inch and 100-inch mirrors of the Mount Wilson Observatory have
taken a series of photographs of the moon far surpassing everything
previously done, as was to be expected from the unique combination of a
tranquil mountain atmosphere with the extraordinary optical power of the
instruments, and a special adaptation of photographic methods. During
lunar eclipses, Pickering has made a photographic search for a possible
satellite of the moon, occultations of stars by the moon have been
recorded by photography, and Russell of Princeton has shown how the
position of the moon among the stars can be determined by the aid of
photography with a high order of precision.
The story of planetary photography is on the whole disappointing. Much
has been done, but there is much that is within reach, or ought to be,
that remains undone. From Mercury nothing ought perhaps to be expected.
On many of the photographs of the transit of Venus, especially those
taken under the writer's direction at the Lick Observatory in 1882, we
have unmistakable evidence of the planet's atmosphere. Here again the
wet plate process, although more clumsy, demonstrated its superiority
over the dry process used by other expeditions.
In spectroscopy, Belopolsky has sought to determine the period of
rotation of Venus on her axis. At the Lowell Observatory, Douglass
succeeded in photographing the faint zodiacal light, and very successful
photographs of Mars were taken at this institution as early as 1905 by
Slipher. Two years later these were much improved upon by the writer's
expedition to the Andes of Chile, when 12,000 exposures of Mars were
made, many of them showing the principal _canali_, and other prominent
features of the planet's disk. At subsequent oppositions of the planet,
Barnard at the Yerkes Observatory and the Mount Wilson observers have
far surpassed all these photographs.
For future oppositions a more sensitive film is highly desired, in
connection with instruments possessing greater light-gathering power, so
permitting a briefer exposure that will be less influenced by
irregularities and defects of the atmosphere. The spectrum of Mars is of
course that of sunlight, very much reduced, and modified to a slight
extent by its passing twice through the atmosphere of Mars. What amount
of aqueous vapor that atmosphere may contai
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