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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?