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+The Project Gutenberg EBook of Astronomy, by David Todd
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever. You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org/license
+
+
+Title: Astronomy
+ The Science of the Heavenly Bodies
+
+Author: David Todd
+
+Release Date: March 15, 2012 [EBook #39142]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK ASTRONOMY ***
+
+
+
+
+Produced by K Nordquist, Tom Cosmas, Brenda Lewis and the
+Online Distributed Proofreading Team at http://www.pgdp.net
+(This file was produced from images generously made
+available by The Internet Archive/American Libraries.)
+
+
+
+
+
+
+
+
+
+ [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 contain is a question that can be
+answered only by critical comparison of the Martian spectrum with the
+spectrum of the moon, and photography affords the only method by which
+this can be done.
+
+Many are the ways in which photography has aided research on the
+asteroid group. Since 1891 more than 600 of them have been discovered by
+photography, and it is many times easier to find the new object on the
+photographic plate than to detect it in the sky as was formerly done by
+means of star charts. The planet by its motion during the exposure of
+the plate produces a trail, whereas the surrounding stars are all round
+dots or images. Or by moving the plate slightly during exposure, as in
+Metcalf's ingenious method, we may catch the planet at that point where
+it will give a nearly circular image, and thus be quite as easy to
+detect, because all the stars on the same plate will then be trails.
+
+Photographic photometry of the asteroids has revealed marked variations
+in their light, due perhaps to irregularities of figure. On account of
+their faint light, the asteroids are especially suited, as Mars is not,
+to exact photography for ascertaining their parallax, and from this the
+sun's distance when the asteroid's distance has been found. Many
+asteroids have been utilized in this way, in particular Eros (433). In
+1931 it approaches the earth within 13 million miles, when the
+photographic method will doubtless give the sun's distance with the
+utmost accuracy.
+
+Photographs of Jupiter have been very successfully taken at the Yerkes
+and Lowell Observatories and elsewhere, but the great depth of the
+planet's atmosphere is highly absorptive, so that the impression is very
+weak in the neighborhood of the limb, if the exposure is correctly timed
+for the center of the disk. The striking detail of the belts, however,
+is excellently shown. Wood of Baltimore has obtained excellent results
+by monochromatic photography of Jupiter and Saturn with the 60-inch
+reflector on Mount Wilson. Jupiter's satellites have not been neglected
+photographically, and Pickering has observed hundreds of the eclipses
+of the satellites by a sort of cinematographic method of repeated
+exposures, around the time of disappearance and reappearance by eclipse.
+The newest outer satellites of Jupiter were all discovered by
+photography, and it is extremely doubtful if they would have been found
+otherwise.
+
+Saturn has long been a favorite object with the astronomical
+photographer, and there are many fine pictures in spite of his yellowish
+light, relatively weak photographically. The marvelous ring system with
+the Cassini division, the oblateness of the ball, the occasional
+markings on it--all are well shown in the best photographs; but the call
+is for more light and a more sensitive photographic process. Pickering's
+ninth satellite (Phoebe) was discovered by photography, one of the
+faintest moons in the solar system. Like the faint outer moons of
+Jupiter, few existing telescopes are powerful enough to show it. Its
+orbit has been found from photographic observations, and its position is
+checked up from time to time by photography.
+
+But the crowning achievement of spectrum photography in the Saturnian
+system is Keeler's application of Doppler's principle in determining the
+rate of orbital motion of particles in different zones of the rings,
+thereby establishing the Maxwellian theory of the constitution of the
+rings beyond the possibility of doubt. For Uranus and Neptune
+photography has availed but little, except to negative the existence of
+additional satellites of these planets, which doubtless would have been
+discovered by the thorough photographic search which has been made for
+them by W. H. Pickering without result.
+
+As with the asteroids, so with comets: several of these bodies have been
+discovered by photography; none more spectacular than the Egyptian comet
+of May 17th, 1882, which impressed itself on the plates of the corona of
+that date. Withdrawal of the sun's light by total eclipse made the comet
+visible, and it had never been seen before, nor is it known whether it
+will ever return. In cometary photography, much the same difficulties
+are present as in photographing the corona: if the plate is exposed long
+enough to get the faint extensions of the tail, the fine filaments of
+the coma or head are obliterated by halation and overexposure.
+
+No one has had greater success in this work than Barnard, whose
+photographs of comets, particularly at the Lick Observatory, are
+numerous and unexcelled. His photographs of the Brooks Comet of 1893
+revealed rapid and violent changes in the tail, as if shattered by
+encounter with meteors; and the tail of Halley's comet in 1910 showed
+the rapid propagation of luminous waves down the tail, similar to
+phenomena sometimes seen in streamers of the aurora. Draper obtained the
+first photograph of a comet's spectrum in 1881, disclosing an identity
+with hydrocarbons burning in a Bunsen flame, also bands in the violet
+due to carbon compounds. The photographic spectra of subsequent comets
+have shown bright lines due to sodium and the vapor of iron and
+magnesium.
+
+Even the elusive meteor has been caught by photography, first by Wolf in
+1891, who was exposing a plate on stars in the Milky Way. On developing
+it, he found a fine, dark nearly uniform line crossing it, due to the
+accidental flight across the field of a meteor of varying brightness.
+Since then meteor trails have been repeatedly photographed, and even
+the trail spectra of meteors have been registered on the Harvard plates.
+At Yale in 1894 Elkin employed a unique apparatus for securing
+photographic trails of meteors: six photographic cameras mounted at
+different angles on a long polar axis driven by clockwork, the whole
+arranged so as to cover a large area of the sky where meteors were
+expected.
+
+When we pass from the solar system to the stellar universe the
+advantages of photography and the amplification of research due to its
+employment as accessory in nearly every line of investigation are
+enormous. So extensively has photography been introduced that plates,
+and to a slight extent films, are now almost exclusively used in
+securing original records. Regrettably so in case of the nebulae, because
+the numerous photographs of the brighter nebulae taken since 1880 when
+Draper got the first photograph of the nebula of Orion, are as a rule
+not comparable with each other. Differences of instruments, of plates,
+of exposure, and development--all have occasioned differences in
+portrayal of a nebula which do not exist. When we consider faithful
+accuracy of portrayal of the nebulae for purposes of critical comparison
+from age to age, many of our nebular photographs of the past forty
+years, fine as they are and marvelous as they are, must fail to serve
+the purpose of revealing progressive changes in nebular features in the
+future.
+
+Roberts and Common in England were among the first to obtain nebular
+photographs with extraordinary detail, also the brothers Henry of Paris.
+As early as 1888 Roberts revealed the true nature of the great nebula in
+Andromeda, which had never been suspected of being spiral; and Keeler
+and Perrine at the Lick Observatory pushed the photographic discovery of
+spiral nebulae so far that their estimates fill the sky with many hundred
+thousands of these objects.
+
+In the southern hemisphere the 24-inch Bruce telescope of Harvard
+College Observatory has obtained many very remarkable photographs of
+nebulae, particularly in the vicinity of Eta Carinae. But the great
+reflectors of the Mount Wilson Observatory, on account of their
+exceptional location and extraordinary power, have surpassed all others
+in the photographic portrayal of these objects, especially of the spiral
+nebulae which appear to show all stages in transition from nebula to
+star. No less remarkable are the photographs of such wonderful clusters
+as Omega Centauri, a perfect visual representation of which is wholly
+impossible. Intercomparison of the photographs of clusters has afforded
+Bailey of Harvard, Shapley of Mount Wilson and others the opportunity of
+discovery that hundreds of the component stars are variable.
+
+What is the longest photographic exposure ever made? At the Cape of Good
+Hope, under the direction of the late Sir David Gill, exposures on
+nebulae were made, utilizing the best part of several nights, and
+totaling as high as seventeen, or even twenty-three hours. But the Mount
+Wilson observers have far surpassed this duration. To study the rotation
+and radial velocity of the central part of the nebula of Andromeda, an
+exposure of no less than 79 hours' total duration was made on the
+exceedingly faint spectrum, and even that record has since been
+exceeded. The eye cannot be removed from the guiding star for a moment
+while the exposure is in progress, and this tedious piece of work was
+rewarded by determining the velocity of the center of the nucleus as a
+motion of approach at the rate of 316 kilometers per second.
+
+But when the stars, their magnitudes and their special peculiarities are
+to be investigated _en masse_, photography provides the facile means for
+researches that would scarcely have been dreamed of without it. The
+international photographic chart of the entire heavens, in progress at
+twenty observatories since 1887, the photographic charts of the northern
+heavens at Harvard and of the southern sky at Cape Town, the manifold
+investigations that have led up to the Harvard photometry, and the
+unparalleled photographic researches of the Henry Draper Memorial,
+enabling the spectra of many hundred thousand stars to be examined and
+classified--all this is but a part of the astronomical work in stellar
+fields that photography has rendered possible.
+
+Then there are the stellar parallaxes, now observed for many stars at
+once photographically, when formerly only one star's parallax could be
+measured at a time and with the eye at the telescope. And photo-electric
+photometry, measuring smaller differences of light than any other
+method, and providing more accurate light-curves of the variable stars.
+And perhaps most remarkable of all, the radial velocity work on both
+stars and nebulae, giving us the distance of whole classes of stars,
+discovering large numbers of spectroscopic binaries and checking up the
+motion of the solar system toward Lyra within a fraction of a mile per
+second.
+
+All told, photography has been the most potent adjunct in astronomical
+research, and it is impossible to predict the future with more powerful
+apparatus and photographic processes of higher sensitiveness. The field
+of research is almost boundless, and the possibilities practically
+without limit.
+
+What would Herschel have done with L100,000--and photography!
+
+
+
+
+CHAPTER XXII
+
+MOUNTAIN OBSERVATORIES
+
+
+The century that has elapsed since the time of Sir William Herschel,
+known as the father of the new or descriptive astronomy, has witnessed
+all the advances of the science that have been made possible by adopting
+the photographic method of making the record, instead of depending upon
+the human eye. Only one eye can be looking at the eyepiece at a time:
+the photograph can be studied by a thousand eyes.
+
+At mountain elevations telescopes are now extensively employed, and
+there the camera is of especial and additional value, because the
+photograph taken on the mountain can be brought down for the expert to
+study, at ease and in the comfort of a lower elevation. We shall next
+trace the movement that has led the astronomer to seek the summits of
+mountains for his observatories, and the photographer to follow him.
+
+Not only did the genius of Newton discover the law of universal
+gravitation, and make the first experiments in optics essential to the
+invention of the spectroscope, but he was the real originator also of
+the modern movement for the occupation of mountain elevations for
+astronomical observatories. His keen mind followed a ray of light all
+the way from its celestial source to the eye of the observer, and
+analyzed the causes of indistinct and imperfect vision.
+
+Endeavoring to improve on the telescope as Galileo and his followers had
+left it, he found such inherent difficulties in glass itself that he
+abandoned the refracting type of telescope for the reflector, to the
+construction of which he devoted many years. But he soon found out, what
+every astronomer and optician knew to their keen regret, that a
+telescope, no matter how perfectly the skill of the optician's hand may
+make it, cannot perform perfectly unless it has an optically perfect
+atmosphere to look through.
+
+So Newton conceived the idea of a mountain observatory, on the summit of
+which, as he thought, the air would be not only cloudless, but so steady
+and equable that the rays of light from the heavenly bodies might reach
+the eye undisturbed by atmospheric tremors and quiverings which are
+almost always present in the lower strata of the great ocean of air that
+surrounds our planet.
+
+This is the way Newton puts the question in his treatise on
+_Opticks_--he says: "The Air through which we look upon the Stars, is in
+a perpetual Tremor; as may be seen by the tremulous Motion of Shadows
+cast from high Towers, and by the twinkling of the fix'd stars.... The
+only remedy is a most serene and quiet Air, such as may perhaps be found
+on the tops of the highest Mountains above the grosser Clouds."
+
+Newton's suggestion is that the _highest_ mountains may afford the best
+conditions for tranquillity; and it is an interesting coincidence that
+the summits of the highest mountains, about 30,000 feet in elevation,
+are at about the same level where the turbulence of the atmosphere most
+likely ceases, according to the indications of recent meteorological
+research. These heights are far above any elevations permanently
+occupied as yet, but a good beginning has been made and results of great
+value have already been reached.
+
+Curiously, investigation of mountain peaks and their suitability for
+this purpose was not undertaken till nearly two centuries after Newton,
+when Piazzi Smyth in 1856 organized his expedition to the summit of a
+mountain of quite moderate elevation, and published his "Teneriffe: an
+Astronomer's Experiment." Teneriffe is an accessible peak of about
+10,000 feet, on an island of the Canaries off the African coast, where
+Smyth fancied that conditions of equability would exist; and on reaching
+the summit with his apparatus and spending a few days and nights there,
+he was not disappointed. Could he have reached an elevation of 13,000
+feet, he would have had fully one-third of all the atmosphere in weight
+below him, and that the most turbulent portion of all. Nevertheless, the
+gain in steadiness of the atmosphere, providing "better seeing," as the
+astronomer's expression is, even at 10,000 feet, was most encouraging,
+and led to attempts on other peaks by other astronomers, a few of whom
+we shall mention.
+
+Davidson, an observer of the United States Coast Survey, with a broad
+experience of many years in mountain observing, investigated the summit
+of the Sierra Nevada mountains as early as 1872, at an elevation of
+7,200 feet. His especial object was to make an accurate comparison
+between elevated stations at different heights. He found the seeing
+excellent, especially on the sun; but the excessive snowfall at his
+station, 45 feet annually, was a condition very adverse to permanent
+occupation.
+
+In the summer of 1872, Young spent several weeks at Sherman, Wyoming, at
+an elevation exceeding 8,300 feet. He carried with him the 9.4-inch
+telescope of Dartmouth College, where he was then professor, and this
+was the first expedition on which a large glass was used by a very
+skillful observer at great elevation. He found the number of good days
+and nights small, but the sky was exceedingly favorable when clear. Many
+7th magnitude stars could be detected with the naked eye. Young's
+observations at Sherman were mainly spectroscopic, however, and they
+demonstrated the immense advantage of a high-level station, far above
+the dust and haze of the lower atmosphere. He pronounced the 9.4-inch
+glass at 8,000 feet the full equivalent of a 12-inch at sea level.
+
+Mont Blanc of 15,000 feet elevation was another summit where the veteran
+Janssen of Paris maintained a station for many years; but the
+continental conditions of atmospheric moisture and circulation were not
+favorable on the whole. Janssen was mainly interested in the sun, and
+the daylight seeing is rarely benefited, owing to the strong upward
+currents of warm air set in motion by the sun itself.
+
+Mountains in the beautiful climate of California were among the earliest
+investigated, and when in 1874 the trustees of Mr. James Lick's estate
+were charged with equipping an observatory with the most powerful
+telescope in existence, they wisely located on the summit of Mount
+Hamilton. It is 4,300 feet above sea level, and Burnham and other
+astronomers made critical tests of the steadiness of vision there by
+observing double stars, which afford perhaps the best means of comparing
+the optical quality of the atmosphere of one region with another. The
+writer was fortunate in having charge of the observations of the transit
+of Venus in 1882 on the mountain, when the Observatory was in process of
+construction, and the quality of the photographs obtained on that
+occasion demonstrated anew the excellence of the site. Particularly at
+night, for about nine months of the year, the seeing is exceptionally
+good, especially when fog banks rolling in from the Pacific, cover the
+valleys below like a blanket, preventing harmful radiation from the soil
+below.
+
+The great telescope mounted in 1888, a 36-inch refractor by Alvan Clark,
+has fulfilled every expectation of its projectors, and justified the
+selection of the site in every particular. The elevation, although
+moderate, is still high enough to secure very marked advantage in
+clearness and steadiness of the air, and at the same time not so high
+that the health and activities of the observers are appreciably affected
+by the thinner air of the summit. This telescope is known the world over
+for the monumental contributions to science made by the able astronomers
+who have worked with it: among them Barnard who discovered the fifth
+satellite of Jupiter in 1892; Burnham, Hussey, and Aitken, who have
+discovered and measured thousands of close double stars; Keeler, who
+spent many faithful years on the summit; and Campbell, the present
+director, whose spectroscopic researches on stellar movements have added
+greatly to our knowledge of the structure of the universe. Among the
+many lines of research now in progress at the Lick Observatory and in
+the D. O. Mills Observatory at Santiago, Chile, are the discoveries of
+stars whose velocities in space are not constant, but variable with the
+spectral type of the star. Mr. Lick's bequest for the Observatory was
+about $700,000. So ably has this scientific trust been administered that
+he might well have endowed it with his entire estate, exceeding
+$4,000,000.
+
+Another California mountain that was early investigated is Mount
+Whitney. Its summit elevation is nearly 15,000 feet, and in 1881 Langley
+made its ascent for the purpose of measuring the solar constant. He
+found conditions much more favorable than on Mount Etna,
+Sicily--elevation about 10,000 feet--which he had visited the year
+before. But the height of Mount Whitney was such as to occasion him much
+inconvenience from mountain sickness, an ailment which is most
+distressing and due partly to lack of oxygen and partly to mere
+diminution of mechanical pressure. Mount Whitney was also visited many
+years after by Campbell for investigating the spectrum of Mars in
+comparison with that of the moon. Langley found on Mount Whitney an
+excellent station lower down, at about 12,000 feet elevation; and by
+equipping the two stations with like apparatus for measuring the solar
+heat, he obtained very important data on the selective absorption of the
+atmosphere.
+
+Returning from the transit of Venus in 1882, Copeland of Edinburgh
+visited several sites in the Andes of Peru, ascending on the railway
+from Mollendo. Vincocaya was one of the highest, something over 14,000
+feet elevation. His report was most enthusiastic, not only as to
+clearness and transparency of the atmosphere, but also as to its
+steadiness, which for planetary and double star observations is almost
+as important. Copeland's investigation of this region of the Andes has
+led many other astronomers to make critical tests in the same general
+region. Climatic conditions are particularly favorable, and the sites
+for high-level research are among the best known, the atmosphere being
+not only clear a large part of the year, but in certain favored spots
+exceedingly steady.
+
+In 1887 the writer ascended the summit of Fujiyama, Japan, 12,400 feet
+elevation. The early September conditions as to steadiness of atmosphere
+were extraordinarily fine, but the mountain is covered by cloud many
+months in each year. There is a saddle on the inside of the crater that
+would form an ideal location for a high-level observatory. This
+expedition was undertaken at the request of the late Professor
+Pickering, director of Harvard College Observatory, which had recently
+received a bequest from Uriah A. Boyden, amounting to nearly a quarter
+of a million dollars, to "establish and maintain, in conjunction with
+others, an astronomical observatory on some mountain peak."
+
+Great elevations were systematically investigated in Colorado and
+California, the Chilean desert of Atacama was visited, and a temporary
+station established at Chosica, Peru, elevation about 5,000 feet.
+Atmospheric conditions becoming unfavorable, a permanent station was
+established in 1891 at Arequipa, Peru, elevation 8,000 feet, which has
+been maintained as an annex to the Harvard Observatory ever since. The
+cloud conditions have been on the whole less favorable than was
+expected, but the steadiness of the air has been very satisfactory. In
+addition to planetary researches conducted there in the earlier years by
+W. H. Pickering, many large programs of stellar research have been
+executed, especially relating to the magnitudes and spectra of the
+stars. In conjunction with the home observatory in the northern
+hemisphere, this afforded a vast advantage in embracing all the stars of
+the entire heavens, on a scale not attempted elsewhere. The Bruce
+photographic telescope of 24-inch aperture has been employed for many
+years at Arequipa, and with it the plates were taken which enabled
+Pickering to discover the ninth satellite of Saturn (Phoebe), and the
+splendid photographs of southern globular clusters in which Bailey has
+found numerous variable stars of very short periods--very faint objects,
+but none the less interesting, and of much significance in modern study
+of the evolution and structure of the stellar universe. The crowning
+research of the observatory is the Henry Draper catalogue of stellar
+spectra, now in process of publication, which is of the first order of
+importance in statistical studies of stellar distribution with reference
+to spectral type, and in studying the relation of parallax and distance,
+proper motion, radial velocity and its variation to the spectral
+characteristics of the stars.
+
+Perrine of Cordova is now establishing on Sierra Chica about twenty-five
+miles southwest of Cordova, a great reflecting telescope comparable in
+size with the instruments of the northern hemisphere, for investigation
+of the southern nebulae and clusters, and motions of the stars. The
+elevation of this new Argentine observatory will be 4,000 feet above sea
+level.
+
+Another observatory at mountain elevation and in a highly favorable
+climate is the Lowell Observatory, located at about 7,000 feet elevation
+at Flagstaff, Arizona. Many localities were visited and the atmosphere
+tested especially for steadiness, an optical quality very essential for
+research on the planetary surfaces. Mexico was one of these stations,
+but local air currents and changes of temperature there were such that
+good seeing was far from prevalent, as had been expected. At Flagstaff,
+on the other hand, conditions have been pretty uniformly good, and an
+enormous amount of work on the planet Mars has been accumulated and
+published. The first successful photographs of this planet were taken
+there in 1905, and Jupiter, Saturn, the zodiacal light and many other
+test objects have been photographed, which demonstrates the excellence
+of the site for astronomical research. Within recent years spectrum
+research by Slipher, especially on the nebulae, has been added to the
+program, and the rotation and radial velocities of many nebulae have been
+determined.
+
+On Mount Wilson, near Pasadena, California, at an elevation of nearly
+6,000 feet, is the Carnegie Solar Observatory, founded and equipped
+under the direction of Professor George E. Hale, as a department of the
+Carnegie Institution of Washington, of which Dr. John Campbell Merriam
+is President. The climatology of the region was carefully investigated
+and tests of the seeing made by Hussey and others. Although equipped
+primarily for study of the sun, the program of the observatory has been
+widely amplified to include the stars and nebulae. The instrumental
+equipment is unique in many respects. To avoid the harmful effect of
+unsteadiness of air strata close to the ground a tower 150 feet high was
+erected, with a dome surmounting it and covering a coelostat with mirror
+for reflecting the sun's rays vertically downward. Underneath the tower
+a dry well was excavated to a depth equal to 1/2 the height of the tower
+above it. In the subterranean chamber is the spectroheliograph of
+exceptional size and power. The sun's original image is nearly 17 inches
+in diameter on the plate, and the solar chromosphere and prominences,
+together with the photosphere and faculae, are all recorded by
+monochromatic light.
+
+Connected with the observatory on Mount Wilson are the laboratories,
+offices and instrument shops in Pasadena, 16 miles distant, where the
+remarkable apparatus for use on the mountain is constructed. A
+reflecting telescope with silver-on-glass mirror 60 inches in diameter
+was first built by Ritchey and thoroughly tested by stellar photographs.
+Also the northern spiral nebulae were photographed, exhibiting an
+extraordinary wealth of detail in apparent star formation. The success
+of this instrument paved the way for one similar in design, but with a
+mirror 100 inches in diameter, provided by gift of the late John D.
+Hooker of Los Angeles. The telescope was completed in 1919.
+Notwithstanding its huge size and enormous weight, the mounting is very
+successful, as well as the mirror. Mercurial bearings counterbalance the
+weight of the polar axis in large part. This great telescope, by far the
+largest and most powerful ever constructed, is now employed on a program
+of research in which its vast light-gathering power will be utilized to
+the full. Under the skillful management of Hale and his enthusiastic and
+capable colleagues, the confines of the stellar heavens will be
+enormously extended, and secrets of evolution of the universe and of its
+structure no doubt revealed.
+
+In all the mountain stations hitherto established, as the Lick
+Observatory at 4,000 feet, the Mount Wilson Observatory at 6,000 feet,
+the Lowell Observatory at 7,000 feet, the Harvard Observatory at 8,000
+feet; and Teneriffe and Etna at 10,000, Fujiyama at 12,000, Pike's Peak
+at 14,000, Mont Blanc and Mount Whitney at 15,000, the researches that
+have been carried on have fully demonstrated the vast advantage of
+increased elevation in localities where climatological conditions as
+well as elevation are favorable. Nevertheless, only one-half of the
+extreme altitude contemplated by Sir Isaac Newton has yet been attained.
+
+Can the greater heights be reached and permanently occupied?
+Geographically and astronomically the most favorably located mountain
+for a great observatory is Mount Chimborazo in Ecuador. Its elevation is
+22,000 feet, and it was ascended by Edward Whymper in 1880. Situated
+very nearly on the earth's equator, almost the entire sidereal heavens
+are visible from this single station, and all the planets are favored by
+circumzenith conditions when passing the meridian. No other mountain in
+the world approaches Chimborazo in this respect. But the summit is
+perpetually snow-capped, exceedingly inaccessible, and the defect of
+barometric pressure would make life impossible up there in the open.
+
+Only one method of occupation appears to be feasible. The permanent snow
+line is at about 16,000 feet, where excellent water power is available.
+By tunneling into the mountain at this point, and diagonally upward to
+the summit, permanent occupation could be accomplished, at a cost not to
+exceed one million dollars.
+
+The rooms of the summit observatory would need to be built as steel
+caissons, and supplied with compressed air at sea-level tension. The
+practicability of this plan was demonstrated by the writer in
+September, 1907, at Cerro de Pasco, Peru. A steel caisson was carried up
+to an elevation exceeding 14,000 feet. Patients suffering acutely with
+mountain sickness were placed inside this caisson, and on restoring the
+atmospheric pressure within it artificially all unfavorable
+symptoms--headache, high respiration and accelerated pulse--disappeared.
+There was every indication that if persons liable to this uncomfortable
+complaint were brought up to this elevation, or indeed any attainable
+elevation, under unreduced pressure, the symptoms of mountain sickness
+would be unknown. Comfortable occupation of the highest mountain summits
+was thereby assured.
+
+The working of astronomical instruments from within air-tight
+compartments does not present any insurmountable difficulties, either
+mechanical or physical. Since the time these experiments were made, the
+Guayaquil-Quito railway has been constructed over a saddle of
+Chimborazo, at an elevation of 12,000 feet; and only six miles of
+railway would need to be built from this station to the point where the
+tunnel would enter the mountain.
+
+Only by the execution of some such plan as this can astronomers hope to
+overcome the baleful effects of an ever mobile atmosphere, and secure
+the advantages contemplated by Sir Isaac Newton in that tranquillity of
+atmosphere, which he conceived as perpetually surrounding the summits of
+the highest mountains.
+
+In Russell's theory of the progressive development of the stars, from
+the giant class to the dwarf, an element of verification from
+observation is lacking, because hitherto no certain method of measuring
+the very minute angular diameters of the stars has been successfully
+applied. The apparent surface brightness corresponding to each spectral
+type is pretty well known, and by dividing it into the total apparent
+brightness, we have the angular area subtended by the star, quite
+independent of the star's distance. This makes it easy to estimate the
+angular diameter of a star, and Betelgeuse is the one which has the
+greatest angular diameter of all whose distances we know. Antares is
+next in order of angular diameter, 0".043, Aldebaran 0".022, Arcturus
+0".020, Pollux 0".013, and Sirius only 0".007.
+
+Can these theoretical estimates be verified by observation? Clearly it
+is of the utmost importance and the exceedingly difficult inquiry has
+been undertaken with the 100-inch reflector on Mount Wilson, employing
+the method of the interferometer developed by Michelson and described
+later on, an instrument undoubtedly capable of measuring much smaller
+angles than can be measured by any other known method. Unquestionably
+the interference of atmospheric waves, or in other words what
+astronomers call "poor seeing," will ultimately set the limit to what
+can be accomplished. "But even if," says Eddington, "we have to send
+special expeditions to the top of one of the highest mountains in the
+world, the attack on this far-reaching problem must not be allowed to
+languish."
+
+
+
+
+CHAPTER XXIII
+
+THE PROGRAM OF A GREAT OBSERVATORY
+
+
+The Mount Wilson Observatory has now been in operation about fifteen
+years. The novelty in construction of its instruments, the
+investigations undertaken with them and the discoveries made, the
+interpretation of celestial phenomena by laboratory experiment, and the
+recent addition to its equipment of a telescope 100 inches in diameter,
+surpassing all others in power, directs especial attention to the
+extensive activities of this institution, whose budget now exceeds a
+million dollars annually. Results are only achieved by a carefully
+elaborated program, such as the following, for which the reader is
+mainly indebted to Dr. Hale, the director of the observatory, who gives
+a very clear idea of the trend of present-day research on the magnetic
+nature of the sun, and the structure and evolution of the sidereal
+universe.
+
+The purpose of the observatory, as defined at its inception, was to
+undertake a general study of stellar evolution, laying especial emphasis
+upon the study of the sun, considered as a typical star; physical
+researches on stars and nebulae; and the interpretation of solar and
+stellar phenomena by laboratory experiments. Recognizing that the
+development of new instruments and methods afforded the most promising
+means of progress, well-equipped machine shops and optical shops were
+provided with this end in view.
+
+The original program of the observatory has been much modified and
+extended by the independent and striking discovery by Campbell and
+Kapteyn of an important relationship between stellar speed and spectral
+type; the demonstration by Hertzsprung and Russell of the existence of
+giant and dwarf stars; the successful application of the 60-inch
+reflector by Van Maanen to the measurement of minute parallaxes of stars
+and nebulae; the important developments of Shapley's investigation of
+globular star clusters; the possibilities of research resulting from
+Seares's studies in stellar photometry; and the remarkable means of
+attack developed by Adams through the method of spectroscopic
+parallaxes.
+
+By this method the absolute magnitude, and hence the distance of a star
+is accurately determined from estimates of the relative intensities of
+certain lines in stellar spectra. Attention was first directed toward
+lines of this character in 1906, when it was inferred that the weakening
+of some lines in the spectra of sun spots and the strengthening of
+others was the result of reduced temperature of the spot vapors. On
+testing this hypothesis by laboratory experiments, it was fully
+verified.
+
+Subsequently Adams, who had thus become familiar with these lines and
+their variability, studied them extensively in the spectra of other
+stars. In this way was discovered the dependence of their relative
+intensities on the star's absolute magnitude, so providing the powerful
+method of spectroscopic parallaxes.
+
+This method, giving the absolute magnitude as well as the distance of
+every star (excepting those of the earliest type) whose spectrum is
+photographed, is no less important from the evolutional than from the
+structural point of view.
+
+Investigations in solar physics which formerly held chief place in the
+research program have developed along unexpected lines. It could not be
+foreseen at the outset that solar magnetic phenomena might become a
+subject of inquiry, demanding special instrumental facilities, and
+throwing light on the complex question of the nature of the sun spots
+and other solar problems of long standing. It is obvious that these
+researches, together with those on the solar rotation and the motions of
+the solar atmosphere, developed by Adams and St. John, must be carried
+to their logical conclusion, if they are to be utilized to the fullest
+in interpreting stellar and nebular phenomena.
+
+The discovery of solar magnetism, like many other Mount Wilson results,
+was the direct outcome of a long series of instrumental developments.
+The progressive improvement and advance in size of the tools of research
+was absolutely necessary. Hale's first spectroheliograph at Kenwood in
+1890 was attached to a 12-inch refractor, and the solar image was but
+two inches in diameter. It was soon found that a larger solar image was
+essential, and a spectrograph of much greater linear dispersion; in
+fact, the spectrograph must be made the prime element in the
+combination, and the telescope so designed as to serve as a necessary
+auxiliary.
+
+Accordingly, successive steps have led through spectrographs of 18 and
+30 feet dimension to a vertical spectrograph 75 feet in focal length.
+The telescope is the 150 feet tower telescope, giving a solar image of
+16.5 inches in diameter. Its spectrograph is massive in construction,
+and by extending deep into the earth, it enjoys the stability and
+constancy of temperature required for the most exacting work.
+
+Another direct outgrowth of the work of sun-spot spectra is a study of
+the spectra of red stars, where the chemistry of these coolest regions
+of the sun is partially duplicated. The combination of titanium and
+oxygen, and the significant changes of line intensity already observed
+in both instances, and also in the electric furnace at reduced
+temperatures, give indication of what may be expected to result from an
+attack on the spectra of the red stars with more powerful instrumental
+means, which is now provided by the 100-inch telescope and its large
+stellar spectrograph.
+
+Other elements in the design of the 100-inch Hooker telescope have the
+same general object in view--that of developing and applying in
+astronomical practice the effective research methods suggested by recent
+advances in physics. Fresh possibilities of progress are constantly
+arising, and these are utilized as rapidly as circumstances permit.
+
+The policy of undertaking the interpretations of celestial phenomena by
+laboratory experiments, an important element in the initial organization
+of Mount Wilson, has certainly been justified by its results. Indeed,
+the development of many of the chief solar investigations would have
+been impossible without the aid of special laboratory studies, going
+hand in hand with the astronomical observations. So indispensable are
+such researches, and so great is the promise of their extension, that
+the time has now come for advancing the laboratory work from an
+accessory feature to full equality with the major factors in the work of
+the observatory. Accordingly a new instrument now under installation is
+an extremely powerful electro-magnet, designed by Anderson for the
+extension of researches on the Zeeman effect, and for other related
+investigations. Within the large and uniform field of this magnet, which
+is built in the form of a solenoid, a special electric furnace, designed
+for this purpose by King, is used for the study of the inverse Zeeman
+effect at various angles with the lines of force. This will provide the
+means of interpreting certain remarkable anomalies in the magnetic
+phenomena of sun spots.
+
+The 100-inch telescope is now in regular use. All the tests so far
+applied show that it greatly surpasses the 60-inch telescope in every
+class of work. For many months most of the observations and photographs
+have been made with the Cassegrain combination of mirrors, giving an
+equivalent focal length of 134 feet and involving three reflections of
+light. The 100-inch telescope is found to give nearly 2.8 times as much
+light as the 60-inch telescope, and therefore extends the scope of the
+instrument to all the stars an entire magnitude fainter. This is a very
+important gain for research on the faint globular clusters, as well as
+the small and faint spiral and planetary nebulae, providing a much larger
+scale for these objects and sufficient light at the same time.
+Photographs of the moon and many other less critical tests have been
+made with very satisfactory results. Those of the moon appear to be
+decidedly superior in definition to any previously taken with other
+instruments.
+
+Another investigation is of great importance in the light of recent
+advances in theoretical dynamics. Darwin, in his fundamental researches
+on the dynamics of rotating masses, dealt with incompressible matter,
+which assumes the well-known pear-shaped figure, and may ultimately
+separate into two bodies. Roche on the other hand discussed the
+evolution of a highly compressible mass, which finally acquires a
+lens-shaped form and ejects matter at its periphery. Both of these are
+extreme cases. Jeans has recently dealt with intermediate cases, such as
+are actually encountered in stars and nebulae. He finds that when the
+density is less than about one-fourth that of water, a lens-shaped
+figure will be produced with sharp edges, as depicted by Roche. Matter
+thrown off at opposite points on the periphery, under the influence of
+small tidal forces from neighboring masses, may take the form of two
+symmetric filaments, though it is not yet entirely clear how these may
+attain the characteristic configuration of spiral nebulae. The
+preliminary results of Van Maanen indicate motion outward along the
+arms, in harmony with Jeans's views.
+
+Jeans further discusses the evolution of the arms, which will break up
+into nuclei (of the order of mass of the sun) if they are sufficiently
+massive, but will diffuse away if their gravitational attraction is
+small. The mass of our solar system is apparently not great enough,
+according to Jeans, to account for its formation in this way. As is
+apparent, these investigations lead to conclusions very different from
+those derived by Chamberlin and Moulton from the planetesimal
+hypothesis.
+
+This is a critical study of spiral nebulae for which the 100-inch
+telescope is of all instruments in existence the best suited. The
+spectra of the spirals must be studied, as well as the motions of the
+matter composing the arms. Their parallaxes, too, must be ascertained.
+A photographic campaign including spiral nebulae of various types will
+settle the question of internal motions. The large scale of the spiral
+nebulae at the principal focus of the Hooker telescope, and the
+experience gained in the measurement of nebular nuclei for parallax
+determination, will help greatly in this research. A multiple-slit
+spectrograph, already applied at Mount Wilson, will be employed, not
+only on spiral nebulae whose plane is directed toward us, but also on
+those whose plane lies at an angle sufficient to permit both components
+of motion to be measured by the two methods.
+
+In dealing with problems of structure and motion in the Galactic system,
+the 100-inch telescope offers especial advantages, because of its vast
+light-gathering power. Studies of radial velocities of the stars have
+hitherto been necessarily confined to the brighter stars, for the most
+part even to those visible to the naked eye. While some of these are
+very distant, most of the stars whose radial velocities are known belong
+to a very limited group, perhaps constituting a distinct cluster of
+which the sun is a member, but in any event of insignificant proportions
+when contrasted with the Galaxy. Current spectrographic work with the
+60-inch telescope includes stars of the eighth magnitude, and some even
+fainter. But while the 60-inch has enabled Adams to measure the
+distances of many remote stars by his new spectroscopic method, and to
+double the known extent (so far as spectroscopic evidence is concerned)
+of the star streams of Kapteyn, a much greater advance into space is
+necessary to find out the community of motion among the stars comprising
+the Galactic system. The Hooker telescope will enable us to determine
+accurate radial velocities to stars of the eleventh magnitude, which
+doubtless truly represent the Galaxy.
+
+In order to secure a maximum return within a reasonable period of time,
+the stars in the selected areas of Kapteyn will be given the preference,
+because of the vast amount of work already done, relating to their
+positions, proper motions, and visual and photographic magnitudes. Such
+consideration as spectral type, the known directions of star-streaming,
+and the position of the chosen regions with reference to the plane of
+the Galaxy are given adequate weight, and it is of fundamental
+importance that the method of spectroscopic parallaxes will permit dwarf
+stars to be distinguished from stars that are in the giant class, but
+rendered faint by their much greater distance. In addition to these
+problems, the stellar spectrograms will provide rich material for study
+of the relationship between stellar mass and speed, and the nature of
+giant stars and dwarf stars.
+
+Shapley's recent studies of globular clusters have indicated the
+significance of these objects in both evolutional and structural
+problems, and the possibility of determining their parallaxes by a
+number of independent methods is of prime importance, both in its
+bearing on the structure of the universe and because it permits a host
+of apparent magnitudes to be at once transformed into absolute
+magnitudes. Here the advantage of the Hooker telescope is two-fold: at
+its 134-foot focus the increased scale of the crowded clusters makes it
+possible to select separate stars for spectrum photography (which could
+not be done with the 60-inch where the images were commingled); and the
+great gain in light is such that the spectra of stars to the 14th
+magnitude have been photographed in less than an hour.
+
+Faint globular clusters, then, will comprise a large part of the early
+program with the 100-inch telescope: the faintest possible stars in them
+must be detected and their magnitudes and colors measured; spectral
+types must be determined, and the radial velocities of individual stars
+and of clusters as a whole; spectroscopic evidence of possible axial
+rotation of globular clusters must be searched for; and the method of
+spectroscopic parallaxes, as well as other methods, must be applied to
+ascertaining the distances of these clusters.
+
+The possibility of dealing with many problems relating to the
+distribution and evolution of the faintest stars depends upon the
+establishment of photographic and photovisual magnitude scales. Below
+the twelfth magnitude, the only existing scale of standard visual or
+photovisual magnitudes is the Mount Wilson sequence, already extended by
+Seares to magnitude 17.5 with the 60-inch telescope.
+
+Extension of this scale to even fainter magnitudes, and its application
+to the faintest stars within its range is an important task for this
+great telescope, as it will doubtless bring within range hundreds of
+millions of stars that are beyond the reach of the 60-inch. The giants
+among them will form for us the outer boundary of the Galactic system,
+while the dwarfs will be of almost equal interest from the evolutional
+standpoint. The photometric program of the 100-inch, then, will deal
+with such questions as the condensation of the fainter stars toward the
+Galactic plane, the color of the most distant stars, and the final
+settlement of the long inquiry regarding the possible absorption of
+light in space.
+
+ [Illustration: GREAT SUN-SPOT GROUP, AUGUST 8, 1917. The disk in the
+ lower left corner represents the comparative size of the earth.
+ (_Photo, Mt. Wilson Solar Observatory._)]
+
+ [Illustration: THE SUN'S DISK. The view shows the "rice grain"
+ structure of the photosphere and brilliant calcium flocculi.
+ (_Photo, Yerkes Observatory._)]
+
+ [Illustration: THE LUNAR SURFACE VISIBLE DURING A TOTAL ECLIPSE OF
+ THE MOON, FEBRUARY 8, 1906. (_Photo, Yerkes Observatory._)]
+
+Another research of exceptional promise will be undertaken, which is of
+great importance in a general study of stellar evolution; and that is
+the determination of the spectral-energy curves of stars of various
+classes, for the purpose of measuring their surface temperatures. A very
+few of the nebulae are found to be variable, and their peculiarities need
+investigation, also special problems of variable stars and temporary
+stars, and the spectra of the components of close double stars which are
+beyond the power of all other instruments to photograph.
+
+Such a program of research conveys an excellent idea of many of the
+great problems that are under investigation by astronomers to-day, and
+gives some notion of the instrumental means requisite in executing
+comprehensive plans of this character. It will not escape notice that
+the climax of instrumental development attained at Mount Wilson has only
+been made possible by an unbroken chain of progress, link by link, each
+antecedent link being necessary to the successful forging of its
+following one. In very large part, and certainly indispensable to these
+instrumental advances, has the art of working in glass and metals been
+the mainstay of research. As we review the history of astronomical
+progress, from Galileo's time to our own, the consummate genius of the
+artisan and his deft handiwork compel our admiration almost equally with
+the keen intelligence of the astronomer who uses these powerful engines
+of his own devising to wrest the secrets of nature from the heavens.
+
+
+
+
+CHAPTER XXIV
+
+OUR SOLAR SYSTEM
+
+
+Now let us go upward in imagination, far, far beyond the tops of the
+highest mountains, beyond the moon and sun, and outward in space until
+we reach a point in the northern heavens millions and millions of miles
+away, directly above and equally distant from all points in the
+ecliptic, or path in which our earth travels yearly round the sun. Then
+we should have that sort of comprehensive view of the solar system which
+is necessary if we are to visualize as a whole the working of the vast
+machine, and the motions, sizes, and distances of all the bodies that
+comprise it. Of such stupendous mechanism our earth is part.
+
+Or in lieu of this, let us attempt to get in mind a picture of the solar
+system by means of Sir William Herschel's apt illustration: "Choose any
+well-leveled field. On it place a globe two feet in diameter. This will
+represent the sun; Mercury will be represented by a grain of mustard
+seed on the circumference of a circle 164 feet in diameter for its
+orbit; Venus, a pea on a circle of 284 feet in diameter; the Earth also
+a pea, on a circle of 430 feet; Mars a rather larger pin's head on a
+circle of 654 feet; the asteroids, grains of sand in orbits of 1,000 to
+1,200 feet; Jupiter, a moderate sized orange in a circle of nearly half
+a mile across; Saturn, a small orange on a circle of four-fifths of a
+mile; Uranus, a full-sized cherry or small plum upon the circumference
+of a circle more than a mile and a half; and finally Neptune, a
+good-sized plum on a circle about two miles and a half in diameter....
+To imitate the motions of the planets in the above mentioned orbits,
+Mercury must describe its own diameter in 41 seconds; Venus in 4
+minutes, 14 seconds; the Earth in 7 minutes; Mars in 4 minutes 48
+seconds; Jupiter in 2 minutes 56 seconds; Saturn in 3 minutes 13
+seconds; Uranus in 2 minutes 16 seconds; and Neptune in 3 minutes 30
+seconds."
+
+Now, let us look earthward from our imaginary station near the north
+pole of the ecliptic. All these planetary bodies would be seen to be
+traveling eastward round the sun, that is, in a counter-clockwise
+direction, or contrary to the motions of the hands of a timepiece. Their
+orbits or paths of motion are very nearly circular, and the sun is
+practically at the center of all of them except Mercury and Mars; of
+Venus and Neptune, almost at the absolute center. The planes of all
+their orbits are very nearly the same as that of the ecliptic, or plane
+in which the earth moves. These and many other resemblances and
+characteristics suggest a uniformity of origin which comports with the
+idea of a family, and so the whole is spoken of as the solar system, or
+the sun and his family of planets.
+
+In addition to the nine bodies already specified, the solar system
+comprises a great variety of other and lesser bodies; no less than
+twenty-six moons or satellites tributary to the planets and traveling
+round them in various periods as the moon does round our earth. Then
+between the orbits of Mars and Jupiter are many thousands of asteroids,
+so called, or minor planets (about 1,000 of them have actually been
+discovered, and their paths accurately calculated). And at all sorts of
+angles with the planetary orbits are the paths of hundreds of comets,
+delicate filmy bodies of a wholly different constitution from the
+planets, and which now and then blaze forth in the sky, their tails
+appearing much like the beam of a searchlight, and compelling for the
+time the attention of everybody. Connected with the comets and doubtless
+originally parts of them are uncounted millions of millions of meteors,
+which for the time become a part of the solar system, their minute
+masses being attracted to the planets, upon which they fall, those
+hitting the earth being visible to us as familiar shooting stars.
+
+We next follow the story of astronomy through the solar system,
+beginning with the sun itself and proceeding outward through his family
+of planets, now much more numerous and vastly more extended than it was
+to the ancient world, or indeed till within a century and a half of our
+own day.
+
+
+
+
+CHAPTER XXV
+
+THE SUN AND OBSERVING IT
+
+
+As lord of day, king of the heavens, mankind in the ancient world adored
+the sun. By their researches into the epoch of the Assyrians, Hittites,
+Phoenicians and other early peoples now passed from earth, archaeologists
+have unearthed many monuments that evidence the veneration in which the
+early peoples who inhabited Egypt and Asia Minor many thousand years ago
+held the sun. A striking example is found in the architecture of early
+Egyptian temples, on the lintels of which are carved representations of
+the winged globe or the winged solar disk, and there is a bare
+possibility that the wings of the globe were suggested by a type of the
+solar corona as glimpsed by the ancients.
+
+Little knew they about the distance and size of the sun; but the effects
+of his light and heat upon all vegetal and animal life were obvious to
+them. Doubtless this formed the basis for their worship of the sun.
+Occasional huge spots must have been visible to the naked eye, and the
+sun's corona was seen at rare intervals. Plutarch and Philostratus
+describe it very much as we see it to-day.
+
+How completely dependent mankind is upon the sun and its powerful
+radiations, only the science of the present day can tell us. By means of
+the sun's heat the forests of early geologic ages were enabled to wrest
+carbon from the atmosphere and store it in forms later converted by
+nature's chemistry into peat and coal. Through processes but imperfectly
+understood, the varying forms of vegetable life are empowered to
+conserve, from air and soil, nitrogen and other substances suitable for
+and essential to the life maintenance of animal creatures. Breezes that
+bring rain and purify the air; the energy of water held under storage in
+stream and dam and fall; trade winds facilitating commerce between the
+continents; oceanic currents modifying coastal climates; the violence of
+tornado, typhoon and water-spout, together with other manifestations of
+natural forces--all can be traced back to their origin in the tremendous
+heating power of the solar rays. In everything material the sun is our
+constant and bountiful benefactor. If his light and heat were withdrawn,
+practically every form of human activity on this planet would come to an
+early end.
+
+How far away is the sun? What is the size of the sun? These are
+questions that astronomers of the present day can answer with accuracy.
+
+So closely do they know the sun's distance that it is employed as their
+yardstick of the sky, or unit of celestial measurement. Many methods
+have been utilized in ascertaining the distance of the sun, and the
+remarkable agreement among them all is very extraordinary. Some of them
+depend upon pure geometry, and the basic measure which we make from the
+earth is not the distance of the sun directly; but we find out how far
+away Venus is during a transit of Venus, for example, or how far away
+Mars is or some of the asteroids are at their closer oppositions. Then
+it is possible to calculate how far away the sun is, because one
+measurement of distance in the solar system affords us the scale on
+which the whole structure is built. But perhaps the simplest method of
+getting the sun's distance is by the velocity of light, 186,300 miles a
+second. From eclipses of Jupiter's moons we know that light takes 8
+minutes 20 seconds to pass from sun to earth. So that the sun's distance
+is the simple product of the two, or 93 millions of miles.
+
+Once this fundamental unit is established, we have a firm basis on which
+to build up our knowledge of the distances, the sizes and motions of the
+heavenly bodies, especially those that comprise the solar system. We can
+at once ascertain the size of the sun, which we do by measuring the
+angle which it fills, that is, the sun's apparent diameter. Finding this
+to be something over a half a degree in arc, the processes of elementary
+trigonometry tell us that the sun's globe is 865,000 miles in diameter.
+For nearly a century this has been accurately measured with the greatest
+care, and diameters taken in every direction are found to be equal and
+invariably the same. So we conclude that the sun is a perfect sphere,
+and so far as our instruments can inform us, its actual diameter is not
+subject to appreciable change.
+
+The vastness of the sun's volume commands our attention. As his diameter
+is 110 times that of the earth, his mere size or volume is 110x110x110
+or 1,300 thousand times that of the earth, because the volumes of
+spheres are in proportion as the cubes of their diameters. If the
+materials that compose the sun were as heavy as those that make up the
+earth, it would take 1,300 thousand earths to weigh as much as the sun
+does. But by a method which we need not detail here, the sun's actual
+weight or mass is found to be only 300 thousand (more nearly 330,000),
+times greater than the earth's. So we must infer that, bulk for bulk,
+the component materials of the sun are about one-fourth lighter than
+those of the earth, that is, about one and one-half times as dense as
+water.
+
+To look at this in another way: it is known that a body falling freely
+toward the earth from outer space would acquire a speed of seven miles a
+second, whereas if it were to fall toward the sun instead, the velocity
+would be 383 miles a second on reaching his surface. If all the other
+bodies of the solar system, that is, the earth and moon, all the planets
+and their satellites, the comets and all were to be fused together in a
+single globe, it would weigh only one-seven hundred and fiftieth as much
+as the sun does.
+
+At the surface, however, the disproportion of gravity is not so great,
+because of the sun's vast size: it is only about twenty-eight times
+greater on the sun than on the earth; and instead of a body falling 16
+feet the first second as here, it would fall 444 feet there. Pendulums
+of clocks on the sun would swing five times for every tick here, and an
+athlete's running high jump would be scaled down to three inches.
+
+Let us next inquire into the amount of the sun's light and heat, and the
+enormously high temperature of a body whose heat is so intense even at
+the vast distance at which we are from it. The intensity of its
+brightness is such that we have no artificial source of light that we
+can readily compare it with. In the sky the next object in brightness is
+the full moon, but that gives less than the half-millionth part as much
+light as the sun. The standard candle used in physics gives so little
+light in comparison that we have to use an enormous number to express
+the quantity of light that the sun gives.
+
+A sperm candle burning 120 grains hourly is the standard, and if we
+compare this with the sun when overhead, and allow for the light
+absorbed by the atmosphere, we get the number 1575 with twenty-four
+ciphers following it, to express the candlepower of the sun's light. If
+we interpose the intense calcium light or an electric arc light between
+the eye and the sun, these artificial sources will look like black spots
+on the disk. Indeed, the sun is nearly four times brighter than the
+"crater," or brightest part of the electric arc. The late Professor
+Langley at a steel works in Pennsylvania once compared direct sunlight
+with the dazzling stream of molten metal from a Bessemer converter; but
+bright as it was, sunlight was found to be five thousand times brighter.
+
+Equally enormous is the heat of the sun. Our intensest sources of
+artificial heat do not exceed 4,000 degrees Fahrenheit, but the
+temperature at the sun's surface is probably not less than 16,000
+degrees F. One square meter of his surface radiates enough heat to
+generate 100,000 horsepower continuously. At our vast distance of 93
+millions of miles, the sun's heat received by the earth is still
+powerful enough to melt annually a layer of ice on the earth more than a
+hundred feet in thickness. If the solar heat that strikes the deck of a
+tropical steamship could be fully utilized in propelling it, the speed
+would reach at least ten knots.
+
+Many attempts have been made in tropical and sub-tropical climates to
+utilize the sun's heat directly for power, and Ericsson in Sweden,
+Mouchot in France, and Shuman in Egypt have built successful and
+efficient solar engines. Necessary intermission of their power at
+night, as well as on cloudy days, will preclude their industrial
+introduction until present fuels have advanced very greatly in cost. All
+regions of the sun's disk radiate heat uniformly, and the sun's own
+atmosphere absorbs so much that we should receive 1.7 times more heat if
+it were removed. So far as is known, solar light and heat are radiated
+equally in all directions, so that only a very minute fraction of the
+total amount ever reaches the earth, that is, 1 2200 millionth part of
+the whole. Indeed all the planets and other bodies of the solar system
+together receive only one one hundred millionth part; the vast remainder
+is, so far as we know, effectively wasted. It is transformed, but what
+becomes of it, and whether it ever reappears in any other form, we
+cannot say.
+
+How is this inconceivably vast output of energy maintained practically
+invariable throughout the centuries? Many theories have been advanced,
+but only one has received nearly universal assent, that of secular
+contraction of the sun's huge mass upon itself. Shrinkage means
+evolution of heat; and it is found by calculation that if the sun were
+to contract its diameter by shrinking only two-hundred and fifty feet
+per year, the entire output of solar heat might thus be accounted for.
+So distant is the sun and so slow this rate of contraction that
+centuries must elapse before we could verify the theory by actual
+measurements. Meanwhile, the progress of physical research on the
+structure and elemental properties of matter has brought to light the
+existence of highly active internal forces which are doubtless
+intimately concerned in the enormous output of radiant energy, though
+the mechanism of its maintenance is as yet known only in part.
+
+Abbot, from many years' observations of the solar constant, at
+Washington, on Mount Wilson, and in Algeria, finds certain evidence of
+fluctuation in the solar heat received by the earth. It cannot be a
+local phenomenon due to disturbances in our atmosphere, but must
+originate in causes entirely extraneous to the earth. Interposition of
+meteoric dust might conceivably account for it, but there is sufficient
+evidence to show that the changes must be attributed to the sun itself.
+The sun, then, is a variable star; and it has not only a period
+connected with the periodicity of the sun spots, but also an irregular,
+nonperiodic variation during a cycle of a week or ten days, though
+sometimes longer, and occasioning irregular fluctuations of two to ten
+per cent of the total radiation. Radiation is found to increase with the
+spottedness.
+
+Attempts have been made on the basis of the contraction theory to find
+out the past history of the sun and to predict its future. Probably 20
+to 50 millions of years in the past represents the life of the sun much
+as it is at present; and if solar radiation in the future is maintained
+substantially as now, the sun will have shrunk to one-half its present
+diameter in the next five million years.
+
+So far then as heat and light from the sun are concerned, the sun may
+continue to support life on the earth not to exceed ten million years in
+the future. But the sun's own existence, independently of the orbs of
+the system dependent upon it, might continue for indefinite millions of
+aeons before it would ever become a cold dead globe; indeed, in the
+present state of science, we cannot be sure that it is destined to reach
+that condition within calculable time.
+
+A few words on observing the sun, an object much neglected by amateurs.
+On account of the intense light, a very slight degree of optical power
+is sufficient. Indeed a piece of window glass, smoked in a candle flame
+with uniform graduation from end to end, will be found worth while in a
+beginner's daily observation of the sun. The glass should be smoked
+densely enough at one end so that the sunlight as seen through it will
+not dazzle the eye on the clearest days. At the other end of the glass,
+the degree of smoke film should not be quite so dense, so that the sun
+can be examined on hazy, foggy or partly cloudy days. An occasional
+naked-eye spot will reward the patient observer.
+
+If a small spyglass, opera glass or field glass is at hand, excellent
+views of the sun may be had by mounting the glass so that it can be held
+steadily pointed on the sun, and then viewing the disk by projection on
+a white card or sheet of paper. Care must be taken to get a good focus
+on the projected image, and then the faculae, or whitish spots, or
+mottling nearer the sun's edge will usually be well seen. By moving the
+card farther away from the eyepiece, a larger disk may be obtained, in
+effect a higher degree of magnification. But care must be used not to
+increase it too much. Keep direct sunlight outside the tube from falling
+on the card where the image is being examined. This is conveniently done
+by cutting a large hole, the size of the brass cell of the object glass,
+through a sheet of corrugated strawboard, and slipping this on over the
+cell. In this way the spots on the sun can be examined with ease and
+safety to the eye.
+
+For large instruments a special type of eyepiece is provided known as a
+helioscope, which disposes of the intense heat rays that are harmful to
+the eye. Frequent examination of the eyepiece should be made and the
+eyepiece cooled if necessary. That part of the sun's surface under
+observation is known as the photosphere, that is, the part which
+radiates light. If the atmosphere admits the use of high magnifying
+powers, the structure of the photosphere will be found more and more
+interesting the higher the power employed. It is an irregularly mottled
+surface showing a species of rice-grain structure under fairly high
+magnification. These grains are grouped irregularly and are about 500
+miles across. Under fine conditions of vision they may be subdivided
+into granules. The faculae, or white spots, are sometimes elevations
+above the general solar level; they have occasionally been seen
+projecting outside the limb, or edge of the disk.
+
+
+
+
+CHAPTER XXVI
+
+SUN SPOTS AND PROMINENCES
+
+
+Dark spots of a deep bluish black will often be seen on the photosphere
+of the sun. Sometimes single, though generally in groups, the larger
+ones will have a dark center, called the umbra, surrounded by the very
+irregular penumbra which is darker near its outer edge and much brighter
+apparently on its inner edge where it joins on the umbra. The penumbra
+often shows a species of thatch-work structure, and systematic sketches
+of sun spots by observers skilled in drawing are greatly to be desired,
+because photography has not yet reached the stage where it is possible
+to compete with visual observation in the matter of fine detail. The
+spots themselves nearly always appear like depressions in the
+photosphere, and on repeated occasions they have been seen as actual
+notches when on the edge of the sun.
+
+Many spots, however, are not depressions: some appear to be actual
+elevations, with the umbra perhaps a central depression, like the crater
+in the general elevation of a volcano. Spots are sometimes of enormous
+size. The largest on record was seen in 1858; it was nearly 150,000
+miles in breadth, and covered a considerable proportion of the whole
+visible hemisphere of the sun. A spot must be nearly 30,000 miles across
+in order to be seen with the naked eye.
+
+In their beginning, development, and end, each spot or group of spots
+appears to be a law unto itself. Sometimes in a few hours they will
+form, though generally it is a question of days and even weeks. Very
+soon after their formation is complete, tonguelike encroachments of the
+penumbra appear to force their way across the umbra, and this splitting
+up of the central spot usually goes on quite rapidly. Sun spots in
+violent disturbance are rarely observed. As the sun turns round on his
+axis, the spots will often be carried across the disk from the center to
+the edge, when they become very much foreshortened. The sun's period of
+rotation is 28 days, so that if a spot lasts more than two weeks without
+breaking up, it may reappear on the eastern limb of the sun after having
+disappeared at the western edge. Two or three months is an average
+duration for a spot; the longest on record lasted through 18 months in
+1840-41.
+
+The position of the sun's axis is well known, its equator being tilted
+about 7 degrees to the ecliptic, and the spots are distributed in zones
+north and south of the equator, extending as far as 30 degrees of solar
+latitude. In very high latitudes spots are never seen; they are most
+abundant in about latitude 15 degrees both north and south, and rather
+more numerous in the northern than in the southern hemisphere of the
+sun. Recent research at Mount Wilson makes the sun a great magnet; and
+its magnetic axis is inclined at an angle of 6 degrees to the axis of
+rotation, around which it revolves in 32 days.
+
+There is a most interesting periodicity of the spots on the sun, for
+months will sometimes elapse with spots in abundance and visible every
+day, while at other periods, days and even weeks will elapse without a
+single spot being seen. There is a well recognized period of eleven and
+one-tenth years, the reason underlying which is not, however, known.
+After passing through the minimum of spottedness, they begin to break
+out again first in latitudes of 25 degrees-30 degrees, rather suddenly,
+and on both sides of the equator, and they move toward the equator as
+their number and individual size decrease.
+
+The last observed epoch of maximum spot activity on the sun was passed
+in 1917.
+
+Many attempts have been made to ascertain the cause of the periodicity
+of sun spots, but the real cause is not yet known. If the spots are
+eruptional in character, the forces held in check during seasons of few
+spots may well break out in period. The brighter streaks and mottlings
+known as faculae are probably elevations above the general photosphere,
+and seem to be crusts of luminous matter, often incandescent calcium,
+protruding through from the lower levels. Generally the faculae are
+numerous around the dark spots, and absorption of the sun's light by his
+own atmosphere affords a darker background for them, with better
+visibility nearer the rim of the solar disk. The spectroheliograph
+reveals vast zones of faculae otherwise invisible, related to the
+sun-spot zones proper on both sides of the equator.
+
+In some intimate way the magnetism of sun and earth are so related that
+outbreaks of solar spots are accompanied with disturbances of electrical
+and other instruments on the earth; also the aurora borealis is seen
+with greater frequency during periods when many spots are visible.
+
+Within very recent years the discovery of a magnetic field in sun spots
+has been made by Hale with powerful instruments of his own design. Sun
+spots had never been investigated before with adequate instrumental
+means. He recognized the necessity of having a spectroscope that would
+record the widened lines of sun-spot spectra, and the strengthened and
+weakened lines on a large scale. Certain changes in relative intensity
+were traced to a reduced temperature of the spot vapors by comparison
+with photographs of the spectrum of iron and other metallic vapors in an
+electric arc at different temperatures. Here the work of the laboratory
+was essential. Sun spots were thus found to be regions of reduced
+temperature in the solar atmosphere. Chemical unions were thus possible,
+and thousands of faint lines in spot-spectra were measured and
+identified as band lines due to chemical compounds. Thus the chemical
+changes at work in sun-spot vapors were recognized.
+
+Then followed the highly significant investigations of solar vortices
+and magnetic fields. Improvements in photographic methods had revealed
+immense vortices surrounding sun spots in the higher part of the
+hydrogen atmosphere; and this led to the hypothesis that a sun spot is a
+solar storm, resembling a terrestrial tornado, and in which the hot
+vapors whirling at high velocity are cooled by expansion. This would
+account for the observed intensity changes of the spectrum lines and the
+presence of chemical compounds. The vortex hypothesis suggested an
+explanation of the widening of many spot lines, and the doubling or
+trebling of some of them. As it is known that electrons are emitted by
+hot bodies, they must be present in vast numbers in the sun; and
+positive or negative electrons, if caught and whirled in a vortex, would
+produce a magnetic field.
+
+Zeeman in 1896 had discovered that the lines in the spectrum of a
+luminous vapor in a magnetic field are widened, or even split into
+several components if the field is strong enough. Characteristic effects
+of polarization appear also. The new apparatus of the observatory in
+conjunction with experiments in the laboratory immediately provided
+evidence that proved the existence of magnetic fields in sun spots, and
+strengthened the view that the spots are caused by electric vortices.
+
+Extended investigations have led Hale to the conclusion that the sun
+itself is a magnet, with its poles situated at or near the poles of
+rotation. In this respect the sun resembles the earth, which has long
+been known to be a magnet. The sun's axial rotation permits
+investigation of the magnetic phenomena of all parts of its surface, so
+that ultimately the exact position of the sun's magnetic poles and the
+intensity of the field at different levels in the solar atmosphere will
+be ascertained. Schuster is of the opinion that not only the sun and
+earth, but every star, and perhaps every rotating body, becomes a magnet
+by virtue of its rotation. Hale is confident that the 100-inch reflector
+will permit the test for magnetism to be applied to a few of the stars.
+
+The sun can be observed at Mount Wilson on at least nine-tenths of all
+the days in the year, and a daily record of the polarities of all spots
+with the 150-foot tower telescope is a part of the routine. A method has
+been devised for classifying sun spots on the basis of their magnetic
+properties, and more than a thousand spots have already been so
+classified. About 60 per cent of all sun spots are found to be binary
+groups, the single or multiple members of which are of opposite magnetic
+polarity. Unipolar spots are very seldom observed without some
+indication of the characteristics of bipolar groups. These are usually
+exhibited in the form of flocculi following the spot. The bipolar spot
+seems to be the dominant type, and the unipolar type a variant of it.
+
+Although devised for quite another purpose, that of photographing the
+hydrogen prominences on the limb of the sun, the spectroheliograph has
+contributed very effectively to many departments of solar research. The
+prominences are dull reddish cloudlets that were first seen during total
+eclipses of the sun. Probably Vassenius, a Swedish astronomer, during
+the total eclipse of 1733, made the earliest record of them, as pinkish
+clouds quite detached from the edge of the moon; and in that day, when
+it had not yet been proved that the moon was without atmosphere, he
+naturally thought they belonged to the moon, not the sun. Undoubtedly
+Ulloa, a Spanish admiral, also saw the prominences in observing the
+total eclipse of 1778; but they seem to have attracted little attention
+till 1842, when a very important total eclipse was central throughout
+Europe, and observed with great care by many of the eminent astronomers
+of all countries.
+
+So different did the prominences appear to different eyes, and so many
+were the theories as to what they were, that no general consensus of
+opinion was reached, and some thought them no part of either sun or
+moon, but a mere mirage or optical illusion. But at the return of this
+eclipse in 1860, photography was employed so as to demonstrate beyond a
+shadow of doubt the real existence and true solar character of the
+prominences. By the slow progress of the moon across the sun and the
+prominences on the edge, a unique series of photographs by De la Rue
+showed the moon's edge gradually cutting off the prominences piecemeal
+on one side of the sun, and equally gradually uncovering them on the
+opposite side.
+
+The prominences, then, were known to be real phenomena of the sun, some
+of them disconnectedly floating in his atmosphere, as if clouds. Their
+forms did not vary rapidly, they were very abundant, and their light was
+so rich in rays of great photographic intensity that many were caught on
+the plate which the eye failed to see; they appeared at every part of
+the sun's limb and their height above it indicated that they must be
+many thousand miles in actual dimension. What they were, however,
+remained an entire mystery, and no one even thought it possible to find
+out what their chemical constitution might be or to measure the speed
+with which they moved.
+
+A few years later came the great Indian eclipse (August 28, 1868), at
+that date the longest total eclipse ever observed. Janssen of France and
+many others went out to India to witness it. Fortunately the prominences
+were very brilliant and this led Janssen to believe it would be possible
+for him to see them the day after the eclipse was over. By modifying the
+adjustment of his apparatus suitably and changing its relation to the
+sun's edge, he found that hydrogen is the main constituent in the light
+of the prominences. In addition to this he was able to trace out the
+shapes of the prominences, and even measure their dimensions. His
+station in India was at Guntoor, many weeks by post from home; so that
+his account of this important discovery reached the Paris Academy of
+Sciences for communication with another from the late Sir Norman Lockyer
+of England, announcing a like discovery, wholly independently.
+
+The principle is simply this, and admirably stated by Young: "Under
+ordinary circumstances the prominences are invisible, for the same
+reason as the stars in the daytime: they are hidden by the intense light
+reflected from the particles of our own atmosphere near the sun's place
+in the sky; and if we could only sufficiently weaken this aerial
+illumination, without at the same time weakening _their_ light, the end
+would be gained. And the spectroscope accomplishes this very thing.
+Since the air-light is reflected sunshine, it of course presents the
+same spectrum as sunlight, a continuous band of color crossed by dark
+lines. Now, this sort of spectrum is greatly weakened by every increase
+of dispersive power, because the light is spread out into a longer
+ribbon and made to cover a more extended area. On the other hand, a
+spectrum of bright lines undergoes no such weakening by an increase in
+the dispersive power of the spectroscope. The bright lines are only more
+widely separated--not in the least diffused or shorn of their
+brightness."
+
+Simultaneous announcement of this great discovery, by astronomers of
+different nations, working in widely separate regions of the earth, led
+to the striking of a gold medal by the French Government in honor of
+both astronomers and bearing their united effigies. Ever since the
+famous Indian eclipse of 1868, it has not been necessary to wait for a
+total eclipse in order to observe the solar prominences, but every
+observer provided with suitable apparatus has been able to observe them
+in full sunlight whenever desired, and the charting of them is part of
+the daily routine at several observatories in different parts of the
+world. So vast has been the accumulation of data about them that we know
+their numbers to fluctuate with the spots on the sun; and their
+distribution over the sun's surface resembles in a way that of the
+spots.
+
+While the spots and protuberances are most numerous around solar
+latitude 20 degrees both north and south, the prominences do not
+disappear above latitude 35 to 40 degrees, as the spots do, but from
+latitude 60 degrees they increase in number to about 75 degrees, and are
+occasionally observed even at the sun's poles. Faculae and prominences
+are more closely related than the sun spots and prominences. There are
+wide variations in both magnitude and type of the prominences. Heights
+above the sun's limb of a few thousand miles are very common, and they
+rarely reach elevations as great as 100,000 miles, though a very
+occasional one reaches even greater heights.
+
+Classification of the prominences divides them into two broad types, the
+quiescent and the eruptive. The former are for the most part hydrogen,
+and the latter metallic. The quiescent prominences resemble closely the
+stratus and cirrus type of terrestrial clouds, and are frequently of
+enormous extent along the sun's edge. They are relatively long-lived,
+persisting sometimes for days without much change. The eruptive
+prominences are more brilliant, changing their form and brightness
+rapidly. Often they appear as brilliant spikes or jets, reaching
+altitudes that average about 25,000 miles. Rarely seen near the sun's
+poles, they are much more numerous nearer the sun spots. Speed of
+motion of their filaments sometimes exceeds one hundred miles a second,
+and the changing variety of shapes of the eruptive prominences is most
+interesting. Oftentimes they change so rapidly that only photography can
+do them justice.
+
+Prominence photography began with Young a half century ago, who obtained
+the first successful impression on a microscope slide with a sensitized
+film of collodion; as was necessary in the earlier wet-plate process of
+photography, which required exposures so long that little progress was
+effected for about twenty years. Then it was taken up by Deslandres of
+Paris and Hale of Chicago independently, both of whom succeeded in
+devising a complex type of apparatus known as the spectroheliograph, by
+which all the prominences surrounding the entire limb of the sun can be
+photographed at any time by light of a single wave-length, together with
+the disk of the sun on the same negative.
+
+The prominences appear to be intimately connected with a gaseous
+envelope surrounding the solar photosphere, in which sodium and
+magnesium are present as well as hydrogen. The depth of the chromosphere
+is usually between 5,000 and 10,000 miles, and its existence was first
+made out during the total solar eclipses of 1605 and 1706, when it
+appeared as an irregular rose-tinted fringe, though not at the time
+recognized as belonging to the sun.
+
+The constitution of the sun and its envelopes are still under
+discussion, and no complete theory of the sun has yet been advanced
+which commands the widest acceptance. Of the interior of the sun we can
+only surmise that it is composed of gases which, because of intense heat
+and compression, are in a state unfamiliar on earth and impossible to
+reproduce in our laboratories. Their consistency may be that of melted
+pitch or tar.
+
+Surrounding the main body of the sun are a series of layers, shells, or
+atmospheres. Outside of all and very irregular in structure, indeed
+probably not a solar atmosphere at all, is the solar corona, parts of
+which behave much as if it were an atmosphere, but it appears to be
+bound up in some way with the sun's radiation. It has streamers that
+vary with the sun-spot period, but its constitution and function are
+very imperfectly known, because it has never been seen or photographed
+except at rare intervals on occasion of total eclipses of the sun.
+
+Beneath the corona we meet the projecting prominences, to which parts of
+the corona are certainly related, and beneath them the first true layer
+or atmosphere of the sun known as the chromosphere, its average depth
+being about one-hundredth part of the sun's diameter. Beneath the
+chromosphere is the layer of the sun from which emanates the light by
+which we see it, called the photosphere. It appears to be composed of
+filaments due to the condensation of metallic vapors, and it is the
+outer extremities of these filaments which are seen as the granular
+structures everywhere covering the disk of the sun. Their light shines
+through the chromosphere and the spots are ruptures in this envelope.
+
+Between photosphere and chromosphere is a very thin envelope, probably
+not over 700 miles in thickness, called the reversing layer. It is this
+relatively thin shell that is responsible for the absorption which
+produces the dark lines in the spectrum of the sun. Under normal
+conditions the filaments of the photosphere are radial, that is vertical
+on the sun; but whenever eruptions take place, as during the occurrence
+of spots, the adjacent filaments are violently swept out of their normal
+vertical lines and these displaced columns then form what we view as the
+spot's penumbra. From the outer surface of the sun's chromosphere rise
+in eruptive columns vapors of hydrogen and the various metals of which
+the sun is composed. These and the spots would naturally occur in
+periods just as we see them.
+
+We have said that the sun is composed of a mass of highly heated or
+incandescent vapors or gases, whose compression on account of gravity
+must render their physical condition quite different from any gaseous
+forms known on the earth or which we can reproduce here. As the result
+of more than half a century of studious observation of the sun and
+mapping of its spectrum in every part, and diligent comparison with the
+spectra of all known chemical elements on the earth, we find that the
+sun contains no elements not already found here, but that a great
+preponderance of elements known to earth are found in the sun.
+
+The intensity of their spectral lines is one prominent indication of the
+presence of elements in the sun, and the number of coincidences of
+spectral lines is another. Iron, nickel, calcium, manganese, sodium,
+cobalt, and carbon are among the elements most strongly identified. A
+few of the rarer terrestrial elements are of doubtful existence in the
+sun, and a very few, as gold, bismuth, antimony, and sulphur are not
+found there, and the existence of oxygen in the sun is regarded by some
+experts as doubtful. But if the whole earth were vaporized by heat,
+probably its spectrum would resemble that of the sun very closely.
+
+What are the effects of the sun, and sun spots in particular, on our
+weather? Is the influence of their periodicity potent or negligible? If
+we investigate conditions pertaining to terrestrial magnetism, as
+fluctuations of the magnetic needle, and the frequency of aurorae, there
+is no occasion for doubt of the sun's direct influence, although we are
+not able to say just how that influence becomes potent. If, however, we
+look into questions of temperature, barometric pressure, rainfall,
+cyclones, crops, and consequent financial conditions, we find fully as
+much evidence against solar influence as for it. The slight variations
+of the sun's light and heat due to the presence or absence of sun spots
+can scarcely be sensible, and much longer periods of closer observation
+are necessary before such questions can be finally decided. The slighter
+such influences are, if they actually exist, and the more veiled they
+are by other influences more or less powerful, the more difficult it is
+to discover their effects with certainty.
+
+The importance of solar radiation in the prediction of terrestrial
+weather has long been recognized, but until very recently no practical
+application has been made. The Smithsonian Astrophysical Observatory at
+Washington, under the direction of Dr. Abbot, has for many years carried
+on at a number of stations a series of determinations of the constant of
+solar radiation by the spectro-bolometric method originated by Langley.
+A new station in Calama, Chile, has recently been inaugurated, at which
+the solar constant is worked out each day, and telegraphed to the
+Argentine weather service, where it is employed in forecasting for the
+day.
+
+Abbot's new method of solar constant determination is based on the fact
+that atmospheric transparency varies oppositely to the variations of
+brightness of the sky. Increase of haziness presents more reflecting
+surface to scatter the solar rays indirectly to the earth. Of course it
+presents also additional surface to obstruct the direct rays from the
+sun. By measuring the brightness of the sky near the sun, it becomes
+possible to infer the coefficients of atmospheric transmission at all
+wave lengths. The direct observations and the complete deduction of the
+solar constant for the day can all be completed within two or three
+hours.
+
+Clayton of Buenos Aires has now employed these results in the Argentine
+weather predictions for two years, and the introduction of this new
+element in forecasting has brought about a pronounced gain in the value
+of the predictions. Its adoption by the weather bureaus of other nations
+will doubtless come in due time, and the new method take a firmly
+established rank in practical meteorology.
+
+Abbot's observations many years ago first called attention to the
+variability of the solar constant through a range of several per cent
+both from year to year, and in irregular short periods of weeks or even
+days. Abbot considers this the more likely explanation than that
+atmospheric changes should take place simultaneously all over the earth.
+The sun is but a star, the stars that are irregularly variable in light
+and heat are numerous, and the sun itself appears to be one of these.
+
+Especially important to the agricultural and vineyard interests of
+Argentina is the question of precipitation, and Clayton finds this very
+dependent on solar radiation. At epochs of practically stationary solar
+intensity, there is little or no precipitation; but quite generally he
+finds that great decrease of solar radiation is followed in from three
+to five days by heavy precipitation. Direct temperature effects are also
+traced in Buenos Aires and other South American cities, lagging from two
+to three days behind the observed solar fluctuations.
+
+The station at Calama yields about 250 determinations of the solar
+constant each year, and the Mount Wilson station about half that number.
+They are the only stations of this character at present in existence,
+and others should be established in widely separated and cloudless
+regions, as Egypt, southern California and Australia. Uniformity in the
+methods of observing would be highly desirable, and the Smithsonian
+Institution has perfected the details of common control of such stations
+which it is expected may be established at an early day.
+
+
+
+
+CHAPTER XXVII
+
+THE INNER PLANETS
+
+
+VULCAN
+
+About the middle of the last century, Le Verrier, a great French
+astronomer, having added the planet Neptune beyond the outside confines
+of the solar system, sought evidence of a lesser planet traveling round
+the sun within the orbit of Mercury. For many years close watch was kept
+on the sun in the hope of discovering such a body in the act of passing
+across the disk, or in transit, as it is technically termed.
+Lescarbault, a French physician, announced that he had actually seen
+such a planet, Vulcan it was called, passing over the sun in 1859. Total
+eclipses of the sun would afford the best opportunity for seeing such a
+body, and on several such occasions astronomers thought they had found
+it. But the signal advantages of photography have been applied so often
+to this search, and always unsuccessfully, that the existence of Vulcan,
+or the intramercurian planet, is now regarded as mythical.
+
+
+MERCURY
+
+This planet is an elusive body that very few, even astronomers, have
+ever seen. It is not very bright, has a rapid motion and never retreats
+far from the sun, so that it was a puzzle to the ancients who saw it,
+sometimes in the twilight after sunset and again in the twilight of
+dawn. When following the sun down in the west, in March or April,
+Mercury is likely to be best seen; twinkling rather violently and nearly
+as bright as a star of the first magnitude.
+
+Very little is to be seen on the minute disk of this planet, except that
+it goes through all the phases of the moon--crescent, gibbous, full,
+gibbous, crescent. Whether Mercury turns round on its axis or not,
+cannot be said to be known, because the markings that are suspected on
+its surface are too indefinite to permit exact observation. More than
+likely the planet presents always the same side or face to the sun, so
+that it turns round on its axis once, while traveling once around the
+sun in its orbit. Mercury's day and year would therefore be equal in
+length. Nor have we much evidence on the question of an atmosphere
+surrounding Mercury; probably it is very thin, if indeed there is any at
+all. When Mercury comes directly between us and the sun, crossing in
+transit, the edge of the planet as projected against the sun is very
+sharply defined, and this would indicate an absence of atmosphere on
+Mercury.
+
+Transits of Mercury can occur in May and November only: there was one on
+November 7, 1914, and there will be one on May 7, 1924. The latter will
+be nearly eight hours in length, which is almost the limit. Mercury's
+distance from the sun averages 36 million miles, the diameter of the
+planet is 3,000 miles, and his orbital speed is 30 miles per second, the
+swiftest of all the planets. No moon of Mercury is known to exist,
+although many times diligently searched for, especially during transits
+of the planet.
+
+
+VENUS
+
+Brightest of all the planets, and the most beautiful of all is Venus.
+Its path is next outside the orbit of Mercury, but within that of the
+earth, so that it partakes of all the phases of the moon. Like Mercury
+it sometimes passes exactly between us and the sun, a rare phenomenon
+which is known as a transit of Venus.
+
+Being without telescopes, the ancients knew nothing about these
+occurrences, but they were puzzled for centuries over the appearance of
+the planet in the west after sunset, when they called it Hesperus, and
+in early dawn in the east when they gave it the name Phosphorus.
+
+Venus is known to be girdled with an atmosphere denser than ours, and it
+seems to be always filled with dense clouds. It is the reflection of
+sunlight from this perpetually cloudy exterior which gives Venus her
+singular radiance. So brilliant is she that even full daylight is not
+strong enough to overpower her rays; and she may often be seen
+glistening in the clear blue daytime sky, if one knows pretty nearly in
+what direction to look for her.
+
+Venus is 67 million miles from the sun, and as our own distance is 93
+million miles, this planet can come within 26 million miles of the
+earth. It is therefore at times our nearest known neighbor in space,
+excepting only the Moon and Eros, one of the erratic little planets that
+travel round the sun between Mars and Jupiter. Also possibly a comet
+might come much nearer.
+
+Astronomers always take advantage of this nearness of Venus to us, if a
+transit across the sun takes place; because it affords an excellent
+method of finding out what the distance of the sun is from the Earth. A
+pair of these transits happens about once a century, there were transits
+in 1874 and 1882, and the next pair occur in 2004 and 2012. In actual
+size, Venus is almost as large a planet as our own, being 7,700 miles
+in diameter, as compared with 7,920 for the earth. Her velocity in her
+orbit is twenty-two miles per second, and she travels all the way round
+the sun in seven and one half months or 225 days.
+
+Venus from her striking brilliancy always leads the novice to expect to
+see great things on applying the telescope. But aside from a brilliant
+disk, now a slender crescent, now half full like the moon at quarter,
+and again gibbous as the moon is between quarter and full, the telescope
+reveals but little. There is pretty good evidence that the markings
+thought to have been seen on the planet's surface are illusory, and so
+it is wholly uncertain in what direction the planet's axis lies; also
+there is great uncertainty about the length of the day on Venus, or the
+period of turning round on its axis. Probably it is the same in length
+as the planet's year.
+
+Once when Venus passed very close to the sun, just barely escaping a
+transit, Lyman of Yale University caught sight of it by hiding the sun
+behind a tall building or church spire. The dark side of Venus was
+turned toward us and he could not of course see that. But the planet was
+clearly there, completely encircled by a narrow delicate luminous ring,
+which was due to sunlight shining through the atmosphere that surrounds
+the planet. Similar ring effects were seen by observers of the transits
+of Venus in 1874 and 1882; and from all their observations it is
+concluded that Venus has an atmosphere probably at least twice as dense
+and extensive as that which encircles the earth. Spurious satellites of
+Venus are many, but no real moon is known to attend this planet.
+
+ [Illustration: THE SURFACE OF THE MOON IN THE REGION OF COPERNICUS.
+ Photograph made with the Hooker 100-inch reflecting telescope.
+ (_Photo, Mt. Wilson Solar Observatory._)]
+
+ [Illustration: A VIEW OF THE SOUTH CENTRAL PORTION OF THE MOON AT
+ LAST QUARTER. (_Photo, Mt. Wilson Solar Observatory._)]
+
+
+
+
+CHAPTER XXVIII
+
+THE MOON AND HER SURFACE
+
+
+As the sun has always reigned as king of day, so is the moon queen of
+night. Observation of her phases, now waxing, now waning, with her
+stately motion always eastward among the stars, began with the earliest
+ages. Often when near the full she must have been seen herself eclipsed,
+and much more rarely the occurrence of total eclipses of the sun are
+certain to have suggested the moon's intervention between earth and sun,
+shutting off the sunlight completely, because these eclipses never took
+place except when the moon was in the same part of the sky with the sun.
+
+If we watch the nightly march of the moon, we shall find that she
+travels over her own breadth in about an hour's time. By using a
+telescope on the stars just eastward or to the left of her, she will now
+and then be seen to pass between us and a star--on very rare occasions a
+planet--extinguishing its light with great suddenness, the most nearly
+instantaneous of all phenomena in nature. Draw a line connecting the
+cusps, or horns of the lunar crescent, and then a line eastward at right
+angles to this, and it will show the direction of the moon's own motion
+in its orbit round the earth quite accurately.
+
+As the phase advances, note the inside edge of the advancing crescent:
+this will be quite rough and jagged, compared to the outside edge which
+is the moon's real contour and relatively very smooth. The position of
+the inside curve will change from night to night, and it marks the line
+of sunrise on the moon during the fortnight elapsing between new moon
+and full; while from full through last quarter and back to new moon,
+this advancing line marks the region of sunset on the moon. The general
+shape of this line is never a circle but always elliptical, and
+astronomers call it the terminator. All along the terminator, sunlight
+strikes the lunar surface at a small angle, whether near sunrise or
+sunset; so that owing to the mountains and other high masses of the
+moon's surface, the terminator is always a more or less jagged and
+irregular line.
+
+Onward from new moon toward full the horns of the crescent are always
+turned upward or eastward. When the general line of the terminator
+becomes a straight line from cusp to cusp, the moon is said to have
+reached first quarter or quadrature. Onward toward full the terminator
+will be seen to bend the other way, and in about a week's time it will
+have merged itself with the moon's limb. The moon is then said to be
+full. Afterward the phase phenomena recur in the reverse order, with
+third quarter midway between full and new moon again; the phase of the
+moon being called gibbous all the way from first quarter to third
+quarter, except when exactly full.
+
+As we know that the moon is, like the earth, a nonluminous body, and
+shines only by virtue of the sunlight falling upon it, clearly an entire
+half of the moon's globe must be perpetually illumined by sunlight. The
+varying phases then are due simply to that part of the illuminated
+hemisphere which is turned toward us. New moon is entirely invisible
+because the sunward hemisphere is turned wholly away from us, while at
+full moon we see the lunar disk complete because we are on the same side
+of the moon that the sun is and practically in line with both sun and
+moon.
+
+If we could visit the moon, we should see the earth in exactly
+complementary phase. At new moon here we should be enjoying full earth
+there, and full moon here would be coincident with new or dark earth
+there. The narrow crescent of new moon here would be the period of
+gibbous earth there; and it is the reflection of sunlight from this
+gibbous earth which illuminates the part of the moon but faintly seen at
+this time, popularly known as the "old moon in the new moon's arms." Its
+greater visibility at some times than at others is due to greater
+prevalence of clouded area in the reflecting regions of the earth turned
+toward the moon, and the higher reflective power of clouds than that
+possessed by mere land and water.
+
+As the moon goes all the way round the sky every month, the same as the
+sun does in a year, and travels in nearly the same path, clearly it must
+also go north and south every month as the sun does. So in midsummer
+when the sun runs high upon the meridian, we expect to find full moons
+running low, and likewise in midwinter the full moon always runs high,
+as almost everyone has sometimes or other noticed.
+
+This eastward or true orbital motion of the moon is responsible for
+another relation which soon comes to light when we begin to observe the
+moon; and that is the later hour of rising or setting each night. Our
+clock time is regulated by the sun, which also is moving eastward about
+1 deg. daily, or twice its own breadth. So the moon's eastward gain on the
+sun amounts to about 12 degrees daily, and one degree being equal to 4
+minutes, the retarded time of moonrise or moonset each day amounts to
+very nearly 50 minutes on the average; though sometimes the delay will
+be less than a half hour and at other times it will exceed an hour and a
+quarter. The season of least retardation of rising of the full moon is
+in the autumn, and so the moon that falls in late September or October
+is known as the Harvest moon, and the next succeeding full moon is
+called the Hunter's moon.
+
+Lunation is a term sometimes given to the moon's period from any
+definite phase round to the same phase again. Its length is the true
+period of the moon's revolution once around the earth, from the sun all
+the way round till it overtakes the sun again. The synodic period is
+another name for lunation, and its true length is 29 and one-half days,
+or very accurately 29 d. 12 h. 44 m. 2.7 s. as calculated by astronomers
+with great exactness from many thousand revolutions of the moon. But if
+we want the true period of the moon round the earth as referred to a
+star, it is much shorter than this, amounting to only 27 days and nearly
+one-third. This is called the moon's sidereal period of revolution,
+because it is the time elapsed while she is traveling eastward from a
+given star around to coincidence with the same star again.
+
+If we study the moon's path in the sky more critically, we shall find
+that it does not quite follow the ecliptic, or the sun's path, but that
+twice each month she deviates from the ecliptic, once to the north and
+once to the south of it, by roughly ten times her own breadth. More
+accurately this angle is 5 deg.8'40", an almost invariable quantity, and it
+is therefore known as an astronomical constant, or the inclination of
+the moon's orbit to the ecliptic. So the moon's orbit must intersect the
+ecliptic, and as both are great circles in the sky, the points of
+intersection are known as the moon's nodes, one ascending and the other
+descending, and the nodes are 180 degrees apart.
+
+The figure of the moon's orbit is not circular, although it deviates
+only slightly from that form. But like the paths of all other satellites
+round their primary planets, and of the planets themselves round the
+sun, the moon's orbit is also an ellipse. The distance of the moon's
+center from the earth's center is therefore perpetually changing; the
+point of nearest approach is called perigee, and that of farthest
+recession, apogee.
+
+The moon's distance from the earth is easier and simpler to be
+ascertained than that of any other heavenly body, because it is the
+nearest. An outline of the method of finding this distance is not
+difficult to present; and it resembles in every particular the method a
+surveyor uses to find the distance of some inaccessible point which he
+cannot measure directly. Up and down a stream, for example, he measures
+the length of a line, and from each end of it he measures the angle
+between the other end of the line and the object on the opposite side of
+the stream whose distance he wishes to find out. Then he applies the
+science of trigonometry to these three measures, two of angles and one
+the length of the side or base included between them, and a few minutes'
+calculation gives the distance of the inaccessible object from either
+end of the base line.
+
+Now in like manner, to transfer the process to the sky, let the two ends
+of the base be represented by two astronomical observatories, for
+example, Greenwich in the northern hemisphere and Cape Town in the
+southern. The base line is the chord or straight line through the earth
+connecting the two observatories, and we know the length of this line
+pretty accurately, because we know the size of the earth. The angles
+measured are somewhat different from those in the terrestrial example,
+but the process amounts to the same thing because the astronomers at the
+two observatories measure the angular distance of the center of the moon
+from the zenith, each using his own zenith at the same time; and the
+same science of trigonometry enables them to figure out the length of
+any side of the triangles involved. The side which belongs to both
+triangles is the distance from the center of the earth to the center of
+the moon, and the average of many hundred measures of this gives 238,800
+miles, or about ten times the distance round the equator of the earth.
+
+We have said that the orbit in which the moon travels round the earth is
+practically a circle, but the earth's center is found not at the center
+of this orbit, but set to one side, or eccentrically, so that the
+distance spanning the centers of the two bodies is sometimes as small as
+221,610 miles at perigee, and 252,970 miles at apogee. The moon's speed
+in this orbit averages rather more than half a mile every second of
+time--more accurately 3,350 feet a second, or 2,290 miles per hour.
+
+Once the moon's distance is known, its size or diameter is easy to
+ascertain. An angular measure is necessary, of course, that of the angle
+which the disk of the moon fills as seen from the earth. There are many
+types of astronomical instruments with which this angle can be measured,
+and its value is something more than half a degree (31'7"). The moon's
+actual diameter figures out from this 2,163 miles; and it would
+therefore require nearly fifty moons merged in one to make a ball the
+size of the earth.
+
+Still, no other planet has a satellite as large in proportion to its
+primary as the moon is in relation to the earth. But the materials that
+compose the moon have less than two-thirds the average density of those
+that make up the earth, so that eighty-one moons fused together would be
+necessary to equal the mass or weight of the earth. If we figure out the
+force of attraction of the moon for bodies on its surface, we find it
+equals about one-sixth that of the earth. Athletes could perform some
+astounding feats there--miracles of high jump and hammer-throw.
+
+Our interest in the moon's physical characteristics never wanes. Her
+nearness to us has always fascinated astronomer and layman alike. Early
+users of the telescope were readily led into error regarding the general
+characteristics of the lunar surface; and it is easy to see why they
+thought the smooth level planes must be seas, and gave them names to
+that effect which persist to-day, as Mare Crisium, Mare Serenitatis and
+so on. We may be sure that no water exists on the moon's surface,
+although some astronomers think that solid water, as ice or snow, may
+still exist there at a temperature too low for appreciable evaporation.
+
+Perhaps water, seas, and oceans were once there, but their secular
+dissemination and loss as vapor have gone on through the millions of
+millions of years till even the moon's atmosphere appears to have
+vanished completely. At least there is much better evidence of absence
+of atmosphere on the moon than of its presence--not enough at any rate
+to equal a thousandth part of the barometric pressure that we have at
+the earth's surface. Frequent observations of stars passing behind the
+moon in occultation have satisfied astronomers on this point.
+
+We often say of the brilliant full moon, it is as bright as day. The
+photometer or instrument for accurate comparison of lights, their amount
+and intensity, tells a different story. Indeed, if the entire dome of
+the sky were filled with full moons, we should be receiving only
+one-eighth of the light the sun gives us, and it would require more than
+600,000 average full moons to equal the light radiation of the sun. Heat
+from the moon, however, is quite different. Early attempts to measure it
+detected none at all, but with modern instruments there is little
+trouble in detecting heat from the moon, though measurement of it is not
+easy.
+
+Much of the moon's heat is sun heat, directly reflected from the moon,
+as sunlight is, but most of it is due to radiation of solar heat
+previously absorbed by the materials of the lunar surface. The actual
+temperature of the moon's surface suffers great variation. A fortnight's
+perpetual shining of the sun upon the lunar rocks would certainly heat
+them above the temperature of boiling water, if the moon had an
+atmosphere to conserve and store this heat; but the entire absence of
+such an air blanket probably permits the sun's heat to be radiated away
+nearly as fast as it is received, leaving the temperature at the surface
+always very low.
+
+What physical influences the moon really has upon the earth must be very
+slight, barring the tides. But there is little hope of getting people
+generally to take that view, because the moon appears to be the planet
+of the people, and opinion that the moon controls the weather, for
+instance, amounts with them to practical certainty. More than likely all
+these notions are but legitimate survivals of superstition and
+astrology. In addition to the tides, our magnetic observatories reveal
+slight disturbances with the swinging of the moon from apogee to perigee
+and back; but long series of weather observations have been faithfully
+interrogated, with negative or contradictory results. If one believes
+that the moon's changes affect the weather, it is easy to remember
+coincidences, and pass over the many times when no change has taken
+place. The moon changes pretty frequently anyhow. As Young well puts it:
+"A change of the moon necessarily occurs about once a week.... _All_
+changes, of the weather for instance, must therefore occur within three
+and three-fourth days of a change of the moon, and fifty per cent of
+them ought to occur within forty-six hours of a change, even if there
+were no causal connection whatever."
+
+When we turn to the strongly diversified surface of the moon itself, we
+find much to rivet the attention, even with slender optical aid.
+Everyone wants to know how near the telescope, the biggest possible
+telescope, brings the moon to us. That will depend on many things, first
+of all on the magnifying power of the eyepiece employed on the
+telescope, and eyepieces are changed on telescopes just as they are on
+microscopes, though not for the same reasons. The theoretical limit of
+the power of a telescope is usually considered as 100 for each inch of
+diameter or aperture of the object glass.
+
+A 40-inch telescope, as that of the Yerkes Observatory, the largest
+refracting telescope in existence, should bear a magnifying power not to
+exceed 4,000. But this limit is practically never reached, one-half of
+it or fifty to the inch of aperture being a good working limit of power,
+even under exceptional conditions of steadiness of atmosphere. If we
+reduce the effective distance of the moon from 240,000 miles to 100
+miles, that is about the utmost that can be expected. But even at that
+distance we can make out only landscape details, nothing whatever like
+buildings or the works of intelligence.
+
+The larger relations of light and shade, so obvious to the naked eye on
+the moon, vanish on looking at it with the telescope, but we are at once
+captivated by the novel character of the surface and the seemingly great
+variety of detail that is clearly visible. As soon as the new moon comes
+out in the west, one may begin to gaze with interest and watch the
+terminator or sunrise line gradually steal over the roughened surface,
+bringing new and striking craters into view each night. Around the time
+of quarter moon, or a little past it, is one of the best times for
+telescopic views of the moon, because the huge craters, Tycho and
+Copernicus, are then in fine illumination. Close to the phase of full
+moon is never a good time, because there are no shadows of the rough
+surface then, and its entire structure seems to be quite flat and
+uninteresting, except for the streaks or rills which radiate from Tycho
+in every direction, and are the only lunar features that are best seen
+near full.
+
+In a broad, general way, the moon's surface, if compared with the
+earth's, differs in having no water. Our extensive oceans are replaced
+there by smooth, level plains which were at first thought to be seas and
+so named. There are ten or twelve of them in all. Then we find mountain
+ranges, so numerous on the earth, relatively few on the moon. Those that
+exist are named, in part, for terrestrial mountain ranges, as the Alps,
+Caucasus, and the Apennines.
+
+But the nearly circular crater, a relatively rare formation on the
+earth, is seen dotted all over the moon in every size, from a fraction
+of a mile in diameter up to sixty, seventy, and in extreme cases a
+hundred miles. No mere description of plains and mountains and craters
+affords an adequate idea of the moon's surface as it actually is; a
+telescopic view is necessary, or some of the modern photographs which
+give an even better notion of the moon than any telescopic view. Many of
+the lunar craters are without doubt volcanic in origin, others seem to
+be ruins of molten lakes. Many thousands of the smaller ones appear as
+if formed by a violent pelting of the surface when semi-plastic, perhaps
+by enormous showers of meteoric matter. More than 30,000 craters cover
+the half of the lunar surface visible from the earth, and hundreds of
+them are named for philosophers and astronomers.
+
+Measurement of the height of lunar mountains has been made in numerous
+instances, especially when their shadows fall on plains or surfaces that
+are nearly level, so that the length of the shadow can be measured. In
+general, the height of lunar peaks is greater than that of terrestrial
+peaks, owing probably to the lesser surface gravity on the moon. About
+forty lunar peaks are higher than Mont Blanc.
+
+Most astronomers regard it as certain that no changes ever take place on
+the moon; probably no very conspicuous changes ever do. Some, however,
+have made out a fair case for comparatively recent changes in surface
+detail. Extreme caution is necessary in drawing conclusions, because the
+varying changes of illumination from one phase to another are themselves
+sufficient to cause the appearance of change. At intervals of a double
+lunation, equal to fifty-nine days, one and one-half hours, the
+terminator goes very nearly through the same objects, so that the
+circumstances of illumination are comparable. In Mare Serenitatis the
+little crater named Linne was announced to have disappeared about a half
+century ago; subsequently it became visible again and other minor
+changes were reported, perhaps due to falling in of the walls of the
+crater.
+
+If one were to visit the moon, he must needs take air and water along
+with him, as well as other sustenance. No atmosphere means no diffused
+light; we could see nothing unless the sun's direct rays were shining
+upon it. Anyone stepping into the shadow of a lunar crag would become
+wholly invisible. No sound, however loud, could be heard; sound in fact
+would become impossible. A rock might roll down the wall of a lunar
+crater, but there would be no noise; though we should know what had
+happened by the tremor produced. So slight is gravity there that a good
+ball player might bat a baseball half a mile or more. Looking upward,
+all the stars would be appreciably brighter than here, and visible
+perpetually in the daytime as well as at night.
+
+If one were to go to the opposite side of the moon, he would lose sight
+of the earth until he came back to the side which is always turned
+toward the earth. Even then the earth would never rise and set at any
+given place, as the moon does to us, but would remain all the time at
+about the same height above the lunar horizon. The earth would go
+through all the phases that the moon shows to us here, full earth
+occurring there when it is new moon here. Our globe would appear to be
+nearly four times broader than the moon seems to us. Its white polar
+caps of ice and snow, its dark oceans, and the vast cloud areas would be
+very conspicuous. Faint stars, the zodiacal light, and the filmy solar
+corona would be visible, probably even close up to the sun's edge; but
+although his rays might shine upon the lunar rocks without intermission
+for a fortnight, probably they would still be too cold to touch with
+safety. On the side of the moon turned away from the sun, the
+temperature of the moon's surface would fall to that of space, or many
+hundred degrees below zero.
+
+
+
+
+CHAPTER XXIX
+
+ECLIPSES OF THE MOON
+
+
+Of all the weird happenings of the nighttime sky, eclipses of the moon
+are the most impressive. Rarely is there a year without one. What is the
+cause? Simply the earth getting in between sun and moon, and thereby
+shutting off the sunlight which at all other times enables us to see the
+moon. As the earth is a dark body it must cast a black shadow on the
+side away from the sun, and it is the moon's passing into this shadow or
+some part of it that causes a lunar eclipse.
+
+Sun and earth being so different in size, the earth's shadow must
+stretch away from it into space, growing smaller and smaller, until at
+length it comes to an end--the apex of a cone 857,000 miles long. If we
+cut off this shadow at the moon's distance from the earth, we find it
+about 6,000 miles in diameter at that point; and this accounts for the
+fact that the curvature on the side of the moon, when the eclipse is
+coming on and where it is dropping into the shadow, is always much less
+rapid than the curvature of the moon's own disk is.
+
+When an eclipse is approaching, the eastern limb will be duskily
+darkened for half an hour or more, because the moon must first pass
+through the outer penumbra, or half-shadow which everywhere surrounds
+the true shadow itself. If the moon hits only the upper or lower part of
+the shadow, the eclipse will be only partial, and during the progress
+of the eclipse it will seem as if the uneclipsed part had swung or
+twisted around in the sky, from the western limb of the moon to the
+eastern. But when the moon passes through the middle regions of the
+shadow, the eclipse is always total, and direct sunlight is wholly cut
+off from every part of the moon's face, for a greater or less length of
+time, according to the part of the shadow through which it passes. When
+passing centrally through the shadow, the total eclipse will last about
+two hours, as the moon's diameter is about one-third of the breadth of
+the shadow; and the eclipse will be partial about two hours longer, an
+hour at beginning and an hour at the end, because the moon moves over
+her own breadth in about an hour.
+
+While the moon is wholly immersed in the shadow, her body is
+nevertheless visible, as a dull tarnished copper disk; and this is
+caused by the reddish sunlight which grazes the earth all around and is
+refracted or bent by our atmosphere into the shadow itself. If this belt
+or ring of terrestrial atmosphere happens to be everywhere filled with
+dense clouds, as was the case in 1886, even the familiar copper moon of
+a total lunar eclipse disappears completely in the black sky.
+
+Quite different from a solar eclipse, all the phases of a lunar eclipse
+are visible at the same time on the earth wherever the moon is above the
+horizon. Eclipses of the moon are therefore seen with great frequency at
+any given place as compared with solar eclipses, which are restricted to
+relatively narrow areas of the earth's surface. Nor are lunar eclipses
+of very much significance to the astronomer, mainly because of the
+slowness and indefiniteness of the phenomena. It is a good time to
+observe occultations of faint stars at the moon's edge or limb, and
+several such programs have been carried out by cooperation of
+observatories in widely separate regions of the world: the object being
+improvement in our knowledge of the distance of the moon, and in the
+accuracy of the mathematical tables of her motion. Search by photography
+for a possible satellite, or moon of the moon, has been made on several
+occasions, though without success.
+
+A lunar eclipse was first observed and photographed from an aeroplane,
+May 2, 1920. At the request of the writer, two aviators of the United
+States navy ascended to a height of 15,000 feet above Rockaway, and
+secured many advantages accruing from great elevation in viewing a
+celestial phenomenon of this character.
+
+
+
+
+CHAPTER XXX
+
+TOTAL ECLIPSES OF THE SUN
+
+
+Primitive peoples indulged in every variety of explanation of mysterious
+happenings in the sky. To the Chinese and all through India, a total
+eclipse of the sun is caused by "a certain dragon with very black
+claws," who, except for their frightening him away by every conceivable
+sort of hideous noise, would most certainly "eat up the sun." The
+eclipse always goes off, the sun has never been eaten yet. Can you
+convince a Chinaman that Rahu, the Dragon, wouldn't have eaten up the
+sun, if his unearthly din hadn't frightened him away?
+
+In Japan the eclipse drops poison from the sky into wells, so the
+Japanese cover them up. Fontenelle relates that in the middle of the
+seventeenth century a multitude of people shut themselves up in cellars
+in Paris during a total eclipse.
+
+In the Shu-king, an ancient Chinese work, occurs the earliest record of
+a total eclipse of the sun, in the year B. C. 2158. The Nineveh eclipse
+of B. C. 763 is perhaps the first of the ancient eclipses of which we
+possess a really clear description on the Assyrian eponym tablets in the
+British Museum. It is the eclipse possibly referred to in the Book of
+Amos, viii.
+
+But of all the ancient eclipses none perhaps exceeds in interest the
+famous eclipse of Thales, B. C. 585, May 28. It is the first eclipse to
+have been predicted, probably by means of the saros, or 18-year period
+of eclipses, which is useful as an approximate method even at the
+present day. But the accident of a war between the Lydians and the Medes
+has added greatly to the historic interest, because the combatants were
+so terrified by the sudden turning of day into night that they at once
+concluded a peace cemented by two marriages.
+
+Very many of the ancient eclipses have been of great use to the
+historian in verifying dates, and mathematical astronomers have employed
+them in correcting the lunar tables, or intricate mathematical data by
+which the motion of the moon is predicted.
+
+Coming down to the middle of the sixth century, we find the first
+eclipse recorded in England, in the "Saxon Chronicle," A. D. 538. During
+the epoch of the Arabian Nights several eclipses were witnessed at
+Bagdad, A. D. 829 to 928, and many a century later by Ibu-Jounis, court
+astronomer of Hakem, the Caliph of Egypt. Nothing is more interesting
+than to search the quaint records of these ancient eclipses. One
+occurring in 1560, when Tycho Brahe was but fourteen, had much to do
+with turning his permanent interest toward mathematics and astronomy.
+The eclipse of 1612 was the first "seen through a tube," the telescope
+having been invented only a few years before. "Paradise Lost" was
+completed about 1665, and the censorship was still in existence; and it
+is matter of record that the oft-quoted passage,
+
+ "As when the Sun, new risen,
+ Looks through the horizontal misty air,
+ Shorn of his beams; or from behind the Moon,
+ In dim eclipse, disastrous twilight sheds
+ On half the nations, and with fear of change
+ Perplexes monarchs."
+ _P. L._, i. 594
+
+was strongly urged as sufficient reason for suppressing the entire epic.
+
+London was favored with the outflashing corona, May 3, 1715, and a
+pamphlet was issued in prediction, entitled "The Black Day, or a
+Prospect of Doomsday."
+
+The first American eclipse expedition was on occasion of the totality of
+Oct. 27, 1780, sent out by Harvard College and the American Academy of
+Arts and Sciences under Professor Samuel Williams to Penobscot. There
+was a fine total eclipse from Albany to Boston on June 16, 1806, and
+many important observations of it were made in this country.
+
+But it was not till the European eclipse of 1842 that research got fully
+under way, because the germ of the new astronomy, particularly as
+applied to the sun, had begun its development; and the significance of
+the corona was obvious, if it could be proved a true appendage of the
+sun. Photography had not long been discovered, and the corona of 1851
+was the first to be automatically registered on a daguerreotype. In 1860
+it was proved that prominences and corona both belong to the sun and not
+to the moon.
+
+The great Indian eclipse of 1868 brought the important discovery that
+the prominences can be observed at any time without an eclipse by means
+of the spectroscope. In 1869 bright lines were found in the spectrum of
+the corona, one line in the green indicating the presence of an element
+not then known on the earth and hence called coronium. In 1870 the
+reversing layer or stratum of the sun was discovered. In 1878 a vast
+ecliptic extension of the streams of the corona many millions of miles
+both east and west of the sun was first seen. This is now known to be
+the type of corona characteristic of minimum spots on the sun. In 1882
+the spectrum of the corona was first photographed and in 1889 excellent
+detail photographs of the corona were taken. In 1893 it was shown that
+the corona quite certainly rotates bodily with the sun. In 1896 actual
+spectrum photographs of the reversing layer established its existence
+beyond doubt--"flash spectrum" it is often called. In 1898 the long
+ecliptic streamers of the corona were successfully photographed for the
+first time. In 1900 the depth of the reversing layer was found to
+average 500 miles, the heat of the corona was first measured by the
+bolometer, and many observations showed that the coronal streamers, in
+part at least, partake of the nature of electric discharges.
+
+All subsequent total eclipses have been carefully observed, in whatever
+part of the world they may happen, and each has added new results of
+significance to our theories of the corona and its relation to the
+radiant energy of the sun. In very recent eclipses the cinematograph has
+been brought into action as an efficient adjunct of observation; in 1914
+the first successful "movie" of the eclipse was secured in Sweden, and
+in 1918 Frost of the Yerkes Observatory first applied the cinematograph
+to registry of the "flash spectrum," and Stebbins tested out his
+photo-electric cell on the corona, making the brightness 0.5 that of the
+full moon. In 1914 (Russia) and again in 1919 (on the Atlantic) the
+obvious advantages of the aeroplane in ecliptic observation and
+photography were sought by the writer, though unsuccessfully. The
+photographic tests, however, conducted in preparation for these
+expeditions proved the entire practicability of securing eclipse results
+of much value, independently of clouds below.
+
+Eclipses in the near future will be total in Australia about six minutes
+on September 21, 1922; in California and Mexico about four minutes on
+September 10, 1923; and along a line from Toronto to Nantucket about two
+minutes on the morning of January 24, 1925.
+
+To all spectators, savage or civilized, scientist or layman, a total
+eclipse is wonderful and impressive. Langley said: "The spectacle is one
+of which, though the man of science may prosaically state the facts,
+perhaps only the poet could render the impression." Very gradually the
+moon steals its way across the face of the sun, the lessened light is
+hardly noticed. If one is near a tree through whose foliage the sunlight
+filters, an extraordinary sight is seen; the ground all about is covered
+with luminous crescents, instead of the overlapping disks which were
+there before the eclipse came on; in both cases they are images of the
+disk of the sun at the time, and the narrowing crescents will be watched
+with interest as totality approaches. Then the shadow bands may be seen
+flitting across the landscape, like "visible wind." They are probably
+related to our atmosphere and the very slender crescent from which true
+sunlight still comes.
+
+Then for a few seconds the moon's actual shadow may be caught in its
+approach, very suddenly the darkness steals over the landscape
+and--totality is on. How lucky if there are no clouds! Every eye is
+riveted on "the incomparable corona, a silvery, soft, unearthly light,
+with radiant streamers, stretching at times millions of uncomprehended
+miles into space, while the rosy flaming protuberances skirt the black
+rim of the moon in ethereal splendor."
+
+Then it is now or never with observer and photographer. Months of
+diligent preparations at home followed by weeks of tedious journey
+abroad, with days of strenuous preparation and rehearsals at the
+station--all go for naught unless the whole is tuned up to perfect
+operation the instant totality begins. It may last but a minute, or even
+less; in 1937, however, total eclipse will last 7 minutes 20 seconds,
+the longest ever observed, and within half a minute of the longest
+possible. All is over as suddenly as it came on. The first thing is to
+complete records, develop plates, and see if everything worked
+perfectly.
+
+There is great utility back of all eclipse research, on account of its
+wide bearing on meteorology and terrestrial physics, and possibly the
+direct use of solar energy for industrial purposes. With this purpose in
+view the astronomer devotes himself unsparingly to the acquisition of
+every possible fact about the sun and his corona.
+
+Considering the earth as a whole, the number of total eclipses will
+average nearly seventy to the century. But at any given place, one may
+count himself very fortunate if he sees a single total eclipse, although
+he may see several partial ones without going from home. Then, too,
+there are annular or ring eclipses, averaging seven in eight years. But
+had one been born in Boston or New York in the latter part of the
+eighteenth century, he might have lived through the entire nineteenth
+century and a long way into the twentieth without seeing more than one
+total eclipse of the sun. In London in 1715 no total eclipse had been
+visible for six centuries. However, taking general averages, and
+recalling the comparatively narrow belt of total eclipse, every part of
+the earth is likely to come within range of the moon's shadow once in
+about three and a half centuries.
+
+The longest total eclipses always occur near the equator; this is
+because an observer on the equator is carried eastward by the earth's
+rotation at a velocity of about 1,000 miles per hour, so that he remains
+longer in the moon's shadow which is passing over him in the same
+direction with a velocity about twice as great.
+
+The general circumstances of total eclipses are readily foretold by
+means of the ancient Chaldean period of eclipses known as the saros. It
+is 18 years and 10 or 11 days in length (according to the number of leap
+years intervening). In one complete saros, forty-one solar eclipses will
+generally happen, but only about one-fourth of them will be total. The
+saros is a period at the end of which the centers of sun and moon return
+very nearly to their relative positions at the beginning of the cycle.
+So, in general, the eclipse of any year will be a repetition of one
+which took place 18 years before, and another very similar in
+circumstances will happen 18 years in the future. Three periods of the
+saros, or 54 years and 1 month, will usually bring about a return of any
+given eclipse to any particular part of the earth, so far as longitude
+is concerned, though the returning track will lie about 600 miles to the
+north or south of the one 54 years earlier.
+
+Paths of total eclipses frequently intersect, if large areas like an
+entire country are considered; Spain, for instance, where total eclipses
+have occurred in 1842, 1860, 1870, 1900 and 1905. Besides crossing
+Spain, the tracks of totality on May 28, 1900, and August 30, 1905, were
+unique in intersecting exactly over a large city--Tripoli in Barbary, on
+both of which occasions the writer's expeditions to that city were
+rewarded with perfect observing conditions in that now Italian province
+on the edge of the great desert.
+
+Kepler was the first astronomer to calculate eclipses with some approach
+to scientific form, as exemplified in his Rudolphine Tables. His method
+was of course geometrical. But La Grange, who applied the methods of
+more refined analysis to the problem, was the first to develop a method
+by which an eclipse and all its circumstances could be accurately
+predicted for any part of the earth. To many minds, the prediction of an
+eclipse affords the best illustration of the superior knowledge of the
+astronomer: it seems little short of the marvelous. But recalling that
+the motion of the moon follows the law of gravitation, and that its
+position in the sky is predictable for years in advance with a high
+degree of precision, it will readily be seen how the arrival of the
+moon's shadow, and hence the total eclipses of the sun, can be foretold
+for any place over which the shadow passes.
+
+All these data derived by the mathematician are known as the elements of
+the eclipse, and they are prepared many years in advance and published
+in the nautical almanacs and astronomical ephemerides issued by the
+leading nations. Buchanan's "Treatise on Eclipses" will supply all the
+technical information regarding the prediction of eclipses that anyone
+desirous of inquiring into this phase of the problem may desire.
+
+So important are total eclipses in the scheme of modern solar research,
+and so necessary are clear skies in order that expeditions may be
+favored with success, that every effort is now made to ascertain the
+weather chances at particular stations along the line of eclipse many
+years in advance. This method of securing preliminary cloud observations
+for a series of years has proved especially useful for the eclipses of
+1893, 1896, 1900, and 1918; and had it been employed in Russia for
+totality of 1914, many well-equipped expeditions might have been spared
+disaster. The California and Mexico totality of 1923 does not require
+this forethought, as the regions visited are quite likely to be free
+from cloud; but observations are now in process of accumulation for the
+total eclipse of 1925. The out-look for clear skies on that occasion,
+the total eclipse nearest New York for more than a century, is not very
+promising. The path of totality passes over Marquette, Michigan,
+Rochester and Poughkeepsie, New York, Newport, Rhode Island, and
+Nantucket about nine in the morning.
+
+Everyone who saw it will remember the last total eclipse in this part of
+the world--on June 8, 1918, visible from Oregon to Florida. Many will
+recall the last total eclipse that was visible before that in the
+eastern part of the United States, on May 28, 1900, visible in a narrow
+path from New Orleans to Norfolk. One's father or grandfather will
+perhaps remember the total eclipse of July 29, 1878, which passed over
+the United States from Pike's Peak to Texas (it was the writer's maiden
+eclipse), and another on August 7, 1869, which passed southeasterly over
+Iowa and Kentucky. On all these occasions the paths of total eclipse
+were dotted with numerous observing parties, many of them equipped with
+elaborate apparatus for studying and photographing the solar corona and
+prominences, together with a multitude of other phenomena which are seen
+only when total eclipses take place.
+
+Looking forward rather than backward, a striking series, or family, of
+eclipses happens in the future: it is the series of May, 1901 and 1919,
+recurring again on June 8, 1937 (over the Pacific Ocean), June 20, 1955
+(through India, Siam, and Luzon), and June 30, 1973 (visible in Sahara,
+Abyssinia, and Somali). Already in 1919 this totality was 6 minutes 50
+seconds in duration; in 1937, as already mentioned, it will be 7 minutes
+20 seconds, and at the subsequent returns even longer yet, approaching
+the estimated maximum of 7 minutes 58 seconds which has never been
+observed. This remarkable series of total eclipses is longer in duration
+than any others during a thousand years. Its next subsequent return is
+in 1991, occurring with the eclipsed sun practically at noon in the
+zenith of Mount Popocatepetl in Mexico.
+
+Whatever may be the progress of solar research during the intervening
+years, it is impossible to imagine the alert astronomer of that remote
+day without incentive for further investigation of the sun's corona, in
+which are concealed no doubt many secrets of the sun's evolution from
+nebula to star.
+
+
+
+
+CHAPTER XXXI
+
+THE SOLAR CORONA
+
+
+"And what is the sun's corona?" mildly asked a college professor of a
+student who might better have answered "Not prepared."
+
+"I did know, Professor, but I have forgotten," was his reply.
+
+"What an incalculable loss to science," returned the professor with a
+twinkle. "The only man who ever knew what the sun's corona is, and he
+has forgotten!"
+
+Only in part has the mystery of the corona been cleared by the research
+of the present day. Our knowledge proceeds but slowly, because the
+corona has never been seen except during total eclipses of the sun; and
+astronomers, as a matter of fact, have never had a fair chance at it.
+Two total eclipses happen on the average of every three years; their
+average duration is only two or three minutes; totality can be seen only
+in a narrow path about a hundred miles wide, though it may be several
+thousand miles long; there is usually about equal chance of cloud with
+clear skies; and fully three-fourths of the totality areas of the globe
+are unavailable because covered by water. So that even if we imagine the
+tracks of eclipses quite thickly populated with astronomers and
+telescopes, at least one every hundred miles, how much solid watching of
+the corona would this permit? Only a little more than one week's time in
+a whole century.
+
+The true corona is at least a triple phenomenon and a very complex one.
+The photographs reveal it much as the eye sees it, with all its
+complexity of interlacing streamers projected into a flat, or plane,
+surrounding the disk of the dark moon which hides the true sun
+completely. But we must keep in mind the fact that the sun is a globe,
+not a disk, and that the streamers of the corona radiate more or less
+from all parts of the surface of the solar sphere, much as quills from a
+porcupine.
+
+From the sun's magnetic poles branch out the polar rays, nearly straight
+throughout their visible extent. Gradually as the coronal rays originate
+at points around the solar disk farther and farther removed from the
+poles, they are more and more curved. Very probably they extend into the
+equatorial regions, but it is not easy to trace them there because they
+are projected upon and confused with the filaments having their origin
+remote from the poles. Then there is the inner equatorial corona,
+apparently connected intimately with truly solar phenomena, quite as the
+polar rays are. The third element in the composite is the outer ecliptic
+corona, for the most part made up of long streamers. This is most fully
+developed at the time of the fewest spots on the sun. It is traceable
+much farther against the black sky with the naked eye than by
+photography. Without any doubt it is a solar appendage and possibly it
+may merge into the zodiacal light.
+
+Naturally this superb spectacle must have been an amazing sight to the
+beholders of antiquity who were fortunate enough to see it. Historical
+references are rare: perhaps the earliest was by Plutarch about A. D.
+100, who wrote of it, "A radiance shone round the rim, and would not
+suffer darkness to become deep and intense." Philostratus a century
+later mentions the death of the emperor Domitian at Ephesus as
+"announced" by a total eclipse.
+
+Kepler thought the corona was evidence of a lunar atmosphere; indeed, it
+was not until the middle of the 19th century that its lack of relation
+to the moon was finally demonstrated. Later observers, Wyberd in 1652
+and Ulloa, got the impression that the corona turned round the disk
+catherine-wheel fashion, "like an ignited wheel in fireworks, turning on
+its center." But no later observer has reported anything of the sort.
+Quite the contrary, there it stands against the black sky in motionless
+magnificence a colorless pearly mass of wisps and streamers for the most
+part nebulous and ill-defined, fading out very irregularly into the
+black sky beyond, but with a complex interlacing of filaments, sometimes
+very sharply defined near the solar poles. It defies the skill of artist
+and draughtsman to sketch it before it is gone.
+
+Photograph it? Yes, but there are troubles. Of course the camera work is
+superior to sketches by hand. As Langley used to say, "The camera has no
+nerves, and what it sets down we may rely on." Foremost among the
+photographic difficulties is the wide variation in intensity of the
+coronal light in different regions of the corona. If a plate is exposed
+long enough to get the outer corona, the exceeding brightness of the
+inner corona overexposes and burns out that part of the plate or film.
+If the exposure is short, we get certain regions of the inner corona
+excellently, but the outer regions are a blank because they can be
+caught only by a long exposure.
+
+So the only way is to take a series of pictures with a wide range of
+exposures, and then by careful and artistic handwork, combine them all
+into a single drawing. Wesley of London has succeeded eminently in work
+of this character, and his drawings of the sun's corona, visible at
+total eclipses from 1871 onward, in possession of the Royal Astronomical
+Society, are the finest in existence. They give a vastly better idea of
+the corona, as the eye sees it, than any single photograph possibly can.
+
+The early observers apparently never thought of the corona as being
+connected with the sun. It was a halo merely, and so drawn. Its real
+structure was neither known, depicted, or investigated. Sketches were
+structureless, as any aureola formed by stray sunlight grazing the moon
+might naturally be. That the rays are curved and far from radial round
+the sun was shown for the first time in the sketches of 1842, and in
+1860 Sir Francis Galton observed that the long arms or streamers "do not
+radiate strictly from the center."
+
+The inner corona had first been recorded photographically on a
+daguerreotype plate during the eclipse of 1851, but the lens belonged to
+a heliometer, and was of course uncorrected for the photographic rays.
+The wet collodion plates of the eclipse of 1860, by De la Rue, proved
+that not only the prominences but the corona were truly solar, because
+his series of technically perfect pictures revealed the steady and
+unchanged character of these phenomena while the moon's disk was passing
+over them as totality progressed. And at the eclipse of 1869, Young put
+the solar theory of the corona beyond the shadow of any further doubt
+by examination of its light with the spectroscope and discovering a
+green line in the spectrum due to incandescent vapor of a substance not
+then identified with anything terrestrial, and therefore called
+coronium.
+
+The total brilliance of the corona was very differently estimated by the
+earlier observers, though pretty carefully measured at later eclipses.
+The standard full moon is used for reference, and at one eclipse the
+corona falls short of, while at another it will exceed the full moon in
+brightness. Variations in brilliancy are quite marked: at one eclipse it
+was nearly four times as bright as the full moon. Much evidence has
+already accumulated on this question; but whether the observed
+variations are real, or due mainly to the varying relative sizes of sun
+and moon at different eclipses, is not yet known. The coronal light is
+largely bluish in tint, and this is the region of the spectrum most
+powerfully absorbed by our atmosphere. Eclipses are observed by
+different expeditions located at stations where the eclipsed sun stands
+at very different altitudes above the horizon; besides this the
+localities of observation are at varied elevations above sea level; so
+that the varying amount of absorption of the coronal light renders the
+problem one of much difficulty.
+
+The long ecliptic streamers of the corona were first seen by Newcomb and
+Langley during the totality of 1878. On one side of the sun there was a
+stupendous extension of at least twelve solar diameters, or nearly 11
+millions of miles. Langley observed from the summit of Pike's Peak, over
+14,000 feet high, and was sure that he was witnessing a "real phenomenon
+heretofore undescribed." The vast advantage of elevation was apparent
+also from the fact that he held the corona for more than four minutes
+after true totality had ended. These streamers are characteristic of the
+epoch of minimum spots on the sun, as Ranyard first suggested. It was
+found that this type of corona had been recorded also in 1867; and it
+has reappeared in 1889, 1900 and 1911, and will doubtless be visible
+again in 1922.
+
+How rapidly the streamers of the corona vary is not known. Occasionally
+an observer reports having seen the filaments vibrate rapidly as in the
+aurora borealis, but this is not verified by others who saw the same
+corona perfectly unmoving. Comparisons of photographs taken at widely
+separate stations during the same eclipse have shown that at least the
+corona remained stationary for hours at a time. Whether it may be
+unchanged at the end of a day, or a week, or a month, is not known;
+because no two total eclipses can ever happen nearer each other than
+within an interval of 173 days, or one-half of the eclipse year. And
+usually the interval between total eclipses is twice or three times this
+period.
+
+Theories of what the solar corona may be are very numerous. The extreme
+inner corona is perhaps in part a sort of gaseous atmosphere of the sun,
+due to matter ejected from the sun, and kept in motion by forces of
+ejection, gravity, and repulsion of some sort. Meteoric matter is likely
+concerned in it, and Huggins suggested the debris of disintegrating
+comets. Schuster was in agreement with Huggins that the brighter
+filaments of the corona might be due to electric discharges, but it
+seems very unlikely that any single hypothesis can completely account
+for the intricate tracery of so complex a phenomenon.
+
+ [Illustration: SOLAR CORONA AND PROMINENCES. Photographed during a
+ total eclipse of the sun, June 8, 1918. (_Courtesy, American
+ Museum of Natural History._)]
+
+ [Illustration: VENUS, SHOWING CRESCENT PHASE OF THE PLANET. Venus
+ is the earth's nearest neighbor on the side toward the sun.
+ (_Photo, Yerkes Observatory._)]
+
+ [Illustration: MARS, THE PLANET NEXT BEYOND THE EARTH. The
+ photograph shows one of the white polar caps. The caps are
+ thought to be snow or ice and may indicate the existence of
+ atmosphere. (_Photo, Yerkes Observatory._)]
+
+Elaborate spectroscopic programs have been carried out at recent
+eclipses, affording evidence that certain regions are due to
+incandescent matter of lower temperature than the sun's surface. A small
+part of the light of the corona is sunlight reflected from dark
+particles possibly meteoric, but more likely dust particles or fog of
+some sort. This accounts for the weakened solar spectrum with Fraunhofer
+absorption lines, and this part of the light is polarized.
+
+Many have been the attempts to see, or photograph, the corona without an
+eclipse. None of them has, however, succeeded as yet. Huggins got very
+promising results nearly forty years ago, and success was thought to
+have been reached; but subsequent experiments on the Riffelberg in 1884
+and later convinced him that his results related only to a spurious
+corona. In 1887 the writer made an unsuccessful attempt to visualize the
+corona from the summit of Fujiyama, and Hale tried both optical and
+photographic methods on Pike's Peak in 1893 without success. He devised
+later a promising method by which the heat of the corona in different
+regions can be measured by the bolometer, and an outline corona
+afterward sketched from these results.
+
+Still another method of attacking the problem occurred to the writer in
+1919, which has not yet been carried out. It would take advantage of
+recent advances in aeronautics, and contemplates an artificial eclipse
+in the upper air by means of a black spherical balloon. This would be
+sent up to an altitude of perhaps 40,000 feet, where it would partake
+of the motion of the air current in which it came to equilibrium. Then a
+snapshot camera would be mounted on an aeroplane, in which the aviator
+would ascend to such a height that the balloon just covered the sun, as
+the moon does in a total eclipse. With the center of the balloon in line
+with the sun's center, he would photograph the regions of the sky
+immediately surrounding the sun, against which the corona is projected.
+As the entire apparatus would be above more than an entire half of the
+earth's atmosphere, the experiment would be well worth the attempt, as
+pretty much everything else has been tried and found wanting. Needless
+to say, the importance of seeing the corona at regular intervals
+whenever desired, without waiting for eclipses of the sun, remains as
+insistent as ever.
+
+
+
+
+CHAPTER XXXII
+
+THE RUDDY PLANET
+
+
+Mars is a planet next in order beyond the earth, and its distance from
+the sun averages 141-1/2 million miles. It has a relatively rapid motion
+among the stars, its color is reddish, and, when nearest to us, it is
+perhaps the most conspicuous object in the sky.
+
+Mars appeared to the ancients just as it does to us to-day. Aristotle
+recorded an observation of Mars, 356 B. C., when the moon passed over
+the planet, or occulted it, as our expression is. Galileo made the first
+observations of Mars with a telescope in 1610, and his little instrument
+was powerful enough to enable him to discover that the planet had
+phases, though it did not pass through all the phases that Mercury and
+Venus do. This was obvious from the fact that Mars is always at a
+greater distance from the sun than we are, and the phase can only be
+gibbous, or about like the moon when midway between full and quarter.
+
+Many observers in the seventeenth century followed up the planet with
+such feeble optical power as the telescopes of that epoch provided:
+Fontana (who made the first sketch), Riccioli and Bianchini in Italy,
+Cassini in France, Huygens in Holland, and later Sir William Herschel in
+England.
+
+It was Cassini who first made out the whitish spots or polar caps of
+Mars in 1666, but not until after Huygens had noted the fact that Mars
+turned round on an axis in a period but little longer than the earth's.
+Cassini followed it up later with a more accurate value; and
+observations in our own day, when combined with these early ones, enable
+us to say that the Martian day is equal to 24 hours 37 minutes 22.67
+seconds, accurate probably to the hundredth part of a second.
+
+When we know that a planet turns round on an axis, we know that it has a
+day. When we know the direction of the axis in space or in relation to
+the plane of its path round the sun, we know that it has seasons: we can
+tell their length and when they begin and end. It did not take many
+years of observation to prove that the axis round which Mars turns is
+tilted to the plane of its path round the sun by an angle practically
+the same as that at which the earth's axis is tilted. So there is the
+immediate inference that on Mars the order and perhaps the character of
+the seasons is much the same as here on the earth.
+
+At least two things, however, tend to modify them. First, the year of
+Mars is not 365 days like ours, but 687 days. Each of the four seasons
+on Mars, therefore, is proportionally longer than our seasons are. Then
+comes the question of atmosphere--how much of an atmosphere does Mars
+really possess in proportion to ours, and how would its lesser amount
+modify the blending of the seasons into one another?
+
+All discussion of Mars and the problems of existence of life upon that
+planet hinge upon the character and extent of Martian atmosphere. The
+planet seems never to be covered, as the earth usually is, with
+extensive areas of cloud which to an observer in space would completely
+mask its oceans and continents. Nearly all the time Mars in his
+equatorial and temperate zones is quite clear of clouds. A few whitish
+spots are occasionally seen to change their form and position in both
+northern and southern latitudes, and they vary with the progress of the
+day on Mars, as clouds naturally would. But Schiaparelli, perhaps the
+best of all observers, thought them to be not low-lying clouds of the
+nimbus type that would produce rains, but rather a veil of fog, or
+perhaps a temporary condensation of vapor, as dew or hoar frost. But the
+strongest argument for an atmosphere is based on the temporary darkening
+or obscuration of well known and permanent markings on the surface of
+Mars. These are more or less frequently observed and clouds afford the
+best explanation of their occurrence.
+
+So much for evidence supplied by the telescope alone. When, however, we
+employ the spectroscope in conjunction with the telescope, another sort
+of evidence is at hand. Several astronomers have reached the conclusion
+that watery vapor exists in the atmosphere of Mars, while other
+astronomers equipped with equal or superior apparatus, and under equally
+favorable or even better conditions, have reached the remarkable
+conclusion that the spectra of Mars and the moon are identical in every
+particular. From this we should be led to infer that Mars has perhaps no
+more atmosphere than the moon has, that is to say, none whatever that
+present instruments and methods of investigation have enabled us to
+detect.
+
+What then, shall we conclude? Simply that the atmosphere of Mars is
+neither very dense nor extensive. Probably its lower strata close to
+the planet's surface are about as dense as the earth's atmosphere is at
+the summits of our highest mountains.
+
+This conclusion is not unwelcome, if we keep a few fundamental facts in
+clear and constant view. Mars is a planet of intermediate size between
+the earth and the moon: twice the moon's diameter (2,160 miles) very
+nearly equals the diameter of Mars (4,200 miles), and twice the diameter
+of Mars does not greatly exceed the earth's diameter (7,920 miles). As
+to the weights or masses of these bodies, Mars is about one-ninth, and
+the moon one-eightieth of the earth. The atmospheric envelope of the
+earth is abundant, the moon has none as far as we can ascertain; so it
+seems safe to infer that Mars has an atmosphere of slight density: not
+dense enough to be detected by spectroscopic methods, but yet dense
+enough to enable us to explain the varying telescopic phenomena of the
+planet's disk which we should not know how to account for, if there were
+no atmosphere whatever. One astronomer has, indeed, gone so far as to
+calculate that in comparison with our planet Mars is entitled to
+one-twentieth as much atmosphere as we have, and that the mercurial
+barometer at "sea level" would run about five and a half inches, as
+against thirty inches on the earth.
+
+In general, then, the climate of Mars is probably very much like that of
+a clear season on a very high terrestrial table land or mountain--a
+climate of wide extremes, with great changes of temperature from day to
+night. The inequality of Martian seasons is such that in his northern
+hemisphere the winter lasts 381 days and the summer only 306 days.
+
+Now, the polar caps of Mars, which are reasonably assumed to be due to
+snow or hoar frost, attain their maximum three or four months after the
+winter solstice, and their minimum about the same length of time after
+the summer solstice. This lagging should be interpreted as an argument
+for a Martian atmosphere with heat-storing qualities, similar to that
+possessed by the earth.
+
+Upon this characteristic, indeed, depends the climate at the surface of
+Mars: whether it is at all similar to our own, and whether fluid water
+is a possibility on Mars or not. While the cosmic relations of the
+planet in its orbit are quite the same as ours, nevertheless the greater
+distance of Mars diminishes his supply of direct solar heat to about
+half what we receive. On the other hand, his distance from the sun
+during his year of motion around it varies much more widely than ours,
+so that he receives when nearest the sun about one-half more of solar
+heat than he does when farthest away.
+
+Southern summers on Mars, therefore, must be much hotter, and southern
+winters colder than the corresponding seasons of his northern
+hemisphere. Indeed, the length of the southern summer, nearly twice that
+of the terrestrial season, sometimes amply suffices to melt all the
+polar ice and snow, as in October, 1894, when the southern polar cap of
+Mars dwindled rapidly and finally vanished completely.
+
+Very interesting in this connection are the researches of Stoney on the
+general conditions affecting planetary atmospheres and their
+composition. According to the kinetic theory, if the molecules of gases
+which are continually in motion travel outward from the center of a
+planet, as they frequently must, and with velocities surpassing the
+limit that a planet's gravity is capable of controlling, these molecules
+will effect a permanent escape from the planet, and travel through space
+in orbits of their own.
+
+So the moon is wholly without atmosphere because the moon's gravity is
+not powerful enough to retain the molecules of its component gases. So
+also the earth's atmosphere contains no helium or free hydrogen. So,
+too, Mars is possessed of insufficient force of gravity to retain water
+vapor, and the Martian atmosphere may therefore consist mainly of
+nitrogen, argon, and carbon dioxide.
+
+As everyone knows, the axis of the earth if extended to the northern
+heavens would pass very near the north polar star, which on that account
+is known as Polaris. In a similar manner the axis of Mars pierces the
+northern heavens about midway between the two bright stars Alpha Cephei
+and Alpha Cygni (Deneb). The direction of this axis is pretty accurately
+known, because the measurement of the polar caps of the planet as they
+turn round from night to night, year in and year out, has enabled
+astronomers to assign the inclination of the axis with great precision.
+
+These caps are a brilliant white, and they are generally supposed to be
+snow and ice. They wax and wane alternately with the seasons on Mars,
+being largest at the end of the Martian winter and smallest near the end
+of summer. The existence of the polar caps together with their seasonal
+fluctuations afford a most convincing argument for the reality of a
+Martian atmosphere, sufficiently dense to be capable of diffusing and
+transporting vapor.
+
+The northern cap is centered on the pole almost with geometric
+exactness, and as far as the 85th parallel of latitude. On the other
+hand, the south polar cap is centered about 200 miles from the true
+pole, and this distance has been observed to vary from one season to
+another. No suggestion has been made to account for this singular
+variation. On one occasion it stretched down to Martian latitude 70
+degrees and was over 1,200 miles in diameter.
+
+Pickering watched the changing conditions of shrinking of the south
+polar cap in 1892 with a large telescope located in the Andes of Peru.
+Mars was faithfully followed on every night but one from July 13 to
+September 9, and the apparent alterations in this cap were very marked,
+even from night to night. As the snows began to decrease, a long dark
+line made its appearance near the middle of the cap, and gradually grew
+until it cut the cap in two. This white polar area (and probably also
+the northern one in similar fashion) becomes notched on the edge with
+the progress of its summer season; dark interior spots and fissures
+form, isolated patches separate from the principal mass, and later seem
+to dissolve and disappear. Possibly if one were located on Mars and
+viewing our earth with a big telescope, the seasonal variation of our
+north and south polar caps might present somewhat similar phenomena. All
+the recent oppositions of Mars have been critically observed by
+Pickering from an excellent station in Jamaica.
+
+Quite obviously the fluctuations of the polar caps are the key to the
+physiographic situation on Mars, and they are made the subject of the
+closest scrutiny at every recurring opposition of the planet. Several
+observers, Lowell in particular, record a bluish line or a sort of
+retreating polar sea, following up the diminishing polar cap as it
+shrinks with the advance of summer. It is said that no such line is
+visible during the formation of the polar cap with the approach of
+winter. All such results of critical observation, just on the limit of
+visibility, have to be repeated over and over again before they become
+part of the body of accepted scientific fact. And in many instances the
+only sure way is to fall back on the photographic record, which all
+astronomers, whether prejudiced or not, may have the opportunity to
+examine and draw their individual conclusions.
+
+Already the approaching opposition of 1924, the most favorable since the
+invention of the telescope, is beginning to attract attention, and
+preparations are in progress, of new and more powerful instruments, with
+new and more sensitive photographic processes, by means of which many of
+the present riddles of Mars may be solved.
+
+
+
+
+CHAPTER XXXIII
+
+THE CANALS OF MARS
+
+
+Then there are the so-called canals of Mars, about which so much is
+written and relatively little known. Faint markings which resemble them
+in character were first drawn in 1840 and later in 1864, but
+Schiaparelli, the famous Italian astronomer, is probably their original
+discoverer, when Mars was at its least distance from the earth in 1877.
+He made the first accurate detailed map of Mars at this time, and most
+of the important or more conspicuous canals (_canali_, he called them in
+Italian, that is, channels merely, without any reference whatever to
+their being watercourses) were accurately charted by him.
+
+At all the subsequent close approaches of Mars, the canals have been
+critically studied by a wide range of astronomical observers, and their
+conclusions as to the nature and visibility of the canals have been
+equally wide and varied. The most favorable oppositions have occurred in
+1892 and 1894, also in 1907 and 1909. On these occasions a close minimum
+distance of Mars was reached, that is, about 35 millions of miles; but
+in 1924 the planet makes the closest approach in a period of nearly a
+thousand years. Its distance will not much exceed 34 millions of miles.
+
+But although this is a minimum distance for Mars, it must not be
+forgotten that it is a really vast distance, absolutely speaking; it is
+something like 150 times greater than the distance of the moon. With no
+telescopic power at our command could we possibly see anything on the
+moon of the size of the largest buildings or other works of human
+intelligence; so that we seem forever barred from detecting anything of
+the sort on Mars.
+
+Nevertheless, the closest scrutiny of the ruddy planet by observers of
+great enthusiasm and intelligence, coupled with imagination and
+persistence, have built up a system of canals on Mars, covering the
+surface of the planet like spider webs over a printed page, crossing
+each other at intersecting spots known as "lakes," and embodying a
+wealth of detail which challenges criticism and explanation.
+
+To see the canals at all requires a favorable presentation of Mars, a
+steady atmosphere and a perfect telescope, with a trained eye behind it.
+Not even then are they sure to be visible. The training of the eye has
+no doubt much to do with it. So photography has been called in, and very
+excellent pictures of Mars have already been taken, some nearly half as
+large as a dime, showing plainly the lights and shades of the grander
+divisions of the Martian surface, but only in a few instances revealing
+the actual canals more unmistakably than they are seen at the eyepiece.
+
+The appearance and degree of visibility of the canals are variable:
+possibly clouds temporarily obscure them. But there is a certain
+capriciousness about their visibility that is little understood. In
+consequence of the changing physical aspects, as to season, on Mars and
+his orbital position with reference to the earth, some of the canals
+remain for a long time invisible, adding to the intricacy of the
+puzzle.
+
+For the most part the canals are straight in their course and do not
+swerve much from a great circle on the planet. But their lengths are
+very different, some as short as 250 miles, some as long as 4,000 miles;
+and they often join one another like spokes in the hub of a wheel,
+though at various angles. As depicted by Lowell and his corps of
+observers at Flagstaff, Arizona, the canal system is a truly marvelous
+network of fine darkish stripes. Their color is represented as a bluish
+green.
+
+Each marking maintains its own breadth throughout its entire length, but
+the breadth of all the canals is by no means the same: the narrowest are
+perhaps fifteen to twenty miles wide, and the broadest probably ten
+times that. At least that must be the breadth of the Nilosyrtis, which
+is generally regarded as the most conspicuous of all the canals. The
+Lowell Observatory has outstripped all others in the number of canals
+seen and charted, now about 500.
+
+What may be the true significance of this remarkable system of markings
+it is impossible to conclude at present. Schiaparelli from his long and
+critical study of them, their changes of width and color, was led to
+think that they may be a veritable hydrographic system for distributing
+the liquid from the melting polar snows. In this case it would be
+difficult to escape the conviction that the canals have, at least in
+part, been designed and executed with a definite end in view.
+
+Lowell went even farther and built upon their behavior an elaborate
+theory of life on the planet, with intelligent beings constructing and
+opening new canals on Mars at the present epoch. Pickering propounded
+the theory that the canals are not water-bearing channels at all, but
+that they are due to vegetation, starting in the spring when first seen
+and vitalized by the progress of the season poleward, the intensity of
+color of the vegetation coinciding with the progress of the season as we
+observe it.
+
+Extensive irrigation schemes for conducting agricultural operations on a
+large scale seem a very plausible explanation of the canals, especially
+if we regard Mars as a world farther advanced in its life history than
+our own. Erosion may have worn the continents down to their minimum
+elevation, rendering artificial waterways not difficult to build; while
+with the vanishing Martian atmosphere and absence of rains, the
+necessity of water for the support of animal and vegetal life could only
+be met by conducting it in artificial channels from one region of the
+planet to another.
+
+Interesting as this speculative interpretation is, however, we cannot
+pass by the fact that many competent astronomers with excellent
+instruments finely located have been unable to see the canals, and
+therefore think the astronomers who do see them are deceived in some
+way. Also many other astronomers, perhaps on insufficient grounds, deny
+their existence _in toto_.
+
+Many patient years of labor would be required to consult all the
+literature of investigation of the planet Mars, but much of the detail
+has been critically embodied in maps at different epochs, by Kayser,
+Proctor, Green, and Dreyer. And Flammarion in two classic volumes on
+Mars has presented all the observations from the earliest time, together
+with his own interpretation of them. Areography is a term sometimes
+applied to a description of the surface of Mars, and it is scarcely an
+exaggeration to say that areography is now better known than the
+geography of immense tracts of the earth.
+
+For some reason well recognized, though not at all well understood, Mars
+although the nearest of all the planets, Venus alone excepted, is an
+object by no means easy to observe with the telescope. Possibly its
+unusual tint has something to do with this. With an ordinary opera glass
+examine the moon very closely, and try to settle precise markings,
+colors, and the nature of objects on her surface; Mars under the best
+conditions, scrutinized with our largest and best telescope, presents a
+problem of about the same order of difficulty. There are delicate and
+changing local colors that add much uncertainty. Nevertheless, the
+planet's leading features are well made out, and their stability since
+the time of the earliest observers leaves no room to doubt their reality
+as parts of a permanent planetary crust.
+
+The border of the Martian disk is brighter than the interior, but this
+brightness is far from uniform. Variations in the color of the markings
+often depend on the planet's turning round on its axis, and the relation
+of the surface to our angle of vision. If we keep in mind these
+obstacles to perfect vision in our own day, it is easy to see why the
+early users of very imperfect telescopes failed to see very much, and
+were misled by much that they thought they saw. Then, too, they had to
+contend, as we do, with unsteadiness of atmosphere, which is least
+troublesome near the zenith.
+
+As their telescopes were all located in the northern hemisphere, the
+northern hemisphere of Mars is the one best circumstanced for their
+investigation; because at the remote oppositions of Mars, which always
+happen in our northern winter with the planet in high north declination,
+it is always the north pole of Mars which is presented to our view.
+Whereas the close oppositions of the planet always come in our northern
+midsummer, with Mars in south declination and therefore passing through
+the zenith of places in corresponding south latitude.
+
+With Mars near opposition, high up from the horizon, a fairly steady
+atmosphere, and a magnifying power of at least 200 diameters, even the
+most casual observer could not fail to notice the striking difference in
+brightness of the two hemispheres: the northern chiefly bright and the
+southern markedly dark. Formerly this was thought to indicate that the
+southern hemisphere of Mars was chiefly water and the northern land,
+much as is the case on the earth: with this difference, however, that
+water and land on the earth are proportioned about as eleven to four.
+
+But Mars in its general topography presents no analogy with the present
+relation of land and water on the earth. There seems no reason to doubt
+that the northern regions with their prevailing orange tint, in some
+places a dark red and in others fading to yellow and white, are really
+continental in character. Other vast regions of the Martian surface are
+possibly marshy, the varying depth of water causing the diversity of
+color. If we could ever catch a reflection of sunlight from any part of
+the surface of Mars, we might conclude that deep water exists on the
+planet; but the farther research progresses, the more complete becomes
+the evidence that permanent water areas on Mars, if they exist at all,
+are extremely limited.
+
+Since 1877 Mars has been known to possess two satellites, which were
+discovered in August of that year by Hall at Washington. Moons of this
+planet had long been suspected to exist and on one or two previous
+occasions critically looked for, though without success. In the writings
+of Dean Swift there is a fanciful allusion to the two moons of Mars; and
+if astronomers had chanced to give serious attention to this, Phobos and
+Deimos, as Hall named them, might have been discovered long before.
+
+They are very small bodies, not only faint in the telescope, but
+actually of only ten or twenty miles diameter; and from the strange
+relation that Phobos, the inner moon, moves round Mars three times while
+the planet itself is turning round only once on its axis, some
+astronomers incline to the hypothesis that this moon at least was never
+part of Mars itself, but that it was originally an inner or very
+eccentric member of the asteroid group, which ventured within the sphere
+of gravitation of Mars, was captured by that planet, and has ever since
+been tributary to it as a secondary body or satellite.
+
+
+
+
+CHAPTER XXXIV
+
+LIFE IN OTHER WORLDS
+
+
+Popular interest in astronomy is exceedingly wide, but it is very
+largely confined to the idea of resemblances and differences between our
+earth and the bodies of the sky. The question most frequently asked the
+astronomer is, "Have any of the stars got people on them?" Or more
+specifically, "Is Mars inhabited?" The average questioner will not
+readily be turned off with yes or no for an answer. He may or may not
+know that it is quite impossible for astronomers to ascertain anything
+definite in this matter, most interesting as it is. What he wants to
+find out is the view of the individual astronomer on this absorbing and
+ever recurring inquiry.
+
+We ought first to understand what is meant by the manifestation here on
+the earth called life, and agree concerning the conditions that render
+it possible. Apparently they are very simple. We may or may not agree
+that a counterpart of life, or life of a wholly different type from
+ours, may exist on other planets under conditions wholly diverse from
+those recognized as essential to its existence here. The problem of the
+origin of life is, in the present state of knowledge, highly speculative
+and hardly within the domain of science. Here on earth, life is
+intimately associated with certain chemical compounds, in which carbon
+is the common element without which life would not exist. Also
+hydrogen, oxygen, and nitrogen are present, with iron, sulphur,
+phosphorus, magnesium and a few less important elements besides. But
+carbon is the only substance absolutely essential. Protoplasm cannot be
+built without it, and protoplasm makes up the most of the living cell.
+Closely related to carbon is silica also, as a substitution in certain
+organic compounds. Protoplasm is able to stand very low temperatures,
+but its properties as a living cell cease when the temperature reaches
+150 Fahrenheit.
+
+Animal life as it exists on the earth to-day appears to have been here
+many million years. The palaeontologists agree that all life originated
+in the waters of the earth. It has passed through evolutionary stages
+from the lowest to the highest. Throughout this vast period the
+astronomer is able to say that the conditions of the earth which appear
+to be essential to the maintenance of life have been pretty constantly
+what they are to-day. The higher the type of life, the narrower the
+range of conditions under which it thrives. Man can exist at the frigid
+poles even if the temperature is 75 degrees below Fahrenheit zero; and
+in the deserts and the tropics, he swelters under temperatures of 115
+degrees, but he still lives. At these extremes, however, he can scarcely
+be said to thrive.
+
+We have, then, a relatively narrow range of temperatures which seems to
+be essential to his comfortable existence and development: we may call
+it 150 degrees in extent. Had not the surface temperature of the earth
+been maintained within this range for indefinite ages, in the regions
+where the human race has developed, quite certainly man would not be
+here. How this equability of temperature has been maintained does not
+now matter. Clearly the earth must have existed through indefinite ages
+in the process of cooling down from temperatures of at least 6,000
+degrees.
+
+During this stage the temperature of the surface was earth-controlled.
+Then this period merged very gradually into the stage where life became
+possible, and the temperature of the surface became, as it now is,
+sun-controlled. How many years are embraced in this span of periods, or
+ages, we have no means of knowing. But of the sequence of periods and
+the secular diminution of temperature, we may be certain.
+
+Then there is the equally important consideration of water necessary for
+the origination, support, and development of life. We cannot conceive of
+life existing without it. On the earth water is superabundant, and has
+been for indefinite ages in the past. There is little evidence that the
+oceans are drying up; although the commonly accepted view is that the
+waters of the earth will very gradually disappear. Water can exist in
+the fluid state, which is essential to life, at all temperatures between
+32 degrees and 680 degrees F.
+
+Air to breathe is essential to life also. The atmosphere which envelops
+the earth is at least 100 miles in depth, and its own weight compresses
+it to a tension of nearly 15 pounds to the square inch at sea level.
+This atmosphere and its physical properties have had everything to do
+with the development of animal life on the planet. Without it and its
+remarkable property of selective absorption, which imprisons and
+diffuses the solar heat, it is inconceivable that the necessary
+equability of surface temperature could be maintained. This appears to
+be quite independent of the chemical constituents of the atmosphere, and
+is perhaps the most important single consideration affecting the
+existence of life on a planet. If the surface of a planet is partly
+covered with water, it will possess also an atmosphere containing
+aqueous vapor.
+
+Heat, water, and air: these three essentials determine whether there is
+life on a planet or not. Of course there must be nutrition suitable to
+the organism; mineral for the vegetal, and vegetal for the animal. But
+the narrow range of variation appears to be the striking thing:
+relatively but a few degrees of temperature, and a narrow margin of
+atmospheric pressure. If this pressure is doubled or trebled, as in
+submarine caissons, life becomes insupportable. If, on the other hand,
+it is reduced even one-third, as on mountains even 13,000 feet high, the
+human mechanism fails to function, partly from lack of oxygen necessary
+in vitalizing the blood, but mainly because of simple reduction of
+mechanical pressure.
+
+If, then, we conceive of life in other worlds and it is agreed that life
+there must manifest itself much as it does here, our answer to the
+question of habitability of the planets must follow upon an
+investigation of what we know, or can reasonably surmise, about the
+surface temperatures of these bodies, whether they have water, and what
+are the probable physical characteristics of their atmospheres.
+
+We may inquire about each planet, then, concerning each of these
+details.
+
+The case of Mercury is not difficult. At an average distance of only 36
+million miles from the sun, and with a large eccentricity of orbit which
+brings it a fifth part nearer, conditions of temperature alone must be
+such as to forbid the existence of life. The solar heat received is
+seven times greater than at the earth, and this is perhaps sufficient
+reason for a minimum of atmosphere, as indicated by observation. If no
+air, then quite certainly no water, as evaporation would supply a slight
+atmosphere. But according to the kinetic theory of gases, the mass of
+Mercury, only a very small fraction of that of the sun, is inadequate to
+retain an atmospheric envelope. If, however, the planet's day and year
+are equal, so that it turns a constant face to the sun, surface
+conditions would be greatly complicated, so that we cannot regard the
+planet as absolutely uninhabitable on the hemisphere that is always
+turned away from the sun.
+
+Venus at 67 millions of miles from the sun presents conditions that are
+quite different. She receives double the solar heat that we do, but
+possessing an atmosphere perhaps threefold denser than ours, as reliably
+indicated by observations of transits of Venus, the intensity of the
+heat and its diffusion may be greatly modified. What the selective
+absorption of the atmosphere of Venus may be, we do not know. Nor is the
+rotation time of the planet definitely ascertained: if equal to her
+year, as many observations show and as indicated by the theory of tidal
+evolution, there may well be certain regions on the hemisphere
+perpetually turned away from the sun where temperature conditions are
+identical with those on the tropical earth, and where every condition
+for the origin and development of life is more fully met than anywhere
+else in the solar system. Whether Venus has water distributed as on the
+earth we do not know, as her surface is never seen, owing to dense
+clouds under which she is always enshrouded. Her cloudy condition
+possibly indicates an overplus of water.
+
+Is the moon inhabited? Quite certainly not: no appreciable air, no
+water, and a surface temperature unmodified by atmosphere--rising
+perhaps to 100 degrees F. during the day, which is a fortnight in
+length, and falling at night to 300 degrees below zero, if not lower.
+
+Is Mars inhabited? The probable surface temperature is much lower than
+the earth's, because Mars receives only half as much solar heat as we
+do; and more important still, the atmosphere of Mars is neither so dense
+nor so extensive as our own. Seasons on Mars are established, much the
+same as here, except that they are nearly twice as long as ours; and
+alternate shrinking and enlarging of the polar caps keeps even pace with
+the seasons, thereby indicating a certainty of atmosphere whose
+equatorial and polar circulation transports the moisture poleward to
+form the snow and ice of which the polar caps no doubt consist.
+
+There is a variety of evidence pointing to an atmosphere on Mars of
+one-third to one-half the density of our own: an atmosphere in which
+free hydrogen could not exist, although other gases might. The
+spectroscopic evidence of water vapor in the Martian atmosphere is not
+very strong. It is very doubtful whether water exists on Mars in large
+bodies: quite certainly not as oceans, though the evidence of many small
+"lakes" is pretty well made out. With very little water, a thin
+atmosphere and a zero temperature, is Mars likely to be inhabited at the
+present time? The chances are rather against it. If, however, the past
+development of the planet has progressed in the way usually considered
+as probable, we may be practically certain that Mars has been inhabited
+in the past, when water was more abundant, and the atmosphere more dense
+so as to retain and diffuse the solar heat.
+
+Biologists tell me that they hardly know enough regarding the extreme
+adaptability of organisms to environment to enable them to say whether
+life on such a planet as Mars would or would not keep on functioning
+with secular changes of moisture and temperature. The survival of a race
+might be insured against extremely low temperatures by dwelling in
+sub-Martian caves, and sufficient water might be preserved by
+conceivable engineering and mechanical schemes; but the secular
+reduction of the quantity and pressure of atmosphere--it is not easy to
+see how a race even more advanced than ourselves could maintain itself
+alive under serious lack of an element so vital to existence. Both
+Wallace, the great biologist, and Arrhenius, the eminent chemist (but
+biologist, astronomer, and physicist as well), both reject the
+habitation theory of Mars, regarding the so-called canals as quite like
+the luminous streaks on the moon; that is, cracks in the volcanic crust
+caused by internal strains due to the heated interior. Wallace, indeed,
+argues that the planet is absolutely uninhabitable.
+
+The asteroids, or minor planets? We may dismiss them with the simple
+consideration that their individual masses are so insignificant and
+their gravity so slight that no atmosphere can possibly surround them.
+Their temperatures must be exceedingly low, and water, if present at
+all, can only exist in the form of ice.
+
+Jupiter, the giant planet, presents the opposite extreme. His mass is
+nearly a thousandth part of the sun's, and is sufficient to retain a
+very high temperature, probably approximating to the condition we call
+red-hot. This precludes the possibility of life at the outset, although
+the indications of a very dense atmosphere many thousand miles in depth
+are unmistakable.
+
+Of Saturn, one thirty-five hundredth the mass of the sun, practically
+the same may be said. Proctor thought it quite likely that Saturn might
+be habitable for living creatures of some sort, but he regarded the
+planet as on many accounts unsuitable as a habitation for beings
+constituted like ourselves. Mere consideration of surface temperature
+precludes the possibility of life in the present stage of Saturn's
+development; but the consensus of opinion is to the effect that life may
+make its appearance on these great planets at some inconceivably remote
+epoch in the future when the surface temperature is sufficiently reduced
+for life processes to begin. Discoveries of algae flourishing in hot
+springs approaching 200 degrees Fahrenheit make it possible that these
+beginnings may take place earlier and at much higher temperatures than
+have hitherto been thought possible.
+
+A century ago, when the ring of Saturn was believed to be a continuous
+plane, this was a favorite corner of the solar system for speculation as
+to habitability; but now that we know the true constitution of the
+rings, no one would for a moment consider any such possibility.
+Conditions may, however, be quite different with Saturn's huge satellite
+Titan, the giant moon of the solar system. Its diameter makes it
+approximately the size of the planet Mars; and although it is much
+farther removed from the sun, its relative nearness to the highly
+heated globe of Saturn may provide that equability of temperature which
+is essential to life processes.
+
+Also the three inner Galilean moons of Jupiter, especially III which is
+about the size of Titan, are excellently placed for life possibilities,
+as far as probable temperature is concerned, but we have of course no
+basis for surmising what their conditions may be as to air and water,
+except that their small mass would indicate a probable deficiency of
+those elements.
+
+Uranus and Neptune are planets so remote, and their apparent disks are
+so small, that very little is known about their physical condition. They
+are each about one-third the diameter of Jupiter, and the spectrum of
+Uranus shows broad diffused bands, indicating strong absorption by a
+dense atmosphere very different from that of the earth. Indications are
+that Neptune has a similar atmosphere.
+
+It is possible that the denser atmospheres of these remote planets may
+be so conditioned as to selective absorption that the relatively slender
+supply of solar heat may be conserved, and thus insure a relatively high
+surface temperature when the sun comes into control. If our theories of
+origin of the planets are to be trusted, we may rather suppose that
+Uranus and Neptune are still in a highly heated condition; that life has
+not yet made its appearance on them, but that it will begin its
+development ages before Saturn and Jupiter have cooled to the requisite
+temperature.
+
+Comets? In his _Lettres Cosmologiques_ (1765) Lambert considers the
+question of habitability of the comets, naturally enough in his day,
+because he thought them solid bodies surrounded by atmosphere, and
+related to the planets. The extremes of temperature at perihelia and
+aphelia to which comets are subjected did not bother him particularly.
+
+After calculating that the comet of 1680, "being 160 times nearer to the
+sun than we are ourselves, must have been subjected to a degree of heat
+25,600 times as great as we are," Lambert goes on to say: "Whether this
+comet was of a more compact substance than our globe, or was protected
+in some other way, it made its perihelion passage in safety, and we may
+suppose all its inhabitants also passed safely. No doubt they would have
+to be of a more vigorous temperament and of a constitution very
+different from our own. But why should all living beings necessarily be
+constituted like ourselves? Is it not infinitely more probable that
+amongst the different globes of the universe a variety of organizations
+exist, adapted to the wants of the people who inhabit them, and fitting
+them for the places in which they dwell, and the temperatures to which
+they will be subjected? Is man the only inhabitant of the earth itself?
+And if we had never seen either bird or fish, should we not believe that
+the air and water were uninhabitable? Are we sure that fire has not its
+invisible inhabitants, whose bodies, made of asbestos, are impenetrable
+to flame? Let us admit that the nature of the beings who inhabit comets
+is unknown to us; but let us not deny their existence, and still less
+the possibility of it."
+
+Little enough is really known about the physical nature of comets even
+now, but what we do know indicates incessant transformation and
+instability of conditions that would render life of any type exceedingly
+difficult of maintenance.
+
+A word about Sir William Herschel's theory of the sun and its
+habitability. He thought the core of the sun a dark, solid body, quite
+cold, and surrounded by a double layer, the inner one of which he
+conceived to act as a sort of fire screen to shield the sun proper
+against the intense heat of the outer layer, or photosphere by which we
+see it. Viewed in this light, the sun, he says, "appears to be nothing
+else than a very eminent, large and lucid planet, evidently the first,
+or, in strictness of speaking, the only primary one of our system.... It
+is most probably also inhabited, like the rest of the planets, by beings
+whose organs are adapted to the peculiar circumstances of that vast
+globe." But physics and biology were undeveloped sciences in Herschel's
+days.
+
+Herschel knew, however, that the stars are all suns, so that he must
+have conceived that they are inhabited also, quite independently of the
+question whether they possess retinues of planets, after the manner of
+our solar system.
+
+This again is a question to which the astronomer of the present day can
+give no certain answer. So immensely distant are even the nearest of
+these multitudinous bodies that no telescope can ever be built large
+enough or powerful enough to reveal a dark planet as large as Jupiter,
+alongside even the nearest fixed star. Whatever may be the process of
+stellar evolution, there doubtless is an era of many hundreds of
+millions of years in the life of a star when it is passing through a
+planet-maintaining stage. This would likely depend upon spectral type,
+or to be indicated by it; and as about half of the stars are of the
+solar type, it would be a reasonable inference that at least half of the
+stars may have planets tributary to them.
+
+In such a case, the chances must be overwhelmingly in favor of vast
+numbers of the planets of other stellar systems being favorably
+circumstanced as to heat and moisture for the maintenance of life at the
+present time. That is, they are habitable, and if habitable, then
+thousands of them are no doubt inhabited now. But astronomers know
+absolutely nothing about this question, nor are they able to conceive at
+present any way that may lead them to any definite knowledge of it.
+There is, indeed, one piece of quasi-evidence which might reasonably be
+interpreted as implying that it is more likely that the stars are not
+attended by families of planets than that they are.
+
+
+
+
+CHAPTER XXXV
+
+THE LITTLE PLANETS
+
+
+Along toward the end of the eighteenth century and the beginning of the
+nineteenth, astronomers were leading a quiet unexcited life. Sir William
+Herschel had been knighted by King George for his discovery of the outer
+planet Uranus, and practically everything seemed to be known and
+discovered in the solar system with a single exception. Between Mars and
+Jupiter there existed an obvious gap in the planetary brotherhood.
+
+Could it be possible that some time in the remote cosmic past a planet
+had actually existed there, and that some celestial cataclysm had blown
+it to fragments? If so, would they still be traveling round the sun as
+individual small planets? And might it not be possible to discover some
+of them among the faint stars that make up the belt of the zodiac in
+which all the other planets travel?
+
+So interesting was this question that the first international
+association of astronomers banded themselves together to carry on a
+systematic search round the entire zodiacal heavens in the faint hope of
+detecting possible fragments of the original planet of mere hypothesis.
+
+The astronomers of that day placed much reliance on what is known as
+Bode's law--not a law at all, but a mere arithmetical succession of
+numbers which represented very well the relative distances of all the
+planets from the sun. And the distance of the newly found Uranus fitted
+in so well with this law that the utter absence of a planet in the gap
+between Mars and Jupiter became very strongly marked.
+
+Quite by accident a discovery of one of the guessed-at small planetary
+bodies was made, on January 1, 1801, in Palermo, Sicily, by Piazzi, who
+was regularly occupied in making an extensive catalogue of the stars.
+His observations soon showed that the new object he had seen could not
+be a fixed star, because it moved from night to night among the stars.
+He concluded that it was a planet, and named it Ceres (1), for the
+tutelary goddess of Sicily.
+
+Other astronomers kept up the search, and another companion planet,
+Pallas (2) was found in the following year. Juno (3) was found in 1804,
+and Vesta (4), the largest and brightest of all the minor planets, in
+1807. Vesta is sometimes bright enough when nearest the earth to be seen
+with the naked eye; but it was the last of the brighter ones, and no
+more discoveries of the kind were made till the fifth was found in 1845.
+Since then discoveries have been made in great abundance, more and more
+with every year till the number of little planets at present known is
+very near 1,000.
+
+The early asteroid hunters found the search rather tedious, and the
+labor increased as it became necessary to examine the increasing
+thousands of fainter and fainter stars that must be observed in order to
+detect the undiscovered planets, which naturally grow fainter and
+fainter as the chase is prolonged. First a chart of the ecliptic sky had
+to be prepared containing all the stars that the telescope employed in
+the search would show. Some of the most detailed charts of the sky in
+existence were prepared in connection with this work, particularly by
+the late Dr. Peters of Hamilton College. Once such charts are complete,
+they are compared with the sky, night after night when the moon is
+absent. Thousands upon thousands of tedious hours are spent in this
+comparison, with no result whatever except that chart and sky are found
+to correspond exactly.
+
+But now and then the planet hunter is rewarded by finding a new object
+in the sky that does not appear on his chart. Almost certainly this is a
+small planet, and only a few night's observation will be necessary to
+enable the discoverer to find out approximately the orbit it is
+traveling in, and whether it is out-and-out a new planet or only one
+that had been previously recognized, and then lost track of.
+
+Nearly all the minor planets so far found have had names assigned to
+them principally legendary and mythological, and a nearly complete
+catalogue of them, containing the elements of their orbits (that is, all
+the mathematical data that tell us about their distance from the sun and
+the circumstances of their motion around him) is published each year in
+the "Annuaire du Bureau des Longitudes" at Paris. But these little
+planets require a great deal of care and attention, for some astronomers
+must accurately observe them every few years, and other astronomers must
+conduct intricate mathematical computations based on these observations;
+otherwise they get lost and have to be discovered all over again.
+Professor Watson, of the University of Michigan and later of the
+University of Wisconsin, endowed the 22 asteroids of his own discovery,
+leaving to the National Academy of Sciences a fund for prosecuting this
+work perpetually, and Leuschner is now ably conducting it.
+
+ [Illustration: JUPITER, LARGEST OF THE PLANETS. The irregular belts
+ change their mutual relation and shapes because they do not
+ represent land, but are part of the atmosphere. (_Photo, Yerkes
+ Observatory._)]
+
+ [Illustration: THE PLANET NEPTUNE AND ITS SATELLITE. The photograph
+ required an exposure of the plate for one hour. (_Photo, Yerkes
+ Observatory._)]
+
+ [Illustration: SATURN, AS SEEN THROUGH THE 40-INCH REFRACTOR, at
+ the time when only the edge of the rings is visible, showing
+ condensations. (_Photo, Yerkes Observatory._)]
+
+ [Illustration: SATURN, PHOTOGRAPHED THROUGH THE 40-INCH REFRACTOR.
+ The rings appear opened to the fullest extent they can be seen
+ from the earth. The picture was made July 7, 1898. (_Photo,
+ Yerkes Observatory._)]
+
+While the number of the asteroids is gratifyingly large, their
+individual size is so small and their total mass so slight that, even if
+there are a hundred thousand of them (as is wholly possible), they would
+not be comparable in magnitude with any one of the great planets. Vesta,
+the largest, is perhaps 400 miles in diameter, and if composed of
+substances similar to those which make up the earth, its mass may be
+perhaps one twenty-thousandth of the earth's mass. If we calculate the
+surface gravity on such a body, we find it about one-thirtieth of what
+it is here; so that a rifle ball, if fired on Vesta with a muzzle
+velocity of only 2,000 feet a second, might overmaster the gravity of
+the little planet entirely and be projected in space never to return.
+
+If, as is likely, some of the smallest asteroids are not more than ten
+miles in diameter, their gravity must be so feeble a force that it might
+be overcome by a stone thrown from the hand. There is no reliable
+evidence that any of the asteroids are surrounded by atmospheric gases
+of any sort. Probably they are for the most part spherical in form,
+although there is very reliable evidence that a few of the asteroids,
+being variable in the amount of sunlight that they reflect, are
+irregular in form, mere angular masses perhaps.
+
+The network of orbits of the asteroids is inconceivable complicated.
+Nevertheless, there is a wide variation in their average distance from
+the sun, and their periods of traveling round him vary in a similar
+manner, the shortest being only about three years. While the longest is
+nearly nine years in duration, the average of all their periods is a
+little over four years. The gap in the zone of asteroids, at a distance
+from the sun equal to about five-eighths that of Jupiter, is due to the
+excessive disturbing action of Jupiter, whose periodic time is just
+twice as long as that of a theoretical planet at this distance.
+
+The average inclination of their orbits to the plane of the ecliptic is
+not far from 8 degrees. But the orbit of Pallas, for example, is
+inclined 35 degrees, and the eccentricities of the asteroid orbits are
+equally erratic and excessive. Both eccentricity and inclination of
+orbit at times suggest a possible relation to cometary orbits, but
+nothing has ever been definitely made out connecting asteroids and
+comets in a related origin.
+
+No comprehensive theory of the origin of the asteroid group has yet been
+propounded that has met with universal acceptance. According to the
+nebular hypothesis the original gaseous material, which should have been
+so concentrated as to form a planet of ordinary type, has in the case of
+the asteroids collected into a multitude of small masses instead of
+simply one. That there is a sound physical reason for this can hardly be
+denied. According to the Laplacian hypothesis, the nearness of the huge
+planetary mass of Jupiter just beyond their orbits produced violent
+perturbations which caused the original ring of gaseous material to
+collect into fragmentary masses instead of one considerable planet. The
+theory of a century ago that an original great planet was shattered by
+internal explosive forces is no longer regarded as tenable.
+
+To astronomers engaged upon investigation of distances in the solar
+system, the asteroid group has proved very useful. The late Sir David
+Gill employed a number of them in a geometrical research for finding the
+sun's distance, and more recently the discovery of Eros (433) has made
+it possible to apply a similar method for a like purpose when it
+approaches nearest to the earth in 1924 and 1931. Then the distance of
+Eros will be less than half that of Mars or even Venus at their nearest.
+
+When the total number of asteroids discovered has reached 1,000, with
+accurate determination of all their orbits, we shall have sufficient
+material for a statistical investigation of the group which ought to
+elucidate the question of its origin, and bear on other problems of the
+cosmogony yet unsolved. Present methods of discovery of the asteroids by
+photography replace entirely the old method by visual observation alone,
+with the result that discoveries are made with relatively great ease and
+rapidity.
+
+
+
+
+CHAPTER XXXVI
+
+THE GIANT PLANET
+
+
+I can never forget as a young boy my first glimpse of the planet Jupiter
+and his moons; it was through a bit of a telescope that I had put
+together with my own hands; a tube of pasteboard, and a pair of old
+spectacle lenses that chanced to be lying about the house.
+
+In the field of view I saw five objects; four of them looking quite
+alike, and as if they were stars merely (they were Jupiter's moons),
+while the fifth was vastly larger and brighter. It was circular in
+shape, and I thought I could see a faint darkish line across the middle
+of it.
+
+This experience encouraged me immensely, and I availed myself eagerly of
+the first chance to see Jupiter through a bigger and better glass. Then
+I saw at once that I had observed nothing wrongly, but that I had seen
+only the merest fraction of what there was to see.
+
+In the first place, the planet's disk was not perfectly circular, but
+slightly oval. Inquiring into the cause of this, we must remember that
+Jupiter is actually not a flat disk but a huge ball or globe, more than
+ten times the diameter of the earth, which turns swiftly round on its
+axis once every ten hours as against the earth's turning round in
+twenty-four hours. Then it is easy to see how the centrifugal force
+bulges outward the equatorial regions of Jupiter, so that the polar
+regions are correspondingly drawn inward, thereby making the polar
+diameter shorter than the equatorial one, which is in line with the
+moons or satellites. The difference between the two diameters is very
+marked, as much as one part in fifteen. All the planets are slightly
+flattened in this way, but Jupiter is the most so of all except Saturn.
+
+The little darkish line across the planet's middle region or equator was
+found to be replaced by several such lines or irregular belts and spots,
+often seen highly colored, especially with reflecting telescopes; and
+they are perpetually changing their mutual relation and shapes, because
+they are not solid territory or land on Jupiter, but merely the outer
+shapes of atmospheric strata, blown and torn and twisted by atmospheric
+circulation on this planet, quite the same as clouds in the atmosphere
+on the earth are.
+
+Besides this the axial turning of Jupiter brings an entirely different
+part of the planet into view every two or three hours; so that in making
+a map or chart of the planet, an arbitrary meridian must be selected.
+Even then the process is not an easy one, and it is found that spots on
+Jupiter's equator turn round in 9 hours 50 minutes, while other regions
+take a few minutes longer, the nearer the poles are approached. The
+Great Red Spot, about 30,000 miles long and a quarter as much in breadth
+has been visible for about half a century. Bolton, an English observer,
+has made interesting studies of it very recently.
+
+The four moons, or satellites, which a small telescope reveals, are
+exceedingly interesting on many accounts. They were the first heavenly
+bodies seen by the aid of the telescope, Galileo having discovered them
+in 1610. They travel round Jupiter much the same as the moon does round
+the earth, but faster, the innermost moon about four times per week, the
+second moon about twice a week, the third or largest moon (larger than
+the planet Mercury) once a week, and the outermost in about sixteen
+days. The innermost is about 260,000 miles from Jupiter, and the
+outermost more than a million miles. From their nearness to the huge and
+excessively hot globe of Jupiter, some astronomers, Proctor especially,
+have inclined to the view that these little bodies may be inhabited.
+
+Jupiter has other moons; a very small one, close to the planet, which
+goes round in less than twelve hours, discovered by Barnard in 1892.
+Four others are known, very small and faint and remote from the planet,
+which travel slowly round it in orbits of great magnitude. The ninth, or
+outermost, is at a distance of fifteen and one-half million miles from
+Jupiter, and requires nearly three years in going round the planet. It
+was discovered by Nicholson at the Lick Observatory in 1914. The eighth
+was discovered by Melotte at Greenwich in 1908, and is peculiar in the
+great angle of 28 degrees, at which its orbit is inclined to the equator
+of Jupiter. The sixth and seventh satellites revolve round Jupiter
+inside the eighth satellite, but outside the orbit of IV; and they were
+discovered by photography at the Lick Observatory in 1905 by Perrine,
+now director of the Argentine National Observatory at Cordoba.
+
+The ever-changing positions of the Medicean moons, as Galileo called the
+four satellites that he discovered--their passing into the shadow in
+eclipse, their transit in front of the disk, and their occultation
+behind it--form a succession of phenomena which the telescopist always
+views with delight. The times when all these events take place are
+predicted in the "Nautical Almanac," many thousand of them each year,
+and the predictions cover two or three years in advance.
+
+Jupiter, as the naked eye sees him high up in the midnight sky, is the
+brightest of all the planets except Venus; indeed, he is five times
+brighter than Sirius, the brightest of all the fixed stars. His stately
+motion among the stars will usually be visible by close observation from
+day to day, and his distance from the earth, at times when he is best
+seen, is usually about 400 million miles. Jupiter travels all the way
+round the sun in twelve years; his motion in orbit is about eight miles
+a second.
+
+The eclipses of Jupiter's moons, caused by passing into the shadow of
+the planet, would take place at almost perfectly regular intervals, if
+our distance from Jupiter were invariable. But it was early found out
+that while the earth is approaching Jupiter the eclipses take place
+earlier and earlier, but later and later when the earth is moving away.
+The acceleration of the earliest eclipse added to the retardation of the
+latest makes 1,000 seconds, which is the time that light takes in
+crossing a diameter of the earth's orbit round the sun. Now the velocity
+of light is well known to be 186,300 miles per second, so we calculate
+at once and very simply that the sun's distance from the earth, which is
+half the diameter of the orbit, equals 500 times 186,300, or 93,000,000
+miles.
+
+
+
+
+CHAPTER XXXVII
+
+THE RINGED PLANET
+
+
+Saturn is the most remote of all the planets that the ancient peoples
+knew anything about. These anciently known planets are sometimes called
+the lucid or naked-eye planets--five in number: Mercury, Venus, Mars,
+Jupiter, and Saturn. Saturn shines as a first-magnitude star, with a
+steady straw-colored light, and is at a distance of about 800 million
+miles from the earth when best seen. Saturn travels completely round the
+sun in a little short of thirty years, and the telescope, when turned to
+Saturn, reveals a unique and astonishing object; a vast globe somewhat
+similar to Jupiter, but surrounded by a system of rings wholly unlike
+anything else in the universe, as far as at present known; the whole
+encircled by a family of ten moons or satellites. The Saturnian system,
+therefore, is regarded by many as the most wonderful and most
+interesting of all the objects that the telescope reveals.
+
+At first the flattening of the disk of Saturn is not easily made out,
+but every fifteen years (as 1921 and 1936) the earth comes into a
+position where we look directly at the thin edge of the rings, causing
+them to completely disappear. Then the remarkable flattening of the
+poles of Saturn is strikingly visible, amounting to as much as one-tenth
+of the entire diameter. The atmospheric belt system is also best seen at
+these times.
+
+But the rings of Saturn are easily the most fascinating features of the
+system. They can never be seen as if we were directly above or beneath
+the planet so they never appear circular, as they really are in space,
+but always oval or elliptical in shape. The minor axis or greatest
+breadth is about one-half the major axis or length. The latter is the
+outer ring's actual diameter, and it amounts to 170,000 miles, or two
+and one-half times the diameter of Saturn's globe.
+
+There are in fact no less than four rings; an outer ring, sometimes seen
+to be divided near its middle; an inner, broader and brighter ring; and
+an innermost dusky, or crape ring, as it is often called. This comes
+within about 10,000 miles of the planet itself. After the form and size
+of the rings were well made out, their thickness, or rather lack of
+thickness, was a great puzzle.
+
+If a model about a foot in diameter were cut out of tissue paper, the
+relative proportion of size and thickness would be about right. In space
+the thickness is very nearly 100 miles, so that, when we look at the
+ring system edge-on, it becomes all but invisible except in very large
+telescopes. Clearly a ring so thin cannot be a continuous solid object
+and recent observations have proved beyond a doubt that Saturn's rings
+are made up of millions of separate particles moving round the planet,
+each as if it were an individual satellite.
+
+Ever since 1857 the true theory of the constitution of the Saturnian
+ring has been recognized on theoretic grounds, because Clerke-Maxwell
+founded the dynamical demonstration that the rings could be neither
+fluid nor solid, so that they must be made up of a vast multitude of
+particles traveling round the planet independently. But the physical
+demonstration that absolutely verified this conclusion did not come
+until 1895, when, as we have said in a preceding chapter, Keeler, by
+radial velocity measures on different regions of the ring by means of
+the spectroscope, proved that the inner parts of the ring travel more
+swiftly round the planet than the outer regions do. And he further
+showed that the rates of revolution in different parts of the ring
+exactly correspond to the periods of revolution which satellites of
+Saturn would have, if at the same distance from the center of the
+planet. The innermost particles of the dusky ring, for example, travel
+round Saturn in about five hours, while the outermost particles of the
+outer bright ring take 137 hours to make their revolution. For many
+years it was thought that the Saturnian ring system was a new satellite
+in process of formation, but this view is no longer entertained; and the
+system is regarded as a permanent feature of the planet, although
+astronomers are not in entire agreement as to the evolutionary process
+by which it came into existence--whether by some cosmic cataclysm, or by
+gradual development throughout indefinite aeons, as the rest of the
+solar system is thought to have come to its present state of existence.
+Possibly the planetesimal hypothesis of Chamberlin and Moulton affords
+the true explanation, as the result of a rupture due to excessive tidal
+strain.
+
+
+
+
+CHAPTER XXXVIII
+
+THE FARTHEST PLANETS
+
+
+On the 13th of March, 1781, between 10 and 11 P. M., as Sir William
+Herschel was sweeping the constellation Gemini with one of his great
+reflecting telescopes, one star among all that passed through the field
+of view attracted his attention. Removing the eyepiece and applying
+another with a higher magnifying power, he found that, unlike all the
+other stars, this one had a small disk and was not a mere point of
+light, as all the fixed stars seem to be.
+
+A few nights' observation showed that the stranger was moving among the
+stars, so he thought it must be a comet; but a week's observation
+following showed that he had discovered a new member of the planetary
+system, far out beyond Saturn, which from time immemorial had been
+assumed to be the outermost planet of all. This, then, was the first
+real discovery of a planet, as the finding of the satellites of Jupiter
+had been the first of all astronomical discoveries. Herschel's discovery
+occasioned great excitement, and he named the new planet Georgium Sidus
+or the Georgian, after his King. The King created him a knight and gave
+him a pension, besides providing the means for building a huge
+telescope, 40 feet long, with which he subsequently made many other
+astronomical discoveries. The planet that Herschel discovered is now
+called Uranus.
+
+Uranus is an object not wholly impossible to see with the naked eye, if
+the sky background is clear and black, and one knows exactly where to
+look for it. Its brightness is about that of a sixth magnitude star or a
+little fainter. Its average distance from the sun is about 1,800 million
+miles and it takes eighty-four years to complete its journey round the
+sun, traveling only a little more than four miles a second. When we
+examine Uranus closely with a large telescope, we find a small disk
+slightly greenish in tint, very slightly flattened, and at times faint
+bands or belts are apparently seen. Uranus is about 30,000 miles in
+diameter, and is probably surrounded by a dense atmosphere. Its rotation
+time is 10 h. 50 m.
+
+Uranus is attended by four moons or satellites, named Ariel, Umbriel,
+Titania, and Oberon, the last being the most remote from the planet.
+This system of satellites has a remarkable peculiarity: the plane of the
+orbits in which they travel round Uranus is inclined about 80 degrees to
+the plane of the ecliptic, so that the satellites travel backward, or in
+a retrograde direction; or we might regard their motion as forward, or
+direct, if we considered the planes of their orbits inclined at 100
+degrees.
+
+For many years after the discovery of Uranus it was thought that all the
+great bodies of the solar system had surely been found. Least of all was
+any planet suspected beyond Uranus until the mathematical tables of the
+motion of Uranus, although built up and revised with the greatest care
+and thoroughness, began to show that some outside influence was
+disturbing it in accordance with Newton's law of gravitation. The
+attraction of a still more distant planet would account for the
+disturbance, and since no such planet was visible anywhere a
+mathematical search for it was begun.
+
+
+NEPTUNE
+
+Wholly independently of each other, two young astronomers, Adams of
+England and Le Verrier of France, undertook to solve the unique problem
+of finding out the position in the sky where a planet might be found
+that would exactly account for the irregular motion of Uranus. Both
+reached practically identical results. Adams was first in point of time,
+and his announcement led to the earliest observation, without
+recognition of the new planet (July 30, 1846), although it was Le
+Verrier's work that led directly to the new planet's being first seen
+and recognized as such (September 23, 1846). Figuring backward, it was
+found that the planet had been accidentally observed in Paris in 1795,
+but its planetary character had been overlooked.
+
+Neptune is the name finally assigned to this historical planet. It is
+thirty times farther from the sun than the earth, or 2,800 million
+miles; its velocity in orbit is a little over three miles per second,
+and it consumes 164 years in going once completely round the sun. So
+faint is it that a telescope of large size is necessary to show it
+plainly. The brightness equals that of a star of the eighth magnitude,
+and with a telescope of sufficient magnifying power, the tiny disk can
+be seen and measured. The planet is about 30,000 miles in diameter, and
+is not known to possess more than one moon or satellite. If there are
+others, they are probably too faint to be seen by any telescope at
+present in existence.
+
+
+
+
+CHAPTER XXXIX
+
+THE TRANS-NEPTUNIAN PLANET
+
+
+Investigation of the question of a possible trans-Neptunian planet was
+undertaken by the writer in 1877. As Neptune requires 164 years to
+travel completely round the sun, and the period during which it has been
+carefully observed embraces only half that interval, clearly its orbit
+cannot be regarded as very well known. Any possible deviations from the
+mathematical orbit could not therefore be traced to the action of a
+possible unknown planet outside. But the case was different with Uranus,
+which showed very slight disturbances, and these were assumed to be due
+to a possible planet exterior to both Uranus and Neptune. As a position
+for this body in the heavens was indicated by the writer's
+investigation, that region of the sky was searched by him with great
+care in 1877-1878 with the twenty-six-inch telescope at Washington; and
+photographs of the same region were afterward taken by others, though
+only with negative results.
+
+In 1880, Forbes of Edinburgh published his investigation of the problem
+from an entirely independent angle. Families of comets have long been
+recognized whose aphelion distances correspond so nearly with the
+distances of the planets that these comet families are now recognized as
+having been created by the several planets, which have reduced the high
+original velocities possessed by the comets on first entering the solar
+system.
+
+Their orbits have ever since been ellipses with their aphelia in groups
+corresponding to the distances of the planets concerned. Jupiter has a
+large group of such comets, also Saturn. Uranus and Neptune likewise
+have their families of comets, and Forbes found two groups with average
+distances far outside of Neptune; from which he drew the inference that
+there are two trans-Neptunian planets. The position he assigned to the
+inner one agreed fairly well with the writer's planet as indicated by
+unexplained deviations of Uranus.
+
+The theoretical problem of a trans-Neptunian planet has since been taken
+up by Gaillot and Lau of Paris, the late Percival Lowell, and W. H.
+Pickering of Harvard. The photographic method of search will, it is
+expected, ultimately lead to its discovery. On account of the probable
+faintness of the planet, at least the twelfth or thirteenth magnitude,
+Metcalf's method of search is well adapted to this practical problem.
+When near its opposition the motion of Neptune retrograding among the
+stars amounts to five seconds of arc in an hour; while the
+trans-Neptunian planet would move but three seconds. By shifting the
+plate this amount hourly during exposure, the suspected object would
+readily be detected on the photographic plate as a minute and nearly
+circular disk, all the adjacent stars being represented by short trails.
+
+Interest in a possible planet or planets outside the orbit of Neptune is
+likely to increase rather than diminish. To the ancients seven was the
+perfect number, there were seven heavenly bodies already known, so there
+could be no use whatever in looking for an eighth. The discovery of
+Uranus in 1781 proved the futility of such logic, and Neptune followed
+in 1846 with further demonstration, if need be. The cosmogony of the
+present day sets no outer limit to the solar system, and some
+astronomers advocate the existence of many trans-Neptunian planets.
+
+
+
+
+CHAPTER XL
+
+COMETS--THE HAIRY STARS
+
+
+Comets--hairy stars, as the origin of the name would indicate--are the
+freaks of the heavens. Of great variety in shape, some with heads and
+some without, some with tails and some without, moving very slowly at
+one time and with exceedingly high velocity at another, in orbits at all
+possible angles of inclination to the general plane of the planetary
+paths round the sun, their antics and irregularities were the wonder and
+terror of the ancient world, and they are keenly dreaded by
+superstitious people even to the present day.
+
+Down through the Middle Ages the advent of a comet was regarded as:
+
+ Threatening the world with famine, plague and war;
+ To princes, death; to kingdoms, many curses;
+ To all estates, inevitable losses;
+ To herdsmen, rot; to plowmen, hapless seasons;
+ To sailors, storms; to cities, civil treasons.
+
+Comets appeared to be marvelous objects, as well as sinister, chiefly
+because they bid apparent defiance to all law. Kepler had shown that the
+moon and the planets travel in regular paths--slightly elliptical to be
+sure, but nevertheless unvarying. None of the comets were known to
+follow regular paths till the time of Halley late in the seventeenth
+century, when, as we have before told, a fine comet made its
+appearance, and Halley calculated its orbit with much precision.
+Comparing this with the orbits of comets that had previously been seen,
+he found its path about the sun practically identical with that of at
+least two comets previously observed in 1531 and 1607.
+
+So Halley ventured to think all these comets were one and the same body,
+and that it traveled round the sun in a long ellipse in a period of
+about seventy-five or seventy-six years. We have seen how his prediction
+of its return in 1758 was verified in every particular. On the comet's
+return in 1910, Crowell and Crommelin of Greenwich made a thorough
+mathematical investigation of the orbit, indicating that the year 1986
+will witness its next return to the sun.
+
+There is a class of astronomers known as comet-hunters, and they pass
+hours upon hours of clear, sparkling, moonless nights in search for
+comets. They are equipped with a peculiar sort of telescope called a
+comet-seeker, which has an object glass usually about four or five
+inches in diameter, and a relatively short length of focus, so that a
+larger field of view may be included. Regions near the poles of the
+heavens are perhaps the most fruitful fields for search, and thence
+toward the sun till its light renders the sky too bright for the finding
+of such a faint object as a new comet usually is at the time of
+discovery. Generally when first seen it resembles a small circular patch
+of faint luminous cloud.
+
+When a suspect is found, the first thing to do is to observe its
+position accurately with relation to the surrounding stars. Then, if on
+the next occasion when it is seen the object has moved, the chances are
+that it is a comet; and a few days' observation will provide material
+from which the path of the comet in space can be calculated. By
+comparing this with the complete lists of comets, now about 700 in
+number, it is possible to tell whether the comet is a new one, or an old
+one returning. The total number of comets in the heavens must be very
+great, and thousands are doubtless passing continually undetected,
+because their light is wholly overpowered by that of the sun. Of those
+that are known, perhaps one in twelve develops into a naked-eye comet,
+and in some years six or seven will be discovered. With sufficiently
+powerful telescopes, there are as a rule not many weeks in the year when
+no comet is visible. Brilliant naked-eye comets are, however,
+infrequent.
+
+Comets, except Halley's, generally bear the name of their discoverer, as
+Donati (1858), and Pons-Brooks (1893). Pons was a very active discoverer
+of comets in France early in the nineteenth century: he was a doorkeeper
+at the observatory of Marseilles, and his name is now more famous in
+astronomy than that of Thulis, then the director of the Observatory, who
+taught and encouraged him. Messier was another very successful
+discoverer of comets in France, and in America we have had many: Swift,
+Brooks, and Barnard the most successful.
+
+How bright a comet will be and how long it will be visible depends upon
+many conditions. So the comets vary much in these respects. The first
+comet of 1811 was under observation for nearly a year and a half, the
+longest on record till Halley's in 1910. In case a comet eludes
+discovery and observation until it has passed its perihelion, or nearest
+point to the sun, its period of visibility may be reduced to a few
+weeks only. The brightest comets on record were visible in 1843 and
+1882: so brilliant were they that even the effulgence of full daylight
+did not overpower them. In particular the comet of 1843 was not only
+excessively bright, but at its nearest approach to the earth its tail
+swept all the way across the sky from one horizon to the other. It must
+have looked very much like the straight beam of an enormous searchlight,
+though very much brighter.
+
+The tails of comets are to the naked eye the most compelling thing about
+them, and to the ancient peoples they were naturally most terrifying.
+Their tails are not only curved, but sometimes curved with varying
+degrees of curvature, and this circumstance adds to their weirdness of
+appearance. If we examine the tail of a comet with a telescope, it
+vanishes as if there were nothing to it: as indeed one may almost say
+there is not. Ordinarily, only the head of the comet is of interest in
+the telescope. When first seen there is usually nothing but the head
+visible, and that is made up of portions which develop more or less
+rapidly, presenting a succession of phenomena quite different in
+different comets.
+
+When first discovered a comet is usually at a great distance from the
+sun, about the distance of Jupiter; and we see it, not as we do the
+planets, by sunlight reflected from them, but by the comet's own light.
+This is at that time very faint, and nearly all comets at such a
+distance look alike: small roundish hazy patches of faint, cloudlike
+light, with very often a concentration toward the center called the
+nucleus, on the average about 4,000 miles in diameter. Approach toward
+the sun brightens up the comet more and more, and the nucleus usually
+becomes very much brighter and more starlike. Then on the sunward side
+of the nucleus, jetlike streamers or envelopes appear to be thrown off,
+often as if in parallel curved strata, or concentrically. As they expand
+and move outward from the nucleus, these envelopes grow fainter and are
+finally merged in the general nebulosity known as the comet's head,
+which is anywhere from 30,000 to 100,000 miles in diameter. As a rule,
+this is an orderly development which can be watched in the telescope
+from hour to hour and from night to night; but occasionally a cometary
+visitor is quite a law to itself in development, presenting a
+fascinating succession of unpredictable surprises.
+
+Then follows the development of the comet's tail, perhaps more striking
+than anything that has preceded it. Here a genuine repulsion from the
+sun appears to come into play. It may be an electrical repulsion. Much
+of the material projected from the comet's nucleus, seems to be driven
+backward or repelled by the sun, and it is this that goes to form the
+tail. The particles which form the tail then travel in modified paths
+which nevertheless can be calculated. The tail is made up of these
+luminous particles and it expands in space much in the form of a hollow,
+horn-shaped cone, the nucleus being near the tip of the horn.
+
+Some comets possess multiple tails with different degrees of curvature,
+Donati's for example. Usually there is a nearly straight central dark
+space, marking the axis of the comet, and following the nucleus. But
+occasionally this is replaced by a thin light streak very much less in
+breadth than the diameter of the head. Cometary tails are sometimes 100
+million miles in length.
+
+Three different types of cometary tails are recognized. First, the long
+straight ones, apparently made up of matter repelled by the sun twelve
+to fifteen times more powerfully than gravitation attracts it. Such
+particles must be brushed away from the comet's head with a velocity of
+perhaps five miles a second, and their speed is continually increasing.
+Probably these straight tails are due to hydrogen. The second type tails
+are somewhat curved, or plume-like, and they form the most common type
+of cometary tail. In them the sun's repulsion is perhaps twice its
+gravitational attraction, and hydrocarbons in some form appear to be
+responsible for tails of this character. Then there is a third type,
+much less often seen, short and quickly curving, probably due to heavier
+vapors, as of chlorine, or iron, or sodium, in which the repulsive force
+is only a small fraction of that of gravitation.
+
+Many features of this theory of cometary tails are borne out by
+examination of their light with the spectroscope, although the
+investigation is as yet fragmentary. It is evident that the tail of a
+comet is formed at the expense of the substance of the nucleus and head;
+so that the matter repelled is forever dissipated through the regions of
+space which the comet has traveled. Comets must lose much of their
+original substance every time they return to perihelion. Comets actually
+age, therefore, and grow less and less in magnitude of material as well
+as brightness, until they are at last opaque, nonluminous bodies which
+it becomes impossible to follow with the telescope.
+
+
+
+
+CHAPTER XLI
+
+WHERE DO COMETS COME FROM?
+
+
+Where do comets come from? The answer to this question is not yet fully
+made out. Most likely they have not all had a similar origin, and
+theories are abundant. Apparently they come into the solar system from
+outer space, from any direction whatsoever. The depths of interstellar
+space seem to be responsible for most, if not all, of the new ones.
+Whether they have come from other stars or stellar systems we cannot
+say.
+
+While comets are tremendous in size or volume, their mass or the amount
+of real substance in them is relatively very slight. We know this by the
+effect they produce on planets that they pass near, or rather by the
+effect that they fail to produce. The earth's atmosphere weighs about
+one two hundred and fifty thousandth as much as the earth itself, but a
+comet's entire mass must be vastly less than this. Even if a comet were
+to collide with the earth head on, there is little reason to believe
+that dire catastrophe would ensue. At least twice the earth is known to
+have passed through the tail of a comet, and the only effect noticed was
+upon the comet itself; its orbit had been modified somewhat by the
+attraction of the earth. If the comet were a small one, collision with
+any of the planets would result in absorption and dissipation of the
+comet into vapor.
+
+The whole of a large comet has perhaps as much mass or weight as a
+sphere of iron a hundred miles in diameter. Even this could not wreck
+the earth, but the effect would depend upon what part of the earth was
+hit. A comet is very thin and tenuous, because its relatively small mass
+is distributed through a volume so enormous. So it is probable that the
+earth's atmosphere could scatter and burn up the invading comet, and we
+should have only a shower of meteors on an unprecedented scale.
+Diffusion of noxious gases through the atmosphere might vitiate it to
+some extent, though probably not enough to cause the extinction of
+animal life.
+
+Every comet has an interesting history of its own, almost indeed unique.
+One of the smallest comets and the briefest in its period round the sun
+is known as Encke's comet. It is a telescopic comet with a very short
+tail, its time of revolution is about three and a half years, and it
+exhibits a remarkable contraction of volume on approach to the sun.
+
+Biela's comet has a period about twice as long. At one time it passed
+within about 15 million miles of the earth, and somewhere about the year
+1840 this comet divided into two distinct comets, which traveled for
+months side by side, but later separated and both have since completely
+disappeared. Perhaps the most beautiful of all comets is that discovered
+by Donati of Florence in 1858. Its coma presented the development of
+jets and envelopes in remarkable perfection, and its tail was of the
+secondary or hydrocarbon type, but accompanied by two faint streamer
+tails, nearly tangential to the main tail and of the hydrogen type.
+Donati's comet moves in an ellipse of extraordinary length, and it will
+not return to the sun for nearly 2,000 years.
+
+The most brilliant comet of the last half century is known as the great
+comet of 1882. In a clear sky it could readily be seen at midday. On
+September 17 it passed across the disk of the sun and was practically as
+bright as the surface of the sun itself. The comet had a multiple
+nucleus and a hydrocarbon tail of the second type, nearly a hundred
+million miles in length. Doubtless this great comet is a member of what
+is known as a cometary group, which consists of comets having the same
+orbit and traveling tandem round the sun. The comets of 1668, 1843,
+1880, 1882 and 1887 belong to this particular group, and they all pass
+within 300,000 miles of the sun's surface, at a maximum velocity
+exceeding 300 miles a second. They must therefore invade the regions of
+the solar corona, the inference being that the corona as well as the
+comet is composed of exceedingly rare matter.
+
+Photography of comets has developed remarkably within recent years,
+especially under the deft manipulation of Barnard, whose plates, in
+particular during his residence at the Lick Observatory on Mount
+Hamilton, California, show the features of cometary heads and tails in
+excellent definition. Halley's comet, at the 1910 apparition, was
+particularly well photographed at many observatories.
+
+The question is often asked, When will the next comet come? If a large
+bright comet is meant, astronomers cannot tell. At almost any time one
+may blaze into prominence within only a few days. During the latter half
+of the last century, bright comets appeared at perihelion at intervals
+of eight years on the average. Several of the lesser and fainter
+periodic comets return nearly every year, but they are mostly
+telescopic, and are rarely seen except by astronomers who are
+particularly interested in observing them.
+
+
+
+
+CHAPTER XLII
+
+METEORS AND SHOOTING STARS
+
+
+"Falling stars," or "shooting stars," have been familiar sights in all
+ages of the world, but the ancient philosophers thought them scarcely
+worthy of notice. According to Aristotle they were mere nothings of the
+upper atmosphere, of no more account than the general happenings of the
+weather. But about the end of the eighteenth century and the beginning
+of the nineteenth the insufficiency of this view began to be fully
+recognized, and interplanetary space was conceived as tenanted by shoals
+of moving bodies exceedingly small in mass and dimension as compared
+with the planets.
+
+Millions of these bodies are all the time in collision with the outlying
+regions of our atmosphere; and by their impact upon it and their
+friction in passing swiftly through it, they become heated to
+incandescence, thus creating the luminous appearances commonly known as
+shooting stars. For the most part they are consumed or dissipated in
+vapor before reaching the solid surface of the earth; but occasionally a
+luminous cloud or streak is left glowing in the wake of a large meteor,
+which sometimes remains visible for half an hour after the passage of
+the meteor itself. These mistlike clouds projected upon the dark sky
+have been especially studied by Trowbridge of Columbia University.
+
+Many more meteors are seen during the morning hours, say from four to
+six, than at any other nightly period of equal length, because the
+visible sky is at that time nearly centered around the general direction
+toward which the earth is moving in its orbit round the sun; so that the
+number of meteors that would fall upon the earth if at rest is increased
+by those which the earth overtakes by its own motion. Also from January
+to July while the earth is traveling from perihelion to aphelion, fewer
+meteors are seen than in the last half of the year; but this is chiefly
+because of the rich showers encountered in August and November.
+
+Although the descent of meteoric bodies from the sky was pretty
+generally discredited until early in the nineteenth century, such falls
+had nevertheless been recorded from very early times. They were usually
+regarded as prodigies or miracles, and such stones were commonly objects
+of worship among ancient peoples. For example, the Phrygian Stone, known
+as the "Diana of the Ephesians which fell down from Jupiter," was a
+famous stone built into the Kaaba at Mecca, and even to-day it is
+revered by Mohammedans as a holy relic. Perhaps the earliest known
+meteoric fall is that historically recorded in the Parian Chronicle as
+having occurred in the island of Crete, B. C. 1478. Also in the imperial
+museum of Petrograd is the Pallas or Krasnoiarsk iron, perhaps
+three-quarters of a ton in weight, found in 1772 by Pallas, the famous
+traveler, at Krasnoiarsk, Siberia.
+
+But a fall of meteoric stones that chanced upon the department of Orne,
+France, in 1805, led to a critical investigation by Biot, the
+distinguished physicist and academician. According to his report a
+violent explosion in the neighborhood of L'Aigle had been heard for a
+distance of seventy-five miles around, and lasting five or six minutes,
+about 1 P. M. on Tuesday, April 26. From several adjoining towns a
+rapidly moving fireball had been seen in a sky generally clear, and
+there was absolutely no room for doubt that on the same day many stones
+fell in the neighborhood of L'Aigle. Biot estimated their number between
+two and three thousand, and they were scattered over an elliptical area
+more than six miles long, and two and a half miles broad. Thenceforward
+the descent of meteoric matter from outer space upon the earth has been
+recognized as an unquestioned fact.
+
+The origin of these bodies being cosmic, meteors may be expected to fall
+upon the earth without reference to latitude, or season, or day and
+night, or weather. On entering our upper atmosphere their temperature
+must be that of space, many hundred degrees below zero; and their
+velocities range from ten miles per second upward. But atmospheric
+resistance to their flight is so great that their velocity is quickly
+reduced: at ground impact it does not exceed a few hundred feet per
+second. On January 1, 1869, several meteoric stones fell on ice only a
+few inches thick in Sweden, rebounding without either breaking through
+the ice or being themselves fractured.
+
+Naturally the flight of a meteor through the atmosphere will be only a
+few seconds in duration, and owing to the sudden reduction of velocity,
+it will continue to be luminous throughout only the upper part of its
+course. Visibility generally begins at an elevation of about seventy
+miles, and ends at perhaps half that altitude.
+
+What is the origin of meteors? Theories there are in great abundance:
+that they come from the sun, that they come from the moon, that they
+come from the earth in past ages as a result of volcanic action, and so
+on. But there are many difficulties in the way of acceptance of these
+and several other theories. That all meteors were originally parts of
+cometary masses is however a theory that may be accepted without much
+hesitation.
+
+Comets have been known to disintegrate. Biela's comet even disappeared
+entirely, so that during a shower of Biela meteors in November, 1885, an
+actual fragment of the lost comet fell upon the earth, at Mazapil,
+Mexico. And as the Bielid meteors encounter the earth with the
+relatively low velocity of ten miles a second, we may expect to capture
+other fragments in the future. Numerous observers saw the weird
+disintegration of the nucleus of the great comet of 1882, well
+recognized as a member of the family of the comet of 1843. As these
+comets are fellow voyagers through space along the same orbit, probably
+all five members of the family, with perhaps others, were originally a
+single comet of unparalleled magnitude.
+
+The Brooks comet of 1890 affords another instance of fragmentary
+nucleus. The oft-repeated action of solar forces tending to disrupt the
+mass of a comet more and more, and scatter its material throughout
+space, the secular dismemberment of all comets becomes an obvious
+conclusion. During the hundreds of millions of years that these forces
+are known to have been operant, the original comets have been broken up
+in great numbers, so that elliptical rings of opaque meteoric bodies now
+travel round the sun in place of the comets.
+
+These bodies in vast numbers are everywhere through space, each too
+small to reflect an appreciable amount of sunlight, and becoming
+visible only when they come into collision with our outer atmosphere.
+The practical identity of several such meteor streams and cometary
+orbits has already been established, and there is every reason for
+assigning a similar origin to all meteoric bodies. Meteors, then, were
+originally parts of comets, which have trailed themselves out to such
+extent that particles of the primal masses are liable to be picked up
+anywhere along the original cometary paths. The historic records of all
+countries contain trustworthy accounts of meteoric showers. Making due
+allowances for the flowery imagery of the oriental, it is evident that
+all have at one time or another seen much the same thing. In A. D. 472,
+for instance, the Constantinople sky was reported alive with flying
+stars. In October, 1202, "stars appeared like waves upon the sky; and
+they flew about like grasshoppers." During the reign of King William II
+occurred a very remarkable shower in which "stars seemed to fall like
+rain from heaven."
+
+But the showers of November, 1799 and 1833, are easily the most striking
+of all. The sky was filled with innumerable fiery trails and there was
+not a space in the heavens a few times the size of the moon that was not
+ablaze with celestial fireworks. Frequently huge meteors blended their
+dazzling brilliancy with the long and seemingly phosphorescent trails of
+the shooting stars.
+
+The interval of thirty-four years between 1799 and 1833 appeared to
+indicate the possibility of a return of the shower in November of 1866
+or 1867, and all the people of that day were aroused on this subject and
+made every preparation to witness the spectacle. Extemporized
+observatories were established, watchmen were everywhere on the
+lookout, and bells were to be rung the minute the shower began. The
+newspapers of the day did little to allay the fears of the multitude,
+but the critical days of November, 1866, passed with disappointment in
+America. In Europe, however, a fine shower was seen, though it was not
+equal to that of 1833. The astronomers at Greenwich counted many
+thousand meteors. In November of 1867, however, American astronomers
+were gratified by a grand display, which, although failing to match the
+general expectation, nevertheless was a most striking spectacle, and the
+careful preparation for observing it afforded data of observation which
+were of the greatest scientific value. The actual orbits of these bodies
+in space became known with great exactitude, and it was found that their
+general path was identical with that of the first comet of 1866, which
+travels outward somewhat beyond the planet Uranus. When the visible
+paths of these meteors are traced backward, all appear as if they
+originated from the constellation Leo. So they are known as Leonids, and
+a return of the shower was confidently predicted for November,
+1900-1901, which for unknown reasons failed to appear.
+
+ [Illustration: TWO VIEWS OF HALLEY'S COMET. Taken with the same
+ camera from the same position, one on May 12, and the other on
+ May 15, 1910. (_Photo, Mt. Wilson Solar Observatory._)]
+
+ [Illustration: SWIFT'S COMET OF 1892. This comet showed
+ extraordinary and rapid transformations, one day having a dozen
+ streamers in its tail, another only two. (_Photo by Prof. E. E.
+ Barnard._)]
+
+ [Illustration: A LARGE METEOR TRAIL IN THE FIELD WITH FINE NEBULAE.
+ (_Photo, Yerkes Observatory._)]
+
+During the last half century meteors have been pretty systematically
+observed, especially by the astronomers of Italy and Denning of England,
+so that several hundred distinct showers are now known, their radiant
+points fall in every part of the heavens, and there is scarcely a clear
+moonless night when careful watching for meteors will be unrewarded.
+Besides November, the months of August (Perseids), April (Lyrids), and
+December (Geminids) are favorable. Following in tabular form is a fairly
+comprehensive list of the meteoric showers of the year, with the
+positions of the radiant points and the epochs of the showers according
+to Denning:
+
+RADIANT POINT
+
+ ============================================================
+ Name of Shower | R. A. | Decl. | Date of Shower
+ -----------------------+---------+--------+-----------------
+ Quadrantids | 230 deg. | +53 deg. | Jan. 2-4
+ Zeta Cepheids | 331 deg. | +56 deg. | Jan. 25
+ Alpha Leonids | 155 deg. | +14 deg. | Feb. 19-March 1
+ Tau Leonids | 166 deg. | +4 deg. | March 1-4
+ Beta Ursids | 161 deg. | +58 deg. | March 13-24
+ Lyrids | 271 deg. | +33 deg. | April 20-22
+ Gamma Aquarids | 338 deg. | -2 deg. | May 1-6
+ Zeta Herculids | 246 deg. | +29 deg. | May 18-26
+ Eta Pegasids | 330 deg. | +28 deg. | May 30-June 4
+ Theta Booetids | 213 deg. | +53 deg. | June 27-28
+ Alpha Capricornids | 304 deg. | -12 deg. | July 15-28
+ Delta Aquarids | 339 deg. | -11 deg. | July 25-30
+ Perseids | 45 deg. | +57 deg. | Aug. 10-12
+ Omicron Draconids | 291 deg. | +60 deg. | Aug. 15-25
+ Zeta Draconids | 262 deg. | +63 deg. | Aug. 21-Sept. 2
+ Piscids | 348 deg. | +2 deg. | Sept. 4-14
+ Alpha Andromedids | 4 deg. | +28 deg. | Sept. 27
+ Epsilon Arietids | 40 deg. | +20 deg. | Oct. 11-24
+ Orionids | 92 deg. | +15 deg. | Oct. 17-24
+ Epsilon Perseids | 61 deg. | +35 deg. | Nov. 5
+ Leonids | 150 deg. | +23 deg. | Nov. 13-15
+ Epsilon Taurids | 64 deg. | +22 deg. | Nov. 14-25
+ Andromedids | 25 deg. | +43 deg. | Nov. 17-23
+ Beta Geminids | 119 deg. | +31 deg. | Dec. 1-12
+ Geminids | 108 deg. | +33 deg. | Dec. 1-14
+ Alpha Ursae Majorids | 161 deg. | +58 deg. | Dec. 18-21
+ Kappa Draconids | 194 deg. | +68 deg. | Dec. 18-28
+ ------------------------------------------------------------
+
+The year 1916 was exceptional in providing an abundant and previously
+unknown shower on June 28, and its stream has nearly the same orbit as
+that of the Pons-Winnecke periodic comet. Useful observations of meteors
+are not difficult to make, and they are of service to professional
+astronomers investigating the orbits of these bodies, among whom are
+Mitchell and Olivier of the University of Virginia.
+
+
+
+
+CHAPTER XLIII
+
+METEORITES
+
+
+Meteorites, the name for meteors which have actually gone all the way
+through our atmosphere, are never regular in form or spherical. As a
+rule the iron meteorites are covered with pittings or thumb marks, due
+probably to the resistance and impact of the little columns of air which
+impede its progress, together with the unequal condition and fusibility
+of their surface material. The work done by the atmosphere in suddenly
+checking the meteor's velocity appears in considerable part as heat,
+fusing the exterior to incandescence. This thin liquid shell is quickly
+brushed off, making oftentimes a luminous train.
+
+But notwithstanding the exceedingly high temperature of the exterior,
+enforced upon it for the brief time of transit through the atmosphere,
+it is probable that all large meteorites, if they could be reached at
+once on striking the earth, would be found to be cold, because the
+smooth, black, varnishlike crust which always incases them as a result
+of intense heat is never thick. On one occasion a meteor which was seen
+to fall in India was dug out of the ground as quickly as possible, and
+found to be, not hot as was expected, but coated thickly over with ice
+frozen on it from the moisture in the surrounding soil.
+
+As to the composition of shooting stars, and their probable mass, and
+its effect upon the earth, our data are quite insufficient. The lines of
+sodium and magnesium have been hurriedly caught in the spectroscope,
+and, estimating on the basis of the light emitted by them, the largest
+meteors must weigh ounces rather than pounds. Nevertheless, it is
+interesting to inquire what addition the continual fall of many millions
+daily upon the earth makes to its weight: somewhere between thirty and
+fifty thousand tons annually is perhaps a conservative estimate, but
+even this would not accumulate a layer one inch in thickness over the
+entire surface of the earth in less than a thousand million years.
+
+Many hundreds of the meteors actually seen to fall, together with those
+picked up accidentally, are recovered and prized as specimens of great
+value in our collections, the richest of which are now in New York,
+Paris, and London. The detailed investigation of them is rather the
+province of the chemist, the crystallographer and the mineralogist than
+of the astronomer whose interest is more keen in their life history
+before they reach the earth. To distinguish a stony meteorite from
+terrestrial rock substances is not always easy, but there is usually
+little difficulty in pronouncing upon an iron meteorite. These are most
+frequently found in deserts, because the dryness of the climate renders
+their oxidation and gradual disappearance very slow.
+
+The surface of a suspected iron meteorite is polished to a high luster
+and nitric acid is poured upon it. If it quickly becomes etched with a
+characteristic series of lines, or a sort of cross-hatching, it is
+almost certain to be a meteorite. Occasionally carbon has been found in
+meteorites, and the existence of diamond has been suspected. The
+minerals composing meteorites are not unlike terrestrial materials of
+volcanic origin, though many of them are peculiar to meteorites only.
+More than one-third of all the known chemical elements have been found
+by analysis in meteorites, but not any new ones.
+
+Meteoric iron is a rich alloy containing about ten per cent of nickel,
+also cobalt, tin, and copper in much smaller amount. Calcium, chlorine,
+sodium, and sulphur likewise are found in meteoric irons. At very high
+temperatures iron will absorb gases and retain them until again heated
+to red heat. Carbonic oxide, helium, hydrogen, and nitrogen are thus
+imprisoned, or occluded, in meteoric irons in very small quantities; and
+in 1867, during a London lecture by Graham, a room in the Royal
+Institution was for a brief space illuminated by gas brought to earth in
+a meteorite from interplanetary space. Meteorites, too, have been most
+critically investigated by the biologist, but no trace of germs of
+organic life of any type has so far been found. Farrington of Chicago
+has published a full descriptive catalogue of all the North American
+meteorites.
+
+Recent investigations of the radioactivity of meteorites show that the
+average stone meteorite is much less radioactive than the average rock,
+and probably less than one-fourth as radioactive as in average granite.
+The metallic meteorites examined were found about wholly free from
+radioactivity.
+
+From shooting stars, perhaps the chips of the celestial workshop, or
+more possibly related to the planetesimals which the processes of growth
+of the universe have swept up into the vastly greater bodies of the
+universe, transition is natural to the stars themselves, the most
+numerous of the heavenly bodies, all shining by their own light, and all
+inconceivably remote from the solar system, which nevertheless appears
+to be not far removed from the center of the stellar universe.
+
+
+
+
+CHAPTER XLIV
+
+THE UNIVERSE OF STARS
+
+
+Our consideration of the solar system hitherto has kept us quite at home
+in the universe. The outer known planets, Uranus and Neptune, are indeed
+far removed from the sun, and a few of the comets that belong to our
+family travel to even greater distances before they begin to retrace
+their steps sunward. When we come to consider the vast majority of the
+glistening points on the celestial sphere--all in fact except the five
+great planets, Mercury, Venus, Mars, Jupiter, and Saturn--we are dealing
+with bodies that are self-luminous like the sun, but that vary in size
+quite as the bodies of the solar system do, some stars being smaller
+than the sun and others many hundred fold larger than he is; some being
+"giants," and others "dwarfs." But the overwhelming remoteness of all
+these bodies arrests our attention and even taxes our credulity
+regarding the methods that astronomers have depended on to ascertain
+their distances from us.
+
+Their seeming countlessness, too, is as bewildering as are the
+distances; though, if we make actual counts of those visible to the
+naked eye within a certain area, in the body of the "Great Bear," for
+example, the great surprise will be that there are so few. And if the
+entire dome of the sky is counted, at any one time, a clear, moonless
+sky would reveal perhaps 2,500, so that in the entire sky, northern and
+southern, we might expect to find 5,000 to 6,000 lucid stars, or stars
+visible to the naked eye.
+
+But when the telescope is applied, every accession of power increases
+the myriads of fainter and fainter stars, until the number within
+optical reach of present instruments is somewhere between 400 and 500
+millions. But if we were to push the 100-inch reflector on Mount Wilson
+to its limit by photography with plates of the highest sensitiveness,
+millions upon millions of excessively faint stars would be plainly
+visible on the plates which the human eye can never hope to see directly
+with any telescope present or future, and which would doubtless swell
+the total number of stars to a thousand millions. Recent counts of stars
+by Chapman and Melotte of Greenwich tend to substantiate this estimate.
+
+What have astronomers done to classify or catalogue this vast array of
+bodies in the sky? Even before making any attempt to estimate their
+number, there is a system of classification simply by the amount of
+light they send us, or by their apparent stellar magnitudes--not their
+actual magnitudes, for of those we know as yet very little. We speak of
+stars of the "first magnitude," of which there are about 20, Sirius
+being the brightest and Regulus the faintest. Then there are about 65 of
+the second, or next fainter, magnitude, stars like Polaris, for example,
+which give an amount of light two and a half times less than the average
+first magnitude star. Stars of the third magnitude are fainter than
+those of the second in the same ratio, but their number increases to
+200; fourth magnitude, 500; fifth magnitude, 1,400; sixth magnitude,
+5,000, and these are so faint that they are just visible on the best
+nights without telescopic aid.
+
+Decimals express all intermediate graduations of magnitude. Astronomers
+carry the telescopic magnitudes much farther, till a magnitude beyond
+the twentieth is reached, preserving in every case the ratio of two and
+one-half for each magnitude in relation to that numerically next to it.
+Even Jupiter and Venus, and the sun and moon, are sometimes calculated
+on this scale of stellar magnitude, numerically negative, of course,
+Venus sometimes being as bright as magnitude -4.3, and the sun -26.7.
+
+Knowing thus the relation of sun, moon, and stars, and the number of the
+stars of different magnitudes, it is possible to estimate the total
+light from the stars. This interesting relation comes out this way: that
+the stars we cannot see with the naked eye give a greater total of light
+than those we can because of their vastly greater numbers. And if we
+calculate the total light of all the brighter stars down to magnitude
+nine and one-half, we find it equal to 1/80th of the light of the
+average full moon.
+
+Many stars show marked differences in color, and strictly speaking the
+stars are now classified by their colors. The atmosphere affects star
+colors very considerably, low altitudes, or greater thickness of air,
+absorbing the bluish rays more strongly and making the stars appear
+redder than they really are. Aldebaran, Betelgeuse and Antares are
+well-known red stars, Capella and Alpha Ceti yellowish, Vega and Sirius
+blue, and Procyon and Polaris white. Among the telescopic stars are many
+of a deep blood-red tint, variable stars being numerous among them.
+Double stars, too, are often complementary in color. There is evidence
+indicating change of color of a very few stars in long periods of time;
+Sirius, for example, two thousand years ago was a red star, now it is
+blue or bluish white. But the meaning of color, or change of color in a
+star is as yet only incompletely ascertained. It may be connected with
+the radiative intensity of the star, or its age, or both.
+
+The late Professor Edward C. Pickering was famous for his life-long
+study and determination of the magnitudes of the stars. Standards of
+comparison have been many, and have led to much unnecessary work.
+Pickering chose Polaris as a standard and devised the meridian
+photometer, an ingenious instrument of high accuracy, in which the light
+of a star is compared directly with that of the pole star by reflection.
+All the bright stars of both the northern and the southern skies are
+worked into a standard system of magnitudes known as HP, or the Harvard
+Photometry.
+
+Astronomers make use of several different kinds of magnitude for the
+stars: the apparent magnitude, as the eye sees it, often called the
+visual magnitude; the photographic magnitude, as the photographic plate
+records it, and these are now determined with the highest accuracy; the
+photovisual magnitude, quite the same as the visual, but determined
+photographically on an isochromatic plate with a yellow screen or
+filter, so that the intensity is nearly the same as it appears to the
+eye. The difference between the star's visual or photovisual magnitude
+and its photographic magnitude is called its color-index, and is often
+used as a measure of the star's color. Light of the shorter wave
+lengths, as blue and violet, affects the photographic plate more rapidly
+than the reds and yellows of longer wave length by which the eye mainly
+sees; so that red stars will appear much fainter and blue stars much
+brighter on the ordinary photographic plate than the eye sees them.
+
+So great are the differences of color in the stars that well-known
+asterisms, with which the eye is perfectly familiar, are sometimes quite
+unrecognizable on the photographic plate, except by relative positions
+of the stars composing them. White stars affect the eye and the plate
+about equally, so that their visual or photovisual and photographic
+magnitudes are about equal. The studies of the colors of the stars, the
+different methods of determining them, and the relations of color to
+constitution have been made the subject of especial investigation by
+Seares of Mount Wilson and many other astronomers.
+
+Centuries of the work of astronomers have been faithfully devoted to
+mapping or charting the stars and cataloguing them. Just as we have
+geographical maps of countries, so the heavens are parceled out in
+sections, and the stars set down in their true relative positions just
+as cities are on the map. Recent years have added photographic charts,
+especially of detailed regions of the sky; but owing to spectral
+differences of the stars, their photographic magnitudes are often quite
+different from their visual magnitudes. From these maps and charts the
+positions of the stars can be found with much precision; but if we want
+the utmost accuracy, we must go to the star catalogues--huge volumes
+oftentimes, with stellar positions set down therein with the last degree
+of precision.
+
+First there will be the star's name, and in the next column its
+magnitude, and in a third the star's right ascension. This is its
+angular distance eastward around the celestial sphere starting from the
+vernal equinox, and it corresponds quite closely to the longitude of a
+place which we should get from a gazetteer, if we wished to locate it on
+the earth. Then another column of the catalogue will give the star's
+declination, north or south of the equator, just as the gazetteer will
+locate a city by its north or south latitude.
+
+
+
+
+CHAPTER XLV
+
+STAR CHARTS AND CATALOGUES
+
+
+Who made the first star chart or catalogue? There is little doubt that
+Eudoxus (B. C. 200) was the first to set down the positions of all the
+brighter stars on a celestial globe, and he did this from observations
+with a gnomon and an armillary sphere. Later Hipparchus (B. C. 130)
+constructed the first known catalogue of stars, so that astronomers of a
+later day might discover what changes are in progress among the stars,
+either in their relative positions or caused by old stars disappearing
+or new stars appearing at times in the heavens. Hipparchus was an
+accurate observer, and he discovered an apparent and perpetual shifting
+of the vernal equinox westward, by which the right ascensions of the
+stars are all the time increasing. He determined the amount of it pretty
+accurately, too. His catalogue contained 1,080 stars, and is printed in
+the "Almagest" of Ptolemy.
+
+Centuries elapsed before a second star catalogue was made, by Ulugh-Beg,
+an Arabian astronomer, A. D. 1420, who was a son of Tamerlane, the
+Tartar monarch of Samarcand, where the observations for the catalogue
+were made. The stars were mainly those of Ptolemy, and much the same
+stars were reobserved by Tycho Brahe (A. D. 1580) with his greatly
+improved instruments, thus forming the third and last star catalogue of
+importance before the invention of the telescope.
+
+From the end of the seventeenth century onward, the application of the
+telescope to all the types of instruments for making observations of
+star places has increased the accuracy many-fold. The entire heavens has
+been covered by Argelander in the northern hemisphere, and Gould in the
+southern--over 700,000 stars in all. Many government observatories are
+still at work cataloguing the stars. The Carnegie Institution of
+Washington maintains a department of astrometry under Boss of Albany,
+which has already issued a preliminary catalogue of more than 6,000
+stars, and has a great general catalogue in progress, together with
+investigations of stellar motions and parallaxes. This catalogue of star
+positions will include proper motions of stars to the seventh magnitude.
+
+In 1887 on proposal of the late Sir David Gill, an international
+congress of astronomers met at Paris and arranged for the construction
+of a photographic chart of the entire heavens, allotting the work to
+eighteen observatories, equipped with photographic telescopes
+essentially alike. The total number of plates exceeds 25,000. Stars of
+the fourteenth magnitude are recorded, but only those including the
+eleventh magnitude will be catalogued, perhaps 2,000,000 in all. The
+expense of this comprehensive map of the stars has already exceeded
+$2,000,000, and the work is now nearly complete. Turner of Oxford has
+conducted many special investigations that have greatly enhanced the
+progress of this international enterprise.
+
+Other great photographic star charts have been carried through by the
+Harvard Observatory, with the annex at Arequipa, Peru, employing the
+Bruce photographic telescope, a doublet with 24-inch lenses; also
+Kapteyn of Groningen has catalogued about 300,000 stars on plates taken
+at Cape Town. Charting and cataloguing the stars, both visually and
+photographically, is a work that will never be entirely finished.
+Improvements in processes will be such that it can be better done in the
+future than it is now, and the detection of changes in the fainter stars
+and investigation of their motions will necessitate repetition of the
+entire work from century to century.
+
+The origin of the names of individual stars is a question of much
+interest. The constellation figures form the basis of the method, and
+the earliest names were given according to location in the especial
+figure; as for instance, Cor Scorpii, the heart of the Scorpion, later
+known as Antares or Alpha Scorpii. The Arabians adopted many star names
+from the Greeks, and gave about a hundred special names to other stars.
+Some of these are in common use to-day, by navigators, observers of
+meteors and of variable stars. Sirius, Vega, Arcturus, and a few other
+first magnitude stars, are instances.
+
+But this method is quite insufficient for the fainter stars whose
+numbers increase so rapidly. Bayer, a contemporary of Galileo,
+originated our present system, which also employs the names of the
+constellations, the Latin genitive in each case, prefixed by the small
+letters of the Greek alphabet, from alpha to omega, in order of
+decreasing brightness; and followed by the Roman letters when the Greek
+alphabet is exhausted.
+
+If there were still stars left in a constellation unnamed, numbers were
+used, first by Flamsteed, Astronomer Royal; and numbers in the order of
+right ascension in various catalogues are used to designate hundreds of
+other stars. The vast bulk of the stars are, however, nameless; but
+about one million are identifiable by their positions (right ascension
+and declination) on the celestial sphere.
+
+
+
+
+CHAPTER XLVI
+
+THE SUN'S MOTION TOWARD LYRA
+
+
+If Hipparchus or Galileo should return to earth to-night and look at the
+stars and constellations as we see them, there would be no change
+whatever discernible in either the brightness of the stars or in their
+relative positions. So the name fixed stars would appear to have been
+well chosen. Halley in the seventeenth century was the first to detect
+that slow relative change of position of a few stars which is known as
+proper motion, and all the modern catalogues give the proper motions in
+both right ascension and declination. These are simply the small annual
+changes in position athwart the line of vision; and, as a whole, the
+proper motions of the brighter stars exceed the corresponding motions of
+the fainter ones because they are nearer to us. The average proper
+motion of the brightest stars is 0".25, and of stars of the sixth
+magnitude only one-sixth as great.
+
+A few extreme cases of proper motion have been detected, one as large as
+9", of an orange yellow star of the eighth magnitude in the southern
+constellation Pictor, and Barnard has recently discovered a star with a
+proper motion exceeding 10"; several determinations of its parallax give
+0".52, corresponding to a distance of 6.27 light years. Nevertheless,
+two centuries would elapse before these stars would be displaced as much
+as the breadth of the moon among their neighbors in the sky. The proper
+motions of stars are along perfectly straight lines, so far as yet
+observed. Ultimately we may find a few moving in curved paths or orbits,
+but this is hardly likely.
+
+As for a central sun hypothesis, that pointing out Alcyone in
+particular, there is no reliable evidence whatever. Analysis of the
+proper motions of stars in considerable numbers, first by Sir William
+Herschel, showed that they were moving radially from the constellation
+Hercules, and in great numbers also toward the opposite side of the
+stellar sphere. Later investigation places this point, called the sun's
+goal, or apex of the sun's way, over in the adjacent constellation Lyra;
+and the opposite point, or the sun's quit, is about halfway between
+Sirius and Canopus. By means of the radial velocities of stars in these
+antipodal regions of the sky, it is found that the sun's motion toward
+Lyra, carrying all his planetary family along with him, is taking place
+at the rate of about 12 miles in every second.
+
+While the right ascensions of the solar apex as given by the different
+investigations have been pretty uniform, the declination of this point
+has shown a rather wide variation not yet explained. For example, there
+is a difference of nearly ten degrees between the declination (+34 deg..3)
+of the apex as determined by Boss from the proper motions of more than
+6,000 stars, and the declination (+25 deg..3) found by Campbell from the
+radial velocities of nearly 1,200 stars. Several investigations tend to
+show that the fainter the stars are, the greater is the declination of
+the solar apex. More remarkable is the evidence that this declination
+varies with the spectral type of the stars, the later types, especially
+G and K, giving much more northerly values. On the whole the great
+amount of research that has been devoted to the solar motion relative to
+the system of the stars for the past hundred years may be said to
+indicate a point in right ascension 18h. (270 deg.) and declination 34 deg. N.
+as the direction toward which the sun is moving. This is not very far
+from the bright star Alpha Lyrae, and the antipodal point from which the
+sun is traveling is quite near to Beta Columbae.
+
+So swift is this motion (nearly twenty kilometers per second) that it
+has provided a base line of exceptional length, and very great service
+in determining the average distance of stars in groups or classes. After
+thousands of years the sun's own motion combined with the proper motions
+of the stars will displace many stars appreciably from their familiar
+places. The constellations as we know them will suffer slight
+distortions, particularly Orion, Cassiopeia and Ursa Major. Identity or
+otherwise of spectra often indicates what stars are associated together
+in groups, and their community of motion is known as star drift. Recent
+investigation of vast numbers of stars by both these methods have led to
+the epochal discovery of star streaming, which indicates that the stars
+of our system are drifting by, or rather through, each other, in two
+stately and interpenetrating streams. The grand primary cause underlying
+this motion is as yet only surmised.
+
+
+
+
+CHAPTER XLVII
+
+STARS AND THEIR SPECTRAL TYPE
+
+
+When in 1872 Dr. Henry Draper placed a very small wet plate in the
+camera of his spectroscope and, by careful following, on account of the
+necessarily long exposure, secured the first photographic spectrum of a
+star ever taken, he could hardly have anticipated the wealth of the new
+field of research which he was opening. His wife, Anna Palmer Draper,
+was his enthusiastic assistant in both laboratory and observatory, and
+on his death in 1882, she began to devote her resources very
+considerably to the amplification of stellar spectrum photography. At
+first with the cooperation of Professor Young of Princeton, and later
+through extension of the facilities of Harvard College Observatory,
+whose director, the late Professor Edward C. Pickering, devoted his
+energies in very large part to this matter, all the preliminaries of the
+great enterprise were worked out, and a comprehensive program was
+embarked upon, which culminated in the "Henry Draper Memorial," a
+catalogue and classification of the spectra of all the stars brighter
+than the ninth magnitude, in both the northern and southern hemispheres.
+
+One very remarkable result from the investigation of large numbers of
+stars according to their type is the close correlation between a star's
+luminosity and its spectral type. But even more remarkable is the
+connection between spectral type and speed of motion. As early as 1892
+Monck of Dublin, later Kapteyn, and still later Dyson, directed
+attention to the fact that stars of the Secchi type II had on the
+average larger proper motions than those of type I. In 1903 Frost and
+Adams brought out the exceptional character of the Orion stars, the
+radial velocities of twenty of which averaged only seven kilometers per
+second.
+
+Soon after, with the introduction of the two-stream hypothesis, a wider
+generalization was reached by Campbell and Kapteyn, whose radial
+velocities showed that the average linear velocity increases continually
+through the entire series B, A, F, G, K, M, from the earliest types of
+evolution to the latest. The younger stars of early type have velocities
+of perhaps five or six kilometers per second, while the older stars of
+later type have velocities nearly fourfold greater.
+
+The great question that occurs at once is: How do the individual stars
+get their motions? The farther back we go in a star's life history, the
+smaller we find its velocity to be. When a star reaches the Orion stage
+of development, its velocity is only one-third of what it may be
+expected to have finally. Apparently, then, the stars at birth have no
+motion, but gradually acquire it in passing through their several types
+or stages of development.
+
+More striking still is the motion of the planetary nebulae, in excess of
+25 kilometers per second, while type A stars move 11 kilometers, type G
+15 kilometers, and type M 17 kilometers per second. Can the law
+connecting speed of motion and spectral type be so general that the
+planetary nebula is to be regarded as the final evolutionary stage?
+Stars have been seen to become nebulae, and one astronomer at least is
+strongly of the opinion that a single such instance ought to outweigh
+all speculation to the contrary, as that stars originate from nebulae.
+
+In his discussion of stellar proper motions, Boss has reached a striking
+confirmation of the relation of speed to type, finding for the cross
+linear motion of the different types a series of velocities closely
+paralleling those of Kapteyn and Campbell.
+
+Concerning the marked relation of the luminosities of the stars to their
+spectral types, there is a pronounced tendency toward equality of
+brightness among stars of a given type; also the brightness diminishes
+very markedly with advance in the stage of evolution. There has been
+much discussion as to the order of evolution as related to the type of
+spectrum, and Russell of Princeton has put forward the hypothesis of
+giant stars and dwarf stars, each spectral type having these two
+divisions, though not closely related. One class embraces intensely
+luminous stars, the other stars only feebly luminous. When a star is in
+process of contraction from a diffused gaseous mass, its temperature
+rises, according to Lane's law, until that density is reached where the
+loss of heat by radiation exceeds the rise in temperature due to
+conversion of gravitational energy into heat. Then the star begins to
+cool again. So that if the spectrum of a star depends mainly on the
+effective temperature of the body, clearly the classification of the
+Draper catalogue would group stars together which are nearly alike in
+temperature, taking no note as to whether their present temperature is
+rising or falling.
+
+Another classification of stars by Lockyer divides them according to
+ascending and descending temperatures. Russell's theory would assign
+the succession of evolutionary types in the order, M_{1}, K_{1}, G_{1},
+F_{1}, A_{1}, B, A_{2}, F_{2}, G_{2}, K_{2}, M_{2}, the subscript 1
+referring to the "giants," and 2 to the dwarf stars. In large part the
+weight of evidence would appear to favor the order of the Harvard
+classification, independently confirmed as it is by studies of stellar
+velocities, Galactic distribution, and periods of binary stars both
+spectroscopic and visual, where Campbell and Aiken find a marked
+increase in length of period with advance in spectral type. At the same
+time, a vast amount of evidence is accumulating in support of Russell's
+theory. Investigations in progress will doubtless reveal the ground on
+which both may be harmonized.
+
+The publication of the new Henry Draper Catalogue of Stellar Spectra is
+in progress, a work of vast magnitude. The great catalogue of thirty
+years ago embraced the spectra of more than ten thousand stars, and was
+a huge work for that day; but the new catalogue utterly dwarfs it, with
+a classification much more detailed than in the earlier work, and with
+the number of stars increased more than twenty-fold. This work,
+projected by the late director of the Harvard Observatory, has been
+brought to a conclusion by the energy and enthusiasm of Miss Annie J.
+Cannon through six years of close application, aided by many assistants.
+The catalogue ranges over the stars of both hemispheres, and is a
+monument to masterly organization and completed execution which will be
+of the highest importance and usefulness in all future researches on the
+bodies of the stellar universe.
+
+
+
+
+CHAPTER XLVIII
+
+STAR DISTANCES
+
+
+So vast are the distances of the stars that all attempts of the early
+astronomers to ascertain them necessarily proved futile. This led many
+astronomers after Copernicus to reject his doctrine of the earth's
+motion round the sun, so that they clung rather to the Ptolemaic view
+that the earth was without motion and was the center about which all the
+celestial motions took place. The geometry of stellar distances was
+perfectly understood, and many were the attempts made to find the
+parallaxes and distances of the stars; but the art of instrument making
+had not yet advanced to a stage where astronomers had the mechanisms
+that were absolutely necessary to measure very small angles.
+
+About 1835, Bessel undertook the work of determining stellar parallax in
+earnest. His instrument was the heliometer, originally designed for
+measuring the sun's diameter; but as modified for parallax work it is
+the most accurate of all angle-measuring instruments that the
+astronomers employ. The star that he selected was 61 Cygni, not a bright
+star, of the sixth magnitude only, but its large proper motion suggested
+that it might be one of those nearest to us. He measured with the
+heliometer, at opposite seasons of the year, the distance of 61 Cygni
+from another and very small star in the same field of view, and thus
+determined the relative parallax of the two stars. The assumption was
+made that the very faint star was very much more distant than the bright
+one, and this assumption will usually turn out to be sound. Bessel got
+0".35 for his parallax of 61 Cygni, and Struve by applying the same
+method to Alpha Lyrae, about the same time, got 0".25 for the parallax of
+that star.
+
+These classic researches of Bessel and Struve are the most important in
+the history of star distances, because they were the first to prove that
+stellar parallax, although minute, could nevertheless be actually
+measured. About the same time success was achieved in another quarter,
+and Henderson, the British astronomer at the Cape of Good Hope, found a
+parallax of nearly a whole second for the bright star Alpha Centauri.
+
+Although the parallaxes of many hundreds of stars have been measured
+since, and the parallaxes of other thousands of stars estimated, the
+measured parallax of Alpha Centauri, as later investigated by Elkin and
+Sir David Gill, and found to be 0".75, is the largest known parallax,
+and therefore Alpha Centauri is our nearest neighbor among the stars, so
+far as we yet know. This star is a binary system and the light of the
+two components together is about the same as that of Capella (Alpha
+Aurigae). But it is never visible from this part of the world, being in
+60 degrees of south declination: one might just glimpse it near the
+southern horizon from Key West.
+
+How the distances of the stars are found is not difficult to explain,
+although the method of doing it involves a good deal of complication,
+interesting to the practical astronomer only. Recall the method of
+getting the moon's distance from the earth: it was done by measuring
+her displacement among the stars as seen from two widely separated
+observatories, as near the ends of a diameter of the earth as
+convenient. This is the base line, and the angle which a radius of the
+earth as seen from the center of the moon fills, or subtends, is the
+moon's parallax.
+
+So near is the moon that this angle is almost an entire degree, and
+therefore not at all difficult to measure. But if we go to the distance
+of even Alpha Centauri, the nearest of the stars, our earth shrinks to
+invisibility; so that we must seek a longer base line. Fortunately there
+is one, but although its length is 25,000 times the earth's diameter, it
+is only just long enough to make the star distances measurable. We found
+that the sun's distance from the earth was 93 million miles; the
+diameter of the earth's orbit is therefore double that amount. Now
+conceive the diameter of the earth replaced by the diameter of the
+earth's orbit: by our motion round the sun we are transported from one
+extremity of this diameter to the opposite one in six month's time; so
+we may measure the displacement of a star from these two extremities,
+and half this displacement will be the star's parallax, often called the
+annual parallax because a year is consumed in traversing its period. And
+it is this very minute angle which Bessel and Struve were the first to
+measure with certainty, and which Henderson found to be in the case of
+Alpha Centauri the largest yet known.
+
+Evidently the earth by its motion round the sun makes every star
+describe, a little parallactic ellipse; the nearer the star is the
+larger this ellipse will be, and the farther the star the smaller: if
+the star were at an infinite distance, its ellipse would become a
+point, that is, if we imagine ourselves occupying the position of the
+star, even the vast orbit of the earth, 186 million miles across, would
+shrink to invisibility or become a mathematical point.
+
+Measurement of stellar parallax is one of many problems of exceeding
+difficulty that confront the practical astronomer. But the actual
+research nowadays is greatly simplified by photography, which enables
+the astronomer to select times when the air is not only clear, but very
+steady for making the exposures. Development and measurement of the
+plates can then be done at any time. Pritchard of Oxford, England, was
+among the earliest to appreciate the advantages of photography in
+parallax work, and Schlesinger, Mitchell, Miller, Slocum and Van Maanen,
+with many others in this country, have zealously prosecuted it.
+
+How shall we intelligently express the vast distances at which the stars
+are removed from us? Of course we can use miles, and pile up the
+millions upon millions by adding on ciphers, but that fails to give much
+notion of the star's distance. Let us try with Alpha Centauri: its
+parallax of 0".75 means that it is 275,000 times farther from the sun
+than the earth is. Multiplying this out, we get 25 trillion miles, that
+is, 25 millions of million miles--an inconceivable number, and an
+unthinkable distance.
+
+Suppose the entire solar system to shrink so that the orbit of Neptune,
+sixty times 93 million miles in diameter, would be a circle the size of
+the dot over this letter i. On the same scale the sun itself, although
+nearly a million miles in diameter, could not be seen with the most
+powerful microscope in existence; and on the same scale also we should
+have to have a circle ten feet in diameter, if the solar system were
+imagined at its center and Alpha Centauri in its circumference.
+
+So astronomers do not often use the mile as a yardstick of stellar
+distance, any more than we state the distance from London to San
+Francisco in feet or inches. By convention of astronomers, the average
+distance between the centers of sun and earth, or 93 million miles, is
+the accepted unit of measure in the solar system. So the adopted unit of
+stellar distance is the distance traveled by a wave of light in a year's
+time: and this unit is technically called the light-year. This unit of
+distance, or stellar yardstick, as we may call it, is nearly 6 millions
+of million miles in length. Alpha Centauri, then, is four and one-third
+light-years distant, and 61 Cygni seven and one-fifth light-years away.
+
+For convenience in their calculations most astronomers now use a longer
+unit called the parsec, first suggested by Turner. Its length is equal
+to the distance of a star whose parallax is one second of arc; that is,
+one parsec is equal to about three and a quarter light-years. Or the
+light-year is equal to 0.31 parsec. Also the parsec is equal to 206,000
+astronomical units, or about 19 millions of million miles.
+
+We have, then four distinct methods of stating the distance of a star:
+Sirius, for example, has a parallax of 0".38 or its distance is two and
+two-thirds parsecs, or eight and a half light-years, or 50 millions of
+million miles. It is the angle of parallax which is always found first
+by actual measurement and from this the three other estimates of
+distance are calculated.
+
+So difficult and delicate is the determination of a stellar distance
+that only a few hundred parallaxes have been ascertained in the past
+century. The distance of the same star has been many times measured by
+different astronomers, with much seeming duplication of effort.
+Comprehensive campaigns for determining star parallaxes in large numbers
+have been undertaken in a few instances, particularly at the suggestion
+of Kapteyn, the eminent astronomer of Groningen, Holland. His catalogue
+of star parallaxes is the most complete and accurate yet published, and
+is the standard in all statistical investigations of the stars.
+
+That we find relatively large parallaxes for some of the fainter stars,
+and almost no measurable parallax for some of the very bright stars is
+one of the riddles of the stellar universe. We may instance Arcturus, in
+the northern hemisphere and Canopus in the southern; the latter almost
+as bright as Sirius. Dr. Elkin and the late Sir David Gill determined
+exhaustively the parallax of Canopus, and found it very minute, only
+0".03, making its distance in excess of a hundred light-years. The
+stupendous brilliancy of this star is apparent if we remember that the
+intensity of its light must vary inversely as the square of the
+distance; so that if Canopus were to be brought as near us as even 61
+Cygni is, it would be a hundredfold brighter than Sirius, the brightest
+of all the stars of the firmament.
+
+In researches upon the distribution of the more distant stars, the
+method of measuring parallaxes of individual stars fails completely, and
+the secular parallax, or parallactic motion of the stars is employed
+instead. By parallactic motion is meant the apparent displacement in
+consequence of the solar motion which is now known with great accuracy,
+and amounts to 19.5 kilometers per second. Even in a single year, then,
+the sun's motion is twice the diameter of the earth's orbit, so that in
+a hundred or more years, a much longer base line is available than in
+the usual type of observations for stellar parallax. If we ascertain the
+parallactic motion of a group of stars, then we can find their average
+distance. It is found, for example, that the mean parallax of stars of
+the sixth magnitude is 0".014. Also the mean distances of stars thrown
+into classes according to their spectral type have been investigated by
+Boss, Kapteyn, Campbell and others. The complete intermingling of the
+two great star streams has been proved, too, by using the magnitude of
+the proper motions to measure the average distances of both streams.
+These come out essentially the same, so that the streaming cannot be due
+to mere chance relation in the line of sight.
+
+Most unexpected and highly important is the discovery that the peculiar
+behavior of certain lines in the spectrum leads to a fixed relation
+between a star's spectrum and its absolute magnitude, which provides a
+new and very effective method of ascertaining stellar distances. By
+absolute magnitudes are meant the magnitudes the stars would appear to
+have if they were all at the same standard distance from the earth.
+
+Very satisfactory estimates of the distance of exceedingly remote
+objects have been made within recent years by this indirect method,
+which is especially applicable to spiral nebulae and globular clusters.
+The absolute magnitude of a star is inferred from the relative
+intensities of certain lines in its spectrum, so that the observed
+apparent magnitude at once enables us to calculate the distance of the
+star. Adams and Joy have recently determined the luminosities and
+parallaxes of 500 stars by this spectroscopic method. Of these stars 360
+have had their parallaxes previously measured; and the average
+difference between the spectroscopic and the trigonometric values of the
+parallax is only the very small angle 0".0037, a highly satisfactory
+verification.
+
+An indirect method, but a very simple one, and of the greatest value
+because it provides the key to stellar distances with the least possible
+calculation, and we can ascertain also the distances of whole classes of
+stars too remote to be ascertained in any other way at present known.
+
+The problem of spectroscopic determinations of luminosity and parallax
+has been investigated at Mount Wilson with great thoroughness from all
+sides, the separate investigations checking each other. A definitive
+scale for the spectroscopic determination of absolute magnitudes has now
+been established, and the parallaxes and absolute magnitudes have
+already been derived for about 1,800 stars.
+
+
+
+
+CHAPTER XLIX
+
+THE NEAREST STARS
+
+
+Of especial interest are the few stars that we know are the nearest to
+us, and the following table includes all those whose parallax is 0".20
+or greater. There are nineteen in all and nearly half of them are binary
+systems. The radial motions given are relative to the sun. The
+transverse velocities are formed by using the measured parallaxes to
+transform proper motions into linear measures. They are given by
+Eddington in his "Stellar Movements":
+
+ Column Key
+ ====================
+ A) Magnitude
+ B) Parallax in Seconds of Arc
+ C) Proper Motion in Seconds of Arc
+ D) Linear Velocity Km. per sec.
+ E) Radial Velocity Km. per sec.
+ F) Spectral Type
+ G) Luminosity (Sun=1)
+ H) Star Stream
+
+ ====================================================================
+ Star's Name | A | B | C | D | E | F | G | H
+ ---------------+-----+------+------+-----+-----+------+-------+-----
+ Groombridge 34 | 8.2 | 0.28 | 2.85 | 48 | .. | Ma | 0.010 | I
+ Eta Cassiop | 3.6 | 0.20 | 1.25 | 30 | +10 | F8 | 1.4 | I
+ Tau Ceti | 3.6 | 0.33 | 1.93 | 28 | -16 | K | 0.50 | II
+ Epsilon Erid | 3.3 | 0.31 | 1.00 | 15 | +16 | K | 0.79 | II
+ CZ 5h 243 | 8.3 | 0.32 | 8.70 | 129 |+242 | G-K | 0.007 | II
+ Sirius |-1.6 | 0.38 | 1.32 | 16 | -7 | A |48.0 | II
+ Procyon | 0.5 | 0.32 | 1.25 | 19 | -3 | F5 | 9.7 | I ?
+ Lal. 21185 | 7.6 | 0.40 | 4.77 | 57 | .. | Ma | 0.009 | II
+ Lal. 21258 | 8.9 | 0.20 | 4.46 | 106 | .. | Ma | 0.011 | I
+ OA (N) 11677 | 9.2 | 0.20 | 3.03 | 72 | .. | .. | 0.008 | I
+ Alpha Centauri | 0.3 | 0.76 | 3.66 | 23 | -22 | G,K5 |{2.0 | I
+ | | | | | | |{0.6 |
+ OA (N) 17415 | 9.3 | 0.27 | 1.31 | 23 | .. | F | 0.004 | II
+ Pos. Med. 2164 | 8.8 | 0.29 | 2.28 | 37 | .. | K | 0.006 | I
+ Sigma Draco | 4.8 | 0.20 | 1.84 | 43 | +25 | K | 0.5 | II
+ Alpha Aquilae | 0.9 | 0.24 | 0.65 | 13 | -33 | A5 |12.3 | I
+ 61 Cygni | 5.6 | 0.31 | 5.25 | 80 | -39 | K5 | 0.10 | I
+ Epsilon Indi | 4.7 | 0.28 | 4.67 | 79 | -62 | K5 | 0.25 | I
+ Krueger 60 | 9.2 | 0.26 | 0.92 | 17 | .. | .. | 0.005 | II
+ Lacaille 9352 | 7.4 | 0.29 | 7.02 | 115 | +12 | Ma | 0.019 | I
+ --------------------------------------------------------------------
+
+These stars are distant less than five parsecs (about 16 light-years)
+from the sun, so they make up the closest fringe of the stellar universe
+immediately surrounding our system. The large number of binary systems
+is quite remarkable. Why some stars are single and others double is not
+yet known. By the spectroscopic method the proportion is not so large;
+Campbell finding that about one quarter of 1,600 stars examined are
+spectroscopic binaries, and Frost two-fifths to a half. The exceptional
+number of large velocities is very remarkable; the average transverse
+motion of the nineteen stars is fifty kilometers per second, whereas
+thirty is about what would have been expected.
+
+As to star streams to which these nearest stars belong, eleven are in
+Stream I and eight in Stream II, in close accord with the ratio 3:2
+given by the 6,000 stars of Boss's catalogue. "We are not able," says
+Eddington, "to detect any significant difference between the
+luminosities, spectra, or speeds of the stars constituting the two
+streams. The thorough interpenetration of the two star streams is well
+illustrated, since we find even in this small volume of space that
+members of both streams are mingled together in just about the average
+proportion."
+
+ [Illustration: THE RING NEBULA IN _Lyra_. This is the best example
+ of the annular and elliptic nebulae, which are not very abundant.
+ (_Photo, Mt. Wilson Solar Observatory._)]
+
+ [Illustration: THE DUMB-BELL NEBULA OF _Vulpecula_. To take the
+ photograph required an exposure of five hours. (_Photo, Mt.
+ Wilson Solar Observatory._)]
+
+
+
+
+CHAPTER L
+
+ACTUAL DIMENSIONS OF THE STARS
+
+
+We have seen that the distances of the stars from the solar system are
+immense beyond conception, and millions upon millions of them are
+probably forever beyond our power of ascertaining by direct measurement
+what their distance really is. After we had found the sun's distance and
+measured the angle filled by his disk, it was easy to calculate his
+actual size. This direct method, however, fails when we try to apply it
+to the stars, because their distances are so vast that no star's disk
+fills an angle of any appreciable size; and even if we try to get a disk
+with the highest magnifying powers of a great telescope our efforts end
+only in failure. There is, indeed, no instrumentally appreciable angle
+to measure.
+
+How then shall we ascertain the actual dimensions of the vast spheres
+which we know the stars actually are, as they exist in the remotest
+regions of space? Clearly by indirect methods only, and it must be said
+that astronomers have as yet no general method that yields very
+satisfactory results for stellar dimensions. The actual magnitude of the
+variable system of Algol, Beta Persei, is among the best known of all
+the stars, because the spectroscope measures the rate of approach and
+recession of Algol when its invisible satellite is in opposite parts of
+the orbit; the law of gravitation gives the mass of the star and the
+size of its orbit, and so the length of the eclipse gives the actual
+size of the dark, eclipsing body. This figures out to be practically the
+same size as that of our sun, while Algol's own diameter is rather
+larger, exceeding a million miles.
+
+If we try to estimate sizes of stars by their brightness merely, we are
+soon astray. Differences of brightness are due to difference of
+dimensions, of course, or of light-giving area; but differences of
+distance also affect the brightness, inversely as the squares of the
+distances, while differences of temperature and constitution affect, in
+very marked degree, the intrinsic brilliance of the light-emitting
+surface of the star. There are big stars and little stars, stars
+relatively near to us and stars exceedingly remote, and stars highly
+incandescent as well as others feebly glowing.
+
+We have already shown how the angular diameters subtended by many of the
+stars have been estimated, through the relation of surface brightness
+and spectral type. Antares and Betelgeuse appear to be the most inviting
+for investigation, because their estimated angular diameters are about
+one-twentieth of a second of arc. This is the way in which their direct
+measurement is being attempted.
+
+As early as 1890, Michelson of Chicago suggested the application of
+interference methods to the accurate measurement of very small angles,
+such as the diameters of the minor planets, and the satellites of
+Jupiter and Saturn, as well as the arc distance between the components
+of double stars. Two portions of the object glass are used, as far apart
+as possible on the same diameter, and the interference fringes produced
+at the focus of the objective are then the subject of observation. These
+fringes form a series of equidistant interference bands, and are most
+distinct when the light comes from a source subtending an infinitesimal
+angle. If the object presents an appreciable angle, the visibility is
+less and may even become zero.
+
+Michelson tested this method on the satellites of Jupiter at the Lick
+Observatory in 1891, and showed its accuracy and practicability.
+Nevertheless, the method has not been taken up by astronomers, until
+very recently at the Mount Wilson Observatory, where Anderson has
+applied it to the measurement of close double stars. It is found that,
+contrary to general expectation, the method gives excellent results,
+even if the "seeing" is not the best--2 on a scale of 10, for instance.
+
+To simplify the manipulation of the interferometer, a small plate with
+two apertures in it is placed in the converging beam of light coming
+from the telescope objective or mirror. The interference fringes formed
+in the focal plane are then viewed with an eyepiece of very high power,
+many thousand diameters. The resolving power of the interferometer is
+found to be somewhat more than double that of a telescope of the same
+aperture. By applying the interferometer method to Capella, arc
+distances of much less than one-twentieth of a second of arc were
+measured. More recently the method has been applied to the great star
+Betelgeuse in Orion, whose angular diameter was found to be 0".46,
+corresponding to an actual diameter of 260,000,000 miles, if the star's
+parallax is as small as it appears to be.
+
+
+
+
+CHAPTER LI
+
+THE VARIABLE STARS
+
+
+Spectacular as they are to the layman, novae, or temporary stars, are to
+the astronomers simply a class among many thousands of stars which they
+call variables, or variable stars. There are a few objects classified as
+irregular variables, one of which is very remarkable. We refer to Eta
+Argus, an erratic variable in the southern constellation Argo and
+surrounded by a well-known nebula. There is a pretty complete record of
+this star. Halley in 1677 when observing at Saint Helena recorded Eta
+Argus as of the fourth magnitude. During the 18th century, it fluctuated
+between the fourth magnitude and the second. Early in the 19th it
+rapidly waxed in brightness, fluctuating between the first and second
+magnitudes from 1822 to 1836. But two years later its light tripled,
+rivaling all the fixed stars except Canopus and Sirius. In 1843 it was
+even brighter for a few months, but since then it has declined fairly
+steadily, reaching a minimum at magnitude seven and a half in 1886, with
+a slight increase in brightness more recently. A period of half a
+century has been suggested, but it is very doubtful if Eta Argus has any
+regular period of variation.
+
+Another very interesting class of variables is known as the Omicron Ceti
+type. Nearly all the time they are very faint, but quite suddenly they
+brighten through several magnitudes, and then fade away, more or less
+slowly, to their normal condition of faintness. But the extraordinary
+thing is that most of these variables go through their fluctuations in
+regular periods: from six months to two years in length. The type star,
+Omicron Ceti, or Mira, is the oldest known variable, having been
+discovered by Fabricius in 1596. Most of the time it is a relatively
+faint star of the 12th magnitude; but once in rather less than a year
+its brightness runs up to the fourth, third and sometimes even the
+second magnitude, where it remains for a week or ten days, and afterward
+it recedes more slowly to its usual faintness, the entire rise and
+decline in brightness usually requiring about 100 days. The spectrum of
+Omicron Ceti contains many very bright lines, and a large proportion of
+the variable stars are of this type.
+
+Another class of variables is designated as the Beta Lyrae type. Their
+periods are quite regular, but there are two or more maxima and minima
+of light in each period, as if the variation were caused by superposed
+relations in some way. Their spectra show a complexity of helium and
+hydrogen bands. No wholly satisfactory explanation has yet been offered.
+Probably they are double stars revolving in very small orbits compared
+with their dimensions, their plane of motion passing nearly through the
+earth.
+
+But the most interesting of all the variables are those of the Algol
+type, their light curves being just the reverse of the Omicron Ceti
+type; that is, they are at their maximum brightness most of the time,
+and then suffer a partial eclipse for a relatively brief interval. Algol
+goes through its variations so frequently that its period is very
+accurately known; it is 2d. 20h. 48m. 55.4s. For most of this period
+Algol is an easy second magnitude star; then in about four and a half
+hours it loses nearly five-sixths of its light, receding to the fourth
+magnitude. Here at minimum it remains for fifteen or twenty minutes, and
+then in the next three and a half hours it regains its full normal
+brilliancy of the second magnitude. During these fluctuations the star's
+spectrum undergoes no marked changes. The spectra of all the Algol
+variables are of the first or Sirian type.
+
+To explain the variation of the Algol type of variables is easy: a dark,
+eclipsing body, somewhat smaller than the primary is supposed to be
+traveling round it in an orbit lying nearly edgewise to our line of
+sight. The gravitation of this dark companion displaces Algol itself
+alternately toward and from the earth, because the two bodies revolve
+round their common center of gravity. With the spectroscope this
+alternate motion of Algol, now advancing and now receding at the rate of
+26 miles per second, has been demonstrated; and the period of this
+motion synchronizes exactly with the period of the star's variability.
+
+Russell and Shapley have made extended studies of the eclipsing
+binaries, and developed the formulae by which the investigations of their
+orbits are conducted. Heretofore, visual binaries and spectroscopic
+binaries afforded the only means of deriving data regarding double
+systems, but it is now possible to obtain from the orbits of eclipsing
+variables fully as much information relating to binary systems in
+general and their bearing on stellar evolution. After an orbit has been
+determined from the photometric data of the light curve, the addition
+of spectroscopic data often permits the calculation of the masses,
+dimensions and densities in terms of the sun. Shapley's original
+investigation included the orbits of ninety eclipsing variables, and
+with the aid of hypothetical parallaxes, he computed the approximate
+position of each system in space. The relation to the Milky Way is
+interesting, the condensation into the Galactic plane being very marked;
+only thirteen of the ninety systems being found at Galactic latitudes
+exceeding 30 degrees.
+
+If we can suppose the variable stars covered with vast areas of spots,
+perhaps similar to the spots on the sun, and then combine the variation
+of these spot areas with rotation of the star on its axis, there is a
+possibility of explanation of many of the observed phenomena, especially
+where the range of variation is small. But for the Omicron Ceti type, no
+better explanation offers than that afforded by Sir Norman Lockyer's
+collision theory. First he assumes that these stars are not condensed
+bodies, but still in the condition of meteoric swarms, and the
+revolution of lesser swarms around larger aggregations, in elliptic
+orbits of greater or less eccentricity, must produce vast multitudes of
+collisions; and these collisions, taking place at pretty regular
+periods, produce the variable maximum light by raising hosts of meteoric
+particles to a state of incandescence simultaneously.
+
+The catalogues of variable stars now contain many thousands of these
+objects. They are often designated by the letters R, S, T, and so on,
+followed by the genitive form of the name of the constellation wherein
+they are found. Most of the recently found variables have a range of
+less than one magnitude. They are so distributed as to be most numerous
+in a zone inclined about 18 degrees to the celestial equator, and split
+in two near where the cleft in the Galaxy is located. Nearly all the
+temporary stars are in this duplex region. Bailey of Harvard a quarter
+century ago began the investigation of variables in close star clusters,
+where they are very abundant, with marked changes of magnitude within
+only a few hours.
+
+Many amateur astronomers afford very great assistance to the
+professional investigator of variable stars by their cooperation in
+observing these interesting bodies, in particular the American
+Association of Observers of Variable Stars, organized and directed by
+William Tyler Olcott.
+
+For a high degree of accuracy in determining stellar magnitudes the
+photo-electric cell is unsurpassed. Stebbins of Urbana has been very
+successful in its application and he discovered the secondary minimum of
+Algol with the selenium cell. His most recent work was done with a
+potassium cell with walls of fused quartz, perfected after many trial
+attempts. The stars he has recently investigated are Lambda Tauri,
+and Pi Five Orionis. Combining results with those reached by the
+spectroscope, the masses of the two component stars of the former are
+2.5 and 1.0 that of the sun, and the radii are 4.8 and 3.6 times the
+sun's.
+
+Russell of Princeton thinks it probable that similar causes are at work
+in all these variables. In the case of the typical Novae there is
+evidence that when the outburst takes place a shell of incandescent gas
+is actually ejected by the star at a very high velocity. What may be the
+forces that cause such an explosion can only be guessed. Repeated
+outbursts have not, in the case of T Pyxidis, destroyed the star,
+because it has gone through this process three times in the past thirty
+years. Russell inclines to regard it as a standard process occurring
+somewhere in the stellar universe probably as often as once a year.
+
+Novae, then, cannot be due to collisions between two stars, for even if
+we suppose the stars to be a thousand millions in number, no two should
+collide except at average intervals of many million years. The idea is
+gaining ground that the stars are vast storehouses of energy which they
+are gradually transforming into heat and radiating into space. "Under
+ordinary circumstances, it is probable that the rate of generation of
+heat is automatically regulated to balance the loss by radiation. But it
+is quite conceivable that some sudden disturbance in the substance of
+the star, near the surface, might cause an abrupt liberation of a great
+amount of energy, sufficient to heat the surface excessively, and drive
+the hot material off into infinite space, in much the form of a shell of
+gas, as seems to have been observed in the case of Nova Aquilae.... With
+the rapid advance of our knowledge of the properties of the stars on one
+hand, and of the very nuclei of atoms on the other, we may, perhaps
+before many years have passed, find ourselves nearer a solution of the
+problem."
+
+The Cepheid variables increase very rapidly in brightness from their
+least light to their maximum, and then fade out much more slowly, with
+certain irregularities or roughnesses of their light-curves when
+declining. Their spectral lines also shift in period with their
+variations of light. In the case of these variables, whose regular
+fluctuation of light cannot be due to eclipse, and is as a rule embraced
+within a few days, there is a fluctuation in color also between maximum
+and minimum, as if there were a periodic change in the star's physical
+condition. Eddington and Shapley advocate the theory of a mechanical
+pulsation of the star as most plausible. Knowledge of the internal
+conditions of the stars make it possible to predict the period of
+pulsation within narrow limits; and for Delta Cephei this theoretical
+period is between four and ten days. Its observed period is five and
+one-third days, and corresponding agreement is found in all the Cepheids
+so far tested.
+
+Shapley of Mount Wilson finds that the Cepheid variables with periods
+exceeding a day in length all lie close to the Galactic lane. So greatly
+have the studies of these objects progressed that, as before remarked,
+when we know the star's period, we can get its absolute magnitude, and
+from this the star's distance. On all sides of the sun, the distances of
+the Cepheids range up to 4,000 parsecs. So they indicate the existence
+of a Galactic system far greater in extent than any previously dealt
+with.
+
+
+
+
+CHAPTER LII
+
+THE NOVAE, OR NEW STARS
+
+
+New stars, or temporary stars, we have already mentioned in connection
+with variables. They are, next to comets, the most dramatic objects in
+the heavens. They may be variable stars which, in a brief period,
+increase enormously in brightness, and then slowly wane and disappear
+entirely, or remain of a very faint stellar magnitude.
+
+In the ancient historical records are found accounts of several such
+stars. For instance, in the Chinese annals there is an allusion to such
+a stellar outburst in the constellation of Scorpio, B. C. 134. This was
+observed also by Hipparchus and, no doubt, it was the immediate
+incentive which led to his construction of the first known catalogue of
+stars, so that similar happenings might be detected in the future. In
+November, 1572, Tycho Brahe observed the most famous of all new stars,
+which blazed out in the constellation Cassiopeia. In something over a
+year it had completely disappeared.
+
+In 1604-1605 a new star of equal brightness was seen in Ophiuchus by
+Kepler; it also faded out to invisibility in 1606. Kepler and Tycho
+printed very complete records of these remarkable objects. The
+eighteenth century passed without any new stars being seen or recorded.
+There was one of the fifth magnitude in 1848, and another of the
+seventh magnitude in 1860; and in May, 1866, a star of the second
+magnitude suddenly made its appearance in Corona Borealis; and one of
+the third magnitude in Cygnus in November, 1876. The latter was fully
+observed by Schmidt of Athens and became a faint telescopic star within
+a few weeks. It is now of the fifteenth magnitude.
+
+In 1885 astronomers were surprised to find suddenly a new star of the
+sixth magnitude very close to the brightest part of the great nebula in
+Andromeda; it ran its course in about six months, fading with many
+fluctuations in brightness, and no star is now visible in its position
+even with the telescope. Stars of this class are known to astronomers as
+Novae, usually with the genitive of the constellation name, as Nova
+Andromedae.
+
+In 1891-1892 Nova Aurigae made its spectacular appearance and yielded a
+distinctly double and complex spectrum for more than a month. Many pairs
+of lines indicated a community of origin as to substance, and accurate
+measurement showed a large displacement with a relative velocity of more
+than 500 miles per second. For each bright hydrogen line displaced
+toward the red there was a dark companion line or band about equally
+displaced toward the violet much as if the weird light of Nova Aurigae
+originated in a solid globe moving swiftly away from us and plunging
+into an irregular nebulous mass as swiftly approaching us. Parallax
+observations of Nova Aurigae made it immensely remote, perhaps within the
+Galaxy, and it still exists as a faint nebulous star.
+
+In February, 1901, in the constellation Perseus appeared the most
+brilliant nova of recent years. It was first discovered by Dr. Anderson,
+an amateur of Glasgow, and at maximum on February 23 it outshone
+Capella. There were many unusual fluctuations in its waning brightness.
+Its spectrum closely resembled that of Nova Aurigae, with calcium,
+helium, and hydrogen lines. In August, 1901, an enveloping nebula was
+discovered, and a month later certain wisps of this nebulosity appeared
+to have moved bodily, at a speed seventy-fold greater than ever
+previously observed in the stellar universe.
+
+According to Sir Norman Lockyer's meteoritic hypothesis, a vast nebulous
+region was invaded, not by one but by many meteor swarms, under
+conditions such that the effects of collision varied greatly in
+intensity. The most violent of these collisions gave birth to Nova
+Persei itself, and the least violent occurred subsequently in other
+parts of the disturbed nebula, perhaps immeasurably removed. This
+explanation would avoid the necessity of supposing actual motion of
+matter through space at velocities heretofore unobserved and
+inconceivably high. A recent photograph of Nova Persei, by Ritchey,
+reveals a nebulous ring of regular structure surrounding the star. The
+great power of the 60-inch has made it possible to photograph even the
+spectra of many of the novae of years ago which are now very faint. After
+the lapse of years the characteristic lines of the nebular spectrum
+generally vanish, as if the star had passed out of the nebula--a plunge
+into which is generally thought to be the cause of the great and sudden
+outburst of light.
+
+Many novae have recently been found in the spiral nebulae, especially in
+the great nebula of Andromeda.
+
+
+
+
+CHAPTER LIII
+
+THE DOUBLE STARS
+
+
+Examining individual stars of the heavens more in detail, thousands of
+them are found to be double; not the stars that appear double to the
+naked eye, as Theta Tauri, Mizar, Epsilon Lyrae, and others; but pairs of
+stars much closer together, and requiring the power of the telescope to
+divide or separate them. Only a very few seconds apart they are or, in
+many cases, only the merest fraction of a second of arc. Some of them,
+called binaries, are found to be revolving around a common center,
+sometimes in only a few years, sometimes in stately periods of hundreds
+of years. Many such binary systems are now known, and the number is
+constantly increasing. Castor is one, Gamma Virginis another, Sirius
+also is one of these binaries, and a most interesting one, having a
+period of revolution of about 52 years.
+
+Aitken, of the Lick Observatory, in his work on binary stars, directs
+special attention to the correlation between the elements of known
+binary orbits and the star's spectral type, and presents a statistical
+study of the distribution of 54,000 visual double stars, of which the
+spectra of 3919 are known. That the masses of binary systems average
+about twice that of the sun's mass has long been known, and this fact
+can be employed with confidence in estimates of the probable parallax
+of these systems. Aitken applies the test to fourteen visual systems
+for which the necessary data are available, and deduces for them a mean
+mass of 1.76 times that of the sun. For the spectroscopic binaries the
+masses are much greater.
+
+Triple, quadruple and multiple stars are less frequent; but many
+exceedingly interesting objects of this class exist. Epsilon Lyrae is
+one, a double-double, or four stars as seen with slender telescopic
+power, and six or seven stars with larger instruments. Sigma Orionis and
+12 Lyncis, also Theta Cancri and Mu Bootis are good examples of triple
+stars.
+
+
+
+
+CHAPTER LIV
+
+THE STAR CLUSTERS
+
+
+From multiple stars the transition is natural to star clusters although
+the gap between these types of stellar objects is very broad. The
+familiar group of the winter sky known as the Pleiades is a loose
+cluster, showing relatively very few stars even in telescopes or on
+photographic plates. The "Beehive," or cluster known as Praesepe in
+Cancer, and a double group in the sword-handle of Perseus, both just
+visible to the naked eye, are excellent examples of star clusters of the
+average type. When the moon is absent, they are easily recognized
+without a telescope as little patches of nebulous light; but every
+increase of optical power adds to their magnificence.
+
+Then we come in regular succession to the truly marvelous globular
+clusters, that for instance in Hercules. Messier 13, a recent photograph
+of which, taken by Ritchey with the 60-inch reflector on Mount Wilson,
+reveals an aggregation of more than 50,000 stars. But the finest
+specimens are in the southern hemisphere. Sir John Herschel spent much
+time investigating them nearly a century ago at the Cape of Good Hope.
+His description of the cluster in the constellation of Centaurus is as
+follows: "The noble globular cluster Omega Centauri is beyond all
+comparison the richest and largest object of the kind in the heavens.
+The stars are literally innumerable, and as their total light when
+received by the naked eye affects it hardly more than a star of the
+fifth or fourth to fifth magnitude, the minuteness of each star may be
+imagined."
+
+Others of these clusters are so remote that the separate stars are not
+distinguishable, especially at the center, and their distances are
+entirely beyond our present powers of direct measurement, although
+methods of estimating them are in process of development. If gravitation
+is regnant among the uncounted components of stellar clusters, as
+doubtless it is, these stars must be in rapid motion, although our
+photographs of measurements have been made too recently for us to detect
+even the slightest motion in any of the component stars of a cluster.
+The only variations are changes of apparent magnitude, of a type first
+detected in a large number of stars in Omega Centauri, by Bailey of
+Harvard, who by comparison of photographs of the globular clusters was
+the first to find variable stars quite numerous in these objects. Their
+unexplained variations of magnitude take place with great rapidity,
+often within a few hours.
+
+There are about a hundred of these globular clusters, and the radial
+velocities of ten of them have been measured by Slipher and found to
+range from a recession of 410 to an approach of 225 kilometers per
+second. These excessive velocities are comparable with those found for
+the spiral nebulae. Shapley has estimated the distances of many of these
+bodies, which contain a large number of variable stars of the Cepheid
+type. By assuming their absolute magnitudes equal to those of similar
+Cepheids at known distances, he finds their distance represented by the
+inconceivably minute parallax of 0".00012, corresponding to 30,000
+light-years. This research also places the globular clusters far
+outside and independent of our Galactic system of stars. The
+distribution of the globular clusters has also been investigated, and
+these interesting objects are found almost exclusively in but one
+hemisphere of the sky. Its center lies in the rich star clouds of
+Scorpio and Sagittarius. Success in finding the distances of these
+objects has made it possible to form a general idea of their
+distribution in three-dimensional space.
+
+The numerous variable stars in any one cluster are remarkable for their
+uniformity. Accepting variables of this type as a constant standard of
+absolute brightness, and assuming that the differences of average
+magnitude of the variables in different clusters are entirely due to
+differences of distance, the relative distances of many clusters were
+ascertained with considerable accuracy. Then it was found that the
+average absolute magnitude of the twenty-five brightest stars in a
+cluster is also a uniform standard, or about 1.3 magnitudes brighter
+than the mean magnitude of the variables. This new standard was employed
+in ascertaining the distances of other clusters not containing many
+variables.
+
+Shapley further shows that the linear dimensions of the clusters are
+nearly uniform, and the proper relative positions in space are charted
+for sixty-nine of these objects. We can determine the scale of the
+charts, if we know the absolute brightness of our primary standard--the
+variable stars; and this is deduced from a knowledge of the distances of
+variables of the same type in our immediate stellar system.
+
+The most striking of all the globular clusters, Omega Centauri, comes
+out the nearest; nevertheless it is distant 6.5 kiloparsecs. A
+kiloparsec is a thousand parsecs, and is the equivalent of 3,256
+light-years. At the inconceivable distance of sixty-seven kiloparsecs,
+or more than 200,000 light-years, is the most remote of the globular
+clusters, known to astronomers as N.G.C. 7006, from its number in the
+catalogue which records its position in the sky, the New General
+Catalogue of nebulae by Dreyer of Armagh.
+
+The clusters are widely scattered, and their center of diffusion is
+about twenty kiloparsecs on the Galactic plane toward the region of
+Scorpio-Sagittarius. Marked symmetry with reference to this plane makes
+it evident that the entire system of globular clusters is associated
+with the Galaxy itself. But to conceive of this it is necessary to
+extend our ideas of the actual dimensions of the Galactic system. Almost
+on the circumference of the great system of globular clusters our local
+stellar system is found, and it contains probably all the naked-eye
+stars, with millions of fainter ones. Its size seems almost diminutive,
+only about one kiloparsec in diameter. The relative location of our
+local stellar system shows why the globular clusters appear to be
+crowded into one hemisphere only.
+
+Shapley suggests that globular clusters can exist only in empty space,
+and that when they enter the regions of space tenanted by stars, they
+dissolve into the well-known loose clusters and the star clouds of the
+Milky Way. Strangely the radial velocities of the clusters already
+observed show that most of them are traveling toward this region, and
+that some will enter the stellar regions within a period of the order of
+a hundred million years.
+
+The actual dimensions of globular clusters are not easy to determine,
+because the outer stars are much scattered. To a typical cluster,
+Messier 3, Shapley assigns a diameter of 150 parsecs, which makes it
+comparable with the size of the stellar cluster to which the sun
+belongs. Also on certain likely assumptions, he finds that the diameter
+of the great cluster in Hercules, the finest one in our northern sky, is
+about 350 parsecs, and its distance no less than 30,000 parsecs; in
+other words, the staggering distance that light would require 9,750,000
+years to travel over. While these distances can never be verified by
+direct measurement, it lends great weight to the three methods of
+indirect measurement, or estimation, (1) from the diameter of the image
+of the clusters, (2) from the mean magnitude of the twenty-five
+brightest stars, and (3) from the mean magnitude of the short period
+variables, that they are in excellent agreement.
+
+
+
+
+CHAPTER LV
+
+MOVING CLUSTERS
+
+
+Recent researches on the proper motions of stars have brought to light
+many groups of stars whose individual members have equal and parallel
+velocities. Eddington calls these moving clusters. The component stars
+are not exceptionally near to each other, and it often happens that
+other stars not belonging to the group are actually interspersed among
+them. They may be likened to double stars which are permanent neighbors,
+with some orbital motion, though exceedingly slow.
+
+The connection is rather one of origin; occurring in the same region of
+space, perhaps, from a single nebula. They set out with the same motion,
+and have "shared all the accidents of the journey together." Their
+equality of motion is intact because any possible deflections by the
+gravitative pull of the stellar system is the same for both. Mutual
+attraction may tend to keep the stars together, but their community of
+motion persists chiefly because no forces tend to interfere with it. In
+this way physically connected pairs may be separated by very great
+distances.
+
+So with the moving clusters: their component stars may be widely
+separate on the celestial sphere, but equality of their motions affords
+a clue to their association in groups. The Hyades, a loose cluster in
+Taurus, is a group of thirty-nine stars, within an area of about 15
+degrees square, which has been pretty fully investigated, especially by
+the late Professor Lewis Boss; and no doubt many fainter stars in the
+same region will ultimately be found to belong to the same group.
+
+If we draw arrows on a chart representing the amount and direction of
+the proper motions of these stars, these arrows must all converge toward
+a point. This shows that their motions are parallel in space. It is a
+relatively compact group, and the close convergence shows that their
+individual velocities must agree within a small fraction of a kilometer
+per second. Radial velocity measures of six of the component stars are
+in very satisfactory accord, giving 45.6 kilometers per second for the
+entire group.
+
+We can get the transverse velocity, and therefrom the distances of the
+stars, which are among the best known in the heavens, because the proper
+motions are very accurately known. The mean parallax of the group by
+this indirect method comes out 0".025, agreeing almost exactly with the
+direct determination by photography, 0".023, by Kapteyn, De Sitter, and
+others.
+
+Eddington concludes that this Taurus group is a globular cluster with a
+slight central condensation. Its entire diameter is about ten parsecs,
+and its known motion enables us to trace its past and future history. It
+was nearest the sun 800,000 years ago, when it was at about half its
+present distance. Boss calculated that in 65 million years, if the
+present motion is maintained, this group will have receded so far as to
+appear like an ordinary globular cluster 20' in diameter, its stars
+ranging from the ninth to the twelfth apparent magnitude. We may infer
+that the motion will likely continue undisturbed, because there are
+interspersed among the group many stars not belonging to it, and these
+have neither scattered its members nor sensibly interfered with the
+parallelism of their motion.
+
+Another moving cluster, the similarity of proper motion of whose
+component stars was first pointed out by Proctor, is known as the Ursa
+Major system, which embraces primarily Beta, Gamma, Delta, Epsilon, and
+Zeta Ursae Majoris, or five of the seven stars that mark the familiar
+Dipper. But as many as eight other stars widely scattered are thought to
+belong to the same system, including Sirius and Alpha Coronae Borealis.
+The absolute motion amounts to 28.8 kilometers per second, and is
+approximately parallel to the Galaxy. Turner has made a model of the
+cluster, which has the form of a flat disk.
+
+Among stars of the Orion type of spectrum are several examples of moving
+clusters. The Pleiades together with many fainter stars form another
+moving cluster; as also do the brighter stars of Orion, together with
+the faint cloudlike extensions of the great nebula in Orion, whose
+radial velocity agrees with that of the stars in the constellation.
+Still another very remarkable moving cluster is in Perseus, first
+detected by Eddington, and embracing eighteen stars, the brightest of
+which is Alpha Persei.
+
+The further discovery of moving clusters is most important in the future
+development of stellar astronomy, because with their aid we can find out
+the relative distribution, luminosity, and distance of very remote
+stars. So far the stars found associated in groups are of early types of
+spectrum; but the Taurus cluster embraces several members equally
+advanced in evolution with the sun, and in the more scattered system of
+Ursae Major there are three stars of Type F.
+
+"Some of these systems," Eddington concludes, "would thus appear to have
+existed for a time comparable with the lifetime of an average star. They
+are wandering through a part of space in which are scattered stars not
+belonging to their system--interlopers penetrating right among the
+cluster stars. Nevertheless, the equality of motion has not been
+seriously disturbed. It is scarcely possible to avoid the conclusion
+that the chance attractions of stars passing in the vicinity have no
+appreciable effect on stellar motions; and that if the motions change in
+course of time (as it appears they must do) this change is due, not to
+the passage of individual stars, but to the central attraction of the
+whole stellar universe, which is sensibly constant over the volume of
+space occupied by a moving cluster."
+
+
+
+
+CHAPTER LVI
+
+THE TWO STAR STREAMS
+
+
+Consider the ships on the Atlantic voyaging between Europe and America:
+at any one time there may be a hundred or more, all bound either east or
+west, some moving in interpenetrating groups, individuals frequently
+passing each other, but rarely or never colliding. We might say, there
+are two great streams of ships, one moving east and the other west.
+
+Now in place of each ship, imagine a hundred ships, and magnify their
+distances from each other to the vast distances that the stars are from
+each other, and all in motion in two great streams as before. This will
+convey some idea of the relatively recent discovery, called by
+astronomers "star-streaming."
+
+Early in this century the investigation of moving clusters began to
+reveal the fact that the motions of the stars were not at random
+throughout the universe, and about 1904 Kapteyn was the first to show
+that the stellar motions considered in great groups are very far from
+being haphazard, but that the stars tend to travel in two great streams,
+or favored directions. This was ascertained by analyzing the proper
+motions of stars in the sky, many thousands of them, and correcting all
+for the effect which the known motion of the sun would have upon them.
+The corrected motion, or part that is left over, is known as the star's
+own motion, or _motus peculiaris_.
+
+This important investigation was very greatly facilitated by the general
+catalogue of 6,188 stars well distributed over the entire sky, the work
+of the late Professor Boss. It was published by the Carnegie Institution
+of Washington, and includes all stars down to the sixth magnitude. Boss
+was very critical in the matter of stellar positions and proper motions
+and his work is the most accurate at present available. Excluding stars
+of the Orion type and the known members of moving clusters, Kapteyn's
+investigation was based on 5,322 stars, which he divided into seventeen
+regions of the sky, each northern region having an antipodal one in the
+southern hemisphere.
+
+Mathematical analysis of these regions showed them all in substantial
+agreement, with one exception, and enabled Kapteyn to draw the
+conclusion that the stars of one stream, called Drift I, move with a
+speed of thirty-two kilometers per second, while those of the other,
+Drift II, travel with a speed of eighteen kilometers per second. Their
+directions are not, like those of east and west bound ships, 180 degrees
+from each other, but are inclined at an angle of 100 degrees. Drift I
+embraces about three-fifths of the stars, and Drift II the remaining
+two-fifths. Quite as remarkable as the drifts themselves is the fact
+that the relative motion of the two is very closely parallel to the
+plane of the Milky Way.
+
+This epochal research has very great significance in all investigations
+of stellar motions, and it has been verified in various ways,
+particularly by the Astronomer Royal, Sir Frank Dyson, who limited the
+stars under consideration to 1,924 in number, but all having very large
+proper motions. In this way the two streams are even more
+characteristically marked. But radial velocity determinations afford the
+ultimate and most satisfactory test, and Campbell has this investigation
+in hand, classifying the stars in their streaming according to the type.
+
+Type A stars are so far found to be confirmatory. Turning to the
+question of physical differences between the stars of the two streams,
+Eddington inquires into the average magnitude of the stars in both
+drifts, and their spectral type. Also whether they are distributed at
+the same distance from the sun, and in the same proportion in all parts
+of the sky. His conclusion is that there is no important difference in
+the magnitudes of the stars constituting the two drifts. Regarding their
+spectra, stars of early and late types are found in both streams, with a
+somewhat higher proportion of late types among the stars of Drift II
+than those of Drift I. Campbell and Moore of the Lick Observatory have
+investigated seventy-three planetary nebulae which exhibit the phenomena
+of star-streaming, and have motions which are characteristic of the
+stars.
+
+Dealing with the very important question whether the two streams are
+actually intermingled in space, Eddington finds them nearly at the same
+mean distance and thoroughly intermingled, and there is no possible
+hypothesis of Drifts I and II passing one behind the other in the same
+line of sight. A third drift, to which all the Orion stars belong, is
+under investigation, together with comprehensive analysis of the drifts
+according to the spectral type of all the stars included.
+
+The farther research on star-streaming is pushed, the more it becomes
+evident that a third stream, called Drift O, is necessary, especially
+to include B-type stars. The farther we recede from the sun, the more
+this drift is in evidence. At the average distances of B-type stars, the
+observed motions are almost completely represented by Drift O alone.
+Halm of Cape Town concludes from recent investigations that the
+double-drift phenomena (Drifts I and II) is of a distinctly _local_
+character, and concerns chiefly the stars in the vicinity of the solar
+system; while stars at the greatest distances from the sun belong
+preeminently to Drift O.
+
+The 60-inch reflector on Mount Wilson gathers sufficient light so that
+the spectra of very faint stars can be photographed, and a discussion of
+velocities derived in this manner has shown that Kapteyn's two star
+streams extend into space much farther than it was possible to trace
+them with the nearer stars. Star-streaming, then, may be a phenomenon of
+the widest significance in reference to the entire universe.
+
+As to the fundamental causes for the two opposite and nearly equal star
+streams, it is early perhaps to even theorize upon the subject.
+Eddington, however, finds a possible explanation in the spiral nebulae,
+which are so numerous as to indicate the certainty of an almost
+universal law compelling matter to flow in these forms. Why it does so,
+we cannot be said to know; but obviously matter is either flowing into
+the nucleus from the branches of the spiral, or it is flowing out from
+the nucleus into the branches. Which of the two directions does not
+matter, because in either case there would be currents of matter in
+opposite directions at the points where the arms merge in the central
+aggregation. The currents continue through the center, because the
+stars do not interfere with one another's paths. As Eddington concludes:
+"There then we have an explanation of the prevalence of motions to and
+fro in a particular straight line; it is the line from which the spiral
+branches start out. The two star streams and the double-branched spirals
+arise from the same cause."
+
+
+
+
+CHAPTER LVII
+
+THE GALAXY OR MILKY WAY
+
+
+Grandest of all the problems that have occupied the mind of man is the
+distribution of the stars throughout space. To the earliest astronomers
+who knew nothing about the distances of the stars, it was not much of a
+problem because they thought all the fixed stars were attached to a
+revolving sphere, and therefore all at essentially the same distance; a
+very moderate distance, too. Even Kepler held the idea that the
+distances of individual stars from each other are much less than their
+distances from our sun.
+
+Thomas Wright, of Durham, England, seems to have been the first to
+suggest the modern theory of the structure of the stellar universe,
+about the middle of the eighteenth century. His idea was taken up by
+Kant who elaborated it more fully. It is founded on the Galaxy, the
+basal plane of stellar distribution, just as the ecliptic is the
+fundamental circle of reference in the solar system.
+
+What is the Galaxy or Milky Way?
+
+Here is a great poet's view of the most poetic object in all nature:
+
+ A broad and ample road, whose dust is gold,
+ And pavement stars, as stars to thee appear
+ Seen in the Galaxy, that Milky Way
+ Which nightly as a circling zone thou seest
+ Powder'd with stars.
+ _Milton, P. L._ vii, 580.
+
+Were the earth transparent as crystal, so that we could see downward
+through it and outward in all directions to the celestial sphere, the
+Galaxy or Milky Way would appear as a belt or zone of cloudlike
+luminosity extending all the way round the heavens. As the horizon cuts
+the celestial sphere in two, we see at anyone time only one-half of the
+Milky Way, spanning the dome of the sky as a cloudlike arch.
+
+As the general plane of the Galaxy makes a large angle with our equator,
+the Milky Way is continually changing its angle with the horizon, so
+that it rises at different elevations. One-half of the Milky Way will
+always be below our horizon, and a small region of it lies so near the
+south pole of the heavens that it can never be seen from medium northern
+latitudes.
+
+Galileo was the first to explain the fundamental mystery of this belt,
+when he turned his telescope upon it and found that it was not a
+continuous sheet of faint light, as it seemed to be, but was made up of
+countless numbers of stars, individually too faint to be visible to the
+naked eye, but whose vast number, taken in the aggregate, gave the
+well-known effect which we see in the sky. In some regions, as Perseus,
+the stars are more numerous than in others, and they are gathered in
+close clusters. The larger the telescope we employ, the greater the
+number of stars that are seen as we approach the Galaxy on either side;
+and the farther we recede from the Galaxy and approach either of its
+poles fewer and fewer stars are found. Indeed, if all the stars visible
+in a 12-inch telescope could be conceived as blotted out, nearly all the
+stars that are left would be found in the Galaxy itself.
+
+The naked eye readily notes the variations in breadth and brightness of
+the galactic zone. Nearly a third of it, from Scorpio to Cygnus, is
+split into two divisions nearly parallel. In many regions its light is
+interrupted, especially in Centaurus, where a dark starless region
+exists, known as the "coal sack." Sir John Herschel, who followed up the
+stellar researches of his father, Sir William, in great detail, places
+the north pole of the Galactic plane in declination 37 degrees N., and
+right ascension 12 h. 47 m. This makes the plane of the Milky Way lie at
+an angle of about 60 degrees with the ecliptic, which it intersects not
+far from the solstices.
+
+Now Kant, in view of the two great facts about the Galaxy known in his
+time, (1) that it wholly encircles the heavens, and (2) that it is
+composed of countless stars too faint to be individually visible to the
+naked eye, drew the safe conclusions that the system of the stars must
+extend much farther in the direction of the Milky Way than in other
+directions.
+
+This theory of Kant was next investigated from an observational
+standpoint by Sir William Herschel, the ultimate goal of whose
+researches was always a knowledge of the construction of the heavens.
+The present conclusion is that we may regard the stellar bodies of the
+sidereal universe as scattered, without much regard to uniformity,
+throughout a vast space having in general the shape of a thick watch,
+its thickness being perhaps one-tenth its diameter. On both sides of
+this disk of stars, and clustered about the poles of the sidereal system
+are the regions occupied by vast numbers of nebulae. The entire visible
+universe, then, would be spheroidal in general shape. The plane of the
+Milky Way passes through the middle of this aggregation of stars and
+nebulae, and the solar system is near the center of the Milky Way.
+Throughout the watch-form space the stars are clustered irregularly, in
+varied and sometimes fantastic forms, but without approach to order or
+system. If we except some of the star groups and star clusters and
+consider only the naked-eye stars, we find them scattered with fair
+approach to uniformity.
+
+ [Illustration: STAR CLOUDS AND BLACK HOLES IN SAGITTARIUS. The dark
+ rifts and lanes resemble those in the nearby Milky Way. (_Photo,
+ Yerkes Observatory._)]
+
+ [Illustration: THE GREAT NEBULA OF ANDROMEDA, LARGEST (APPARENTLY)
+ OF ALL THE SPIRAL NEBULAE. This nebula can be seen very faintly
+ with the naked eye, but no telescope has yet resolved it into
+ separate stars. (_Photo, Yerkes Observatory._)]
+
+The watch-shaped disk is not to be understood as representing the actual
+form of the stellar system, but only in general the limits within which
+it is for the most part contained.
+
+A vigorous attack on the problem of the evolution and structure of the
+stellar universe as a whole is now being conducted by cooperation of
+many observatories in both hemispheres. It is known as the Kapteyn "Plan
+of Selected Areas," embracing 206 regions which are distributed
+regularly over the entire sky. Besides this a special plan includes
+forty-six additional regions, either very rich or extremely poor in
+stars, or to which other interest attaches.
+
+Of all investigators Kapteyn has gone into the question of our precise
+location in the Milky Way most thoroughly, concluding that the solar
+system lies, not at the center in the exact plane, but somewhat to the
+north of the Galaxy. Discussing the Sirian stars he finds that if stars
+of equal brightness are compared, the Sirians average nearly three times
+more distance from the sun than those of the solar type. So, probably,
+the Sirians far exceed the Solars in intrinsic brightness. Farther,
+Kapteyn concludes that the Galaxy has no connection with our solar
+system, and is composed of a vast encircling annulus or ring of stars,
+far exceeding in number the stars of the great central solar cluster,
+and everywhere exceedingly remote from these stars, as well as differing
+from them in physical type and constitution. So it would be mainly the
+mere element of distance that makes them appear so faint and crowded
+thickly together into that gauzy girdle which we call the Galaxy.
+
+The Milky Way reveals irregularities of stellar density and star
+clustering on a large scale, with deep rifts between great clouds of
+stars. Modern photographs, particularly those of Barnard in Sagittarius,
+make this very apparent. Within the Milky Way, nearly in its plane and
+almost central, is what Eddington terms the inner stellar system, near
+the center of which is the sun. Surrounding it and near its plane are
+the masses of star clouds which make up the Milky Way. Whether these
+star clouds are isolated from the inner system or continuous with it, is
+not yet ascertained.
+
+The vast masses of the Milky Way stars are very faint, and we know
+nothing yet as to their proper motions, their radial motions, or their
+spectra. Probably a few stars as bright as the sixth magnitude are
+actually located in the midst of the Milky Way clusters, the fainter
+ninth magnitude stars certainly begin the Milky Way proper, while the
+stars of the twelfth or thirteenth magnitude carry us into the very
+depths of the Galaxy.
+
+It is now pretty generally believed that many of the dark regions of the
+Milky Way are due not to actual absence of stars so much as to the
+absorption of light by intervening tracts of nebulous matter on the
+hither side of the Galactic aggregations and, probably in fact, within
+the oblate inner stellar system itself. Easton has made many hundred
+counts of stars in galactic regions of Cygnus and Aquila where the range
+of intensity of the light is very marked; in fact, the star density of
+the bright patches of the Galaxy is so far in excess of the density
+adjacent and just outside the Milky Way, that the conclusion is
+inevitable that this excess is due to the star clouds.
+
+Of the distance of the Milky Way we have very little knowledge. It is
+certainly not less than 1,000 parsecs, and more likely 5,000 parsecs, a
+distance over which light would travel in about 16,000 years. Quite
+certainly all parts of the Galaxy are not at the same distance, and
+probably there are branches in some regions that lie behind one another.
+While the general regions of the nebulae are remote from the Galactic
+plane, the large irregular nebulae, as the Trifid, the Keyhole, and the
+Omega nebulae, are found chiefly in the Milky Way.
+
+In addition to the irregular nebulae many types of stellar objects appear
+to be strongly condensed toward the Milky Way, but this may be due to
+the inner stellar system, rather than a real relation to Galactic
+formations. Quite different are the Magellanic clouds, which contain
+many gaseous nebulae and are unique objects of the sky, having no
+resemblance to the true spiral nebulae which, as a rule, avoid the
+Galactic regions. Worthy of note also is the theory of Easton that the
+Milky Way has itself the form of a double-branched spiral, which
+explains the visible features quite well, but is incapable of either
+disproof or verification. The central nucleus he locates in the rich
+Galactic region of Cygnus, with the sun well outside the nucleus itself.
+By combining the available photographs of the Galaxy, he has produced a
+chart which indicates in a general way how the stellar aggregations
+might all be arrayed so as to give the effect of the Galaxy as we see
+it.
+
+Shapley, at Mount Wilson, has studied the structure of the Galactic
+system, in which he has been aided by Mrs. Shapley. An interesting part
+of this work relates to the distribution of the spiral nebulae, and to
+certain properties of their systematic recessional motion, suggesting
+that the entire Galactic system may be rapidly moving through space.
+Apparently the spiral nebulae are not distant stellar organizations or
+"island universes," but truly nebular structures of vast volume which in
+general are actively repelled from stellar systems. A tentative
+cosmogonic hypothesis has been formulated to account for the motions,
+distribution, and observed structure of clusters and spiral nebulae.
+
+An additional great problem of the Galaxy is a purely dynamical one.
+Doubtless it is in some sort of equilibrium, according to Eddington,
+that is to say, the individual stars do not oscillate to and fro across
+the stellar system in a period of 300 million years, but remain
+concentrated in clusters as at present. Poincare has considered the
+entire Milky Way as in stately rotation, and on the assumption that the
+total mass of the inner stellar system is 1,000,000,000 times the sun's
+mass, and that the distance of the Milky Way is 2,000 parsecs, the
+angular velocity for equilibrium comes out 0".5 per century. That is to
+say, a complete revolution would take place in about 250 million years.
+
+
+
+
+CHAPTER LVIII
+
+STAR CLOUDS AND NEBULAE
+
+
+From star clusters to nebulae, only a century ago, the transition was
+thought to be easy and immediate. Accuracy in determining the distances
+of stars was just beginning to be reached, the clusters were obviously
+of all degrees of closeness following to the verge of irresolvability,
+and it was but natural to jump to the conclusion that the mystery of the
+nebulae consisted in nothing but their vaster distance than that of
+clusters, and it was believed that all nebulae would prove resolvable
+into stars whenever telescopes of sufficiently great power could be
+constructed.
+
+But the development of the spectroscope soon showed the error of this
+hypothesis, by revealing bright lines in the nebular spectra showing
+that many nebulae emit light that comes from glowing incandescent gas,
+not from an infinitude of small stars.
+
+In pre-telescope days nothing was known about the nebulae. The great
+nebula in Andromeda, and possibly the great nebula in Orion, are alone
+visible to the naked eye, but as thus seen they are the merest wisps of
+light, the same as the larger clusters are. Galileo, Huygens and other
+early users of the telescope made observations of nebulae, but long-focus
+telescopes were not well adapted to this work. Simon Mayer has left us
+the first drawing of a nebula, the Orion nebula as he saw it in 1612.
+The vast light-gathering power of the reflectors built by Sir William
+Herschel first afforded glimpses of the structure of the nebulae, and if
+his drawings are critically compared with modern ones, no case of motion
+with reference to the stars or of change in the filaments of the nebulae
+themselves has been satisfactorily made out.
+
+Only very recently has the distance of a nebula been determined, and the
+few that have been measured seem to indicate that the nebulae are at
+distances comparable with the stars. Of all celestial objects the nebulae
+fill the greatest angles, so that we are forced to conclude, with regard
+to the actual size of the greater nebulae as they exist in space, that
+they far surpass all other objects in bulk.
+
+Photography invaded the realm of the nebulae in 1880, when Dr. Henry
+Draper secured the first photograph of the nebula of Orion.
+Theoretically photography ought to help greatly in the study of the
+nebulae, and enable us in the lapse of centuries to ascertain the exact
+nature of the changes which must be going on. The differences of
+photographic processes, of plates, of exposure and development produce
+in the finished photograph vastly greater differences than any actual
+changes that might be going on, so that we must rely rather on optical
+drawings made with the telescope, or on drawings made by expert artists
+from photographs with many lengths of exposure on the same object.
+
+The great work on nebulae and star clusters recently concluded by
+Bigourdan of the Paris Observatory and published in five volumes
+received the award of the gold medal of the Royal Astronomical Society.
+While D'Arrest measured about 2,000 nebulae, and Sir John Herschel about
+double that number in both hemispheres, Bigourdan has measured about
+7,000. His work forms an invaluable lexicon of information concerning
+the nebulae.
+
+Classification of the nebulae is not very satisfactory, if made by their
+shapes alone. There are perhaps fifteen thousand nebulae in all that have
+been catalogued, described, and photographed. Dreyer's new general
+catalogue (N.G.C.) is the best and most useful. Many of the nebulae,
+especially the large ones, can only be classified as irregular nebulae.
+The Orion nebula is the principal one of this class, revealing an
+enormous amount of complicated detail, with exceptional brilliancy of
+many regions and filaments. An extraordinary multiple star, Theta
+Orionis, occupies a very prominent position in the nebula, and
+photographs by Pickering have brought to light curved filaments, very
+faint and optically invisible, in the outlying regions which give the
+Orion nebula in part a spiral character. But the delicate optical wisps
+of this nebula are well seen, even in very small telescopes. Its
+spectrum yields hydrogen, helium and nitrogen. The Orion nebula is
+receding from the earth about eleven miles in every second. Keeler and
+Campbell have shown that nearly every line of the nebular spectrum is a
+counterpart of a prominent dark line in the spectrum of the brighter
+stars of the constellation of Orion. A recent investigator of the
+distribution of luminosity in the great nebula of Orion finds that
+radiations from nebulium are confined chiefly to the Huygenian region of
+the nebula and its immediate neighborhood.
+
+Photography has revealed another extraordinary nebula or group of nebulae
+surrounding the stars in the Pleiades, which the deft manipulation of
+Barnard has brought to light. All the stars and the nebula are so
+interrelated that they are obviously bound together physically, as the
+common proper motion of the stars also appears to show. Also in the
+constellation Cygnus, Barnard has discovered very extensive nebulosities
+of a delicate filmy cloudlike nature which are wholly invisible with
+telescopes, but very obvious on highly sensitive plates with long
+exposures.
+
+Another class of these objects are the annular and elliptic nebulae which
+are not very abundant. The southern constellation Grus, the crane,
+contains a fine one, but by far the best example is in the constellation
+Lyra. It is a nearly perfect ring, elliptic in figure, exceedingly faint
+in small telescopes; but large instruments reveal many stars within the
+annulus, one near the center which, although very faint to the eye, is
+always an easy object on the photographic plate, because it is rich in
+blue and violet rays. The parallax of the ring nebula in Lyra comes out
+only one-sixth of that of the planetary nebulae, and the least greatest
+diameters of this huge continuous ring are 250 and 330 times the orbit
+of Neptune.
+
+Planetary nebulae and nebulous stars are yet another class of nebulae, for
+the most part faint and small, resembling in some measure a planetary
+disk or a star with nebulous outline. Practically all are gaseous in
+composition, and have large radial velocities. Probably they are located
+within our own stellar system. The parallaxes of several of them have
+been measured by Van Maanen: one of the very small angle 0".023, which
+enables us to calculate the diameter of this faint but interesting
+object as equal to nineteen times the orbit of Neptune.
+
+
+
+
+CHAPTER LIX
+
+THE SPIRAL NEBULAE
+
+
+Last and most important of all are the spiral nebulae. The finest example
+is in the constellation Canes Venatici, and its spiral configuration was
+first noted by Lord Rosse, an epoch-making discovery. The convolutions
+of its spiral are filled with numerous starlike condensations,
+themselves engulfed in nebulosity. Photography possesses a vast
+advantage over the eye in revealing the marvelous character of this
+object, an inconceivably vast celestial whirlpool. Naturally the central
+regions of the whorl would revolve most swiftly, but no comparison of
+drawings and photographs, separated by intervals of many years, has yet
+revealed even a trace of any such motion.
+
+The number of large spiral nebulae is not very great; the largest of all
+is the great nebula of Andromeda, whose length stretches over an arc of
+seven times the breadth of the moon, and its width about half as great.
+This nebula is a naked-eye object near Eta Andromedae, and it is often
+mistaken for a comet. Optically it was always a puzzle, but photographs
+by Roberts of England first revealed the true spiral, with ringlike
+formations partially distinct, and knots of condensing nebulosity as of
+companion stars in the making. While its spectrum shows the nongaseous
+constitution of this nebula, no telescope has yet resolved it into
+component stars.
+
+Systematic search for spiral nebulae by Keeler, and later continued by
+Perrine, at the Lick Observatory, with the 36-inch Crossley reflector,
+disclosed the existence of vast numbers of these objects, in fact many
+hundreds of thousands by estimation; so that, next to the stars, the
+spiral nebulae are by far the most abundant of all objects in the sky.
+They present every phase according to the angle of their plane with the
+line of sight, and the convolutions of the open ones are very perfectly
+marked. Many are filled with stars in all degrees of condensation, and
+the appearance is strongly as if stars are here caught in every step of
+the process of making.
+
+The vast multitude of the spiral nebulae indicates clearly their
+importance in the theory of the cosmogony, or science of the development
+of the material universe. Curtis of the Lick Observatory has lately
+extended the estimated number of these objects to 700,000. He has also
+photographed with the Crossley reflector many nebulae with lanes or dark
+streaks crossing them longitudinally through or near the center. These
+remarkable streaks appear as if due to opaque matter between us and the
+luminous matter of the nebula beyond. Perhaps a dark ring of absorptive
+or occulting matter encircles the nebula in nearly the same plane with
+the luminous whorls. Duncan has employed the 60-inch Mount Wilson
+reflector in photographing bright nebulae and star clusters in the very
+interesting regions of Sagittarius. One of these shows unmistakable dark
+rifts or lanes in all parts of the nebula, resembling the dark regions
+of the neighboring Milky Way.
+
+Pease of Mount Wilson has recently employed the 60-inch and the 100-inch
+reflectors of the Mount Wilson Observatory to good advantage in
+photographing several hundred of the fainter nebulae. Many of these are
+spirals, and others present very intricate and irregular forms. A search
+was made for additional spirals among the smaller nebulae along the
+Galaxy, but without success. Several of the supposedly variable nebulae
+are found to be unchanging. Many nights in each month when the moon is
+absent are devoted to a systematic survey of the smaller nebulae and
+their spectra by photography. The visible spiral figure of all these
+objects is a double-branched curve, its two arms joining on the nucleus
+in opposing points, and coiling round in the same geometrical direction.
+The spiral nebulae, as to their distribution, are remote from the Galaxy,
+and the north Galactic polar region contains a greater aggregation than
+the south. The distances of the spiral nebulae are exceedingly great.
+They lie far beyond the planetary and irregular gaseous nebulae, like
+that of Orion, which are closely related to the stars forming part of
+our own system. Possibly the spiral nebulae are exterior or separate
+"island universes." If so, they must be inconceivably vast in size, and
+would develop, not into solar systems, but into stellar clusters. The
+enormous radial velocities of the spiral nebulae, averaging 300 to 400
+kilometers per second, or twenty-fold that of the stars, tend to sustain
+the view that they may be "island universes," each comparable in extent
+with the universe of stars to which our sun belongs.
+
+Recent spectroscopic observations of the nebulae applying the principle
+of Doppler have revealed high velocities of rotation. Slipher of the
+Lowell Observatory made the first discovery of this sort and Van Maanen
+of Mount Wilson has detected in the great Ursa Major spiral, No. 101 in
+Messier's catalogue, a speed of rotation at five minutes of arc from
+the center that would correspond to a complete period in 85,000 years.
+As was to be expected, the nebula does not rotate as a rigid body, but
+the nearer the center the greater the angular velocity, and Van Maanen
+finds evidence of motion along the arms and away from the center.
+
+These great velocities appear to belong to the spiral nebulae as a class,
+and not to other nebulae. Thirteen nebulae investigated by Keeler are as a
+whole almost at rest relatively to our system, as are the large
+irregular objects in Orion, and the Trifid nebula. This would seem to
+indicate that the spiral nebulae form systems outside our own and
+independent of it.
+
+Quite different from the spirals in their distribution through space are
+the planetary nebulae. The spirals follow the early general law of nebulae
+arrangement, that is, they are concentrated toward the poles of the
+Galaxy; but the planetary nebulae, on the other hand, are very few near
+the poles and show a marked frequency toward the Galactic plane.
+Campbell and Moore have found spectroscopic evidence of internal
+rotatory motion in a large proportion of the planetary nebulae.
+
+The distribution of the nebulae throughout space, like that of the stars,
+is still under critical investigation, but the location of vast numbers
+of the more compact nebulae on the celestial sphere is very
+extraordinary. The Milky Way appears to be the determining plane in both
+cases; the nearer we approach it the more numerous the stars become,
+whereas this is the general region of fewest nebulae and they increase in
+number outward in both directions from the Galaxy, and toward both poles
+of the Galactic circle. Obviously this relation, or contra-relation of
+stars and nebulae on such a vast scale is not accidental, and it also
+must be duly accounted for in the true theory of the cosmogony. The
+nebulae which are found principally in and near the Milky Way are the
+large irregular nebulae, and vast nebulous backgrounds, like those
+photographed by Barnard in Scorpio, Taurus and elsewhere, as well as the
+Keyhole, Omega, and Trifid nebulae. Allied to these backgrounds are
+doubtless some of the dark Galactic spaces, radiating little or no
+intrinsic light, and absorbing the light of the fainter stars beyond
+them. A peculiar veiled or tinted appearance has been remarked in some
+cases visually, and examination of the photographs strongly confirms the
+existence of absorbing nebulosity.
+
+The spiral nebulae are so abundant, and so much attention is now being
+given to them, both by observers and mathematicians, that their precise
+relation to the stellar systems must soon be known; that is, whether
+they are comparatively small objects belonging to the stellar system, or
+independent systems on the borders of the stellar system, or as seems
+more likely, vast and exceedingly remote galaxies comparable with that
+of the Milky Way itself. Our knowledge of the motions of the spirals,
+both radial and angular, is increasing rapidly, and must soon permit
+accurate general conclusions to be drawn.
+
+
+
+
+CHAPTER LX
+
+COSMOGONY
+
+
+Down to the middle of the last century and later, it was commonly
+believed that in the beginning the cosmos came into being by divine fiat
+substantially as it is. Previously the earth had been "without form and
+void," as in the Scripture. Had it not been for the growth and gradual
+acceptance of the doctrine of evolution, and its reactionary effect upon
+human thought, it is conceivable that the early view might have
+persisted to the present day; but now it is universally held that
+everything in the heavens above and the earth beneath is subject more or
+less to secular change, and is the result of an orderly development
+throughout indefinite past ages, a progressive evolution which will
+continue through indefinite aeons of the future.
+
+In the writings of the Greek philosophers, and down through the Middle
+Ages we find the idea of an original "chaos" prevailing, with no
+indication whatever of the modern view of the process by which the
+cosmos came to be what they saw it and as it is to-day. If we go still
+farther back, there is no glimmer of any ideas that will bear
+investigation by scientific method, however interesting they may be as
+purely philosophical conceptions. Many ancient philosophers, among them
+Anaxagoras, Democritus, and Anaximenes, regarded the earth as the
+product of diffused matter in a state of the original chaos having
+fallen together haphazard, and they even presumed to predict its future
+career and ultimate destiny.
+
+In Anaximander and Anaximenes alone do we find any conception of
+possible progress; their thought was that as the world had taken time to
+become what it is, so in time it would pass, and as the entire universe
+had undergone alternate renewal and destruction in the past, that would
+be its history in the future. Aristotle, Ptolemy, and others appear to
+have held the curious notion that although everything terrestrial is
+evanescent, nevertheless the cosmos beyond the orbit of the moon is
+imperishable and eternal.
+
+By tracing the history of the intellectual development of Europe we may
+find why it was that scientific speculation on the cosmogony was delayed
+until the 18th century, and then undertaken quite independently by three
+philosophers in three different countries. Swedenborg, the theologian,
+set down in due form many of the principles that underlie the modern
+nebular hypothesis. Thomas Wright of Durham whose early theory of the
+arrangement of stars in the Galaxy we have already mentioned, speculated
+also on the origin and development of the universe, and his writings
+were known to Kant, who is now regarded as the author of the modern
+nebular hypothesis. This presents a definite mechanical explanation of
+the development and formation of the heavenly bodies, and in particular
+those composing the solar system.
+
+Kant was illustrious as a metaphysician, but he was a great physicist or
+natural philosopher as well, and he set down his ideas regarding the
+cosmogony with precision. Learned in the philosophy of the ancients, he
+did not follow their speculative conceptions, but merely assumed that
+all the materials from which the bodies of the solar system have been
+fashioned were resolved into their original elements at the beginning,
+and filled all that part of space in which they now move. True, this is
+pretty near the chaos of the Greeks, but Kant knew of the operation of
+the Newtonian law of gravitation, which the Greeks did not.
+
+As a natural result of gravitative processes, Kant inferred that the
+denser portions of the original mass would draw upon themselves the less
+dense portions, whirling motions would be everywhere set up, and the
+process would continue until many spherical bodies, each with a gaseous
+exterior in process of condensation, had taken the place of the original
+elements which filled space. In this manner Kant would explain the
+sameness in direction of motion, both orbital and axial, of all the
+planets and satellites of our system. But many philosophers are of the
+opinion that Kant's hypothesis would result, not in the formation of
+such a collection of bodies as the solar system is, but rather in a
+single central sun formed by common gravitation toward a single center.
+
+From quite another viewpoint the work of the elder Herschel is important
+here. No one knew the nebulae from actual observation better than he did;
+but, while his ideas about their composition were wrong, he nevertheless
+conceived of them as gradually condensing into stars or clusters of
+stars. And it was this speculative aspect of the nebulae, not as a
+possible means of accounting for the birth and development of the solar
+system, which constitutes Herschel's chief contribution to the nebular
+hypothesis. Classifying the nebulae which he had carefully studied with
+his great telescopes, it seemed obvious to him that they were actually
+in all the different stages of condensation, and subsequent research has
+strongly tended to substantiate the Herschelian view.
+
+Then came Laplace, who took up the great hypothesis where Kant and
+Herschel had left it, added new and important conceptions in the light
+of his mature labors as mathematician and astronomer, and put the theory
+in definitive form, such that it has ever since been known under the
+name of Laplacian nebular hypothesis. For reasons like those that
+prevailed with Kant, he began the evolution of the solar system with the
+sun already formed as the center, but surrounded by a vast incandescent
+atmosphere that filled all the space which the sun's family of planets
+now occupy. This entire mass, sun, atmosphere, and all, he conceived to
+have a stately rotation about its axis. With rotation of the mass and
+slow reduction of temperature in its outer regions, there would be
+contraction toward the solar center, and an increase in velocity of
+rotation until the whole mass had been much reduced in diameter at its
+poles and proportionately expanded at its equator.
+
+When the centrifugal force of the outer equatorial masses finally became
+equal to the gravitational forces of the central mass, then these
+conjoined outer portions would be left behind as a ring, still revolving
+at the velocity it had acquired when detached. The revolution of the
+entire inner mass goes on, its velocity accelerating until a similar
+equilibration of forces is again reached, when a second rotating ring
+is left behind. Laplace conceived the process as repeated until as many
+rings had been detached as there are individual planets, all central
+about the sun, or nearly so.
+
+In all, then, we should have nine gaseous rings; the outer ones
+preceding the inner in formation, but not all existing as rings at the
+same time. Radiation from the ring on all sides would lead to rapid
+contraction of its mass, so that many nuclei of condensation would form,
+of various sizes, all revolving round the central sun in practically the
+same period. Laplace conceived the evolution of the ring to proceed
+still farther till the largest aggregation in it had drawn to itself all
+the other separate nuclei in the ring.
+
+This, then, was the planet in embryo, in effect a diminutive sun, a
+secondary incandescent mass endowed with axial rotation in the same
+direction as the parent nebula. With reduction of temperature by
+radiation, polar contraction and equatorial expansion go on, and
+planetary rings are detached from this secondary mass in exactly the
+same way as from the original sun nebula. And these planetary rings are,
+in the Laplacian hypothesis, the embryo moons or planetary satellites,
+all revolving round their several planets in the same direction that the
+planets revolve about the sun.
+
+In the case of one of the planetary rings, its formation was so nearly
+homogeneous throughout that no aggregation into a single satellite was
+possible; all portions of the ring being of equal density, there was no
+denser region to attract the less dense regions, and in this manner the
+rings of Saturn were formed, in lieu of condensation into a separate
+satellite. Similarly in the case of the primal solar ring that was
+detached next after the Jovian ring; there was such a nice balancing of
+masses and densities that, instead of a single major planet, we have the
+well-known asteroidal ring, composed of innumerable discrete minor
+planets.
+
+This, then, in bare outline, is the Laplacian nebular hypothesis, and it
+accounted very well for the solar system as known in his day; the fairly
+regular progression of planetary distances; their orbits round the sun
+all nearly circular and approximately in a single plane; the planetary
+and satellite revolutions in orbit all in the same direction; the axial
+rotations of planets in the same direction as their orbital revolutions;
+and the plane of orbital revolution of the satellites practically
+coinciding with the plane of the planet's axial rotation. But the
+principle of conservation of energy was, of course, unknown to Laplace,
+nor had the mechanical equivalence of heat with other forms of energy
+been established in his day.
+
+In 1870, Lane of Washington first demonstrated the remarkable law that a
+gaseous sphere, in process of losing heat by radiation and contraction
+because of its own gravity, actually grows hotter instead of cooler, as
+long as it continues to be gaseous, and not liquid or solid. So there is
+no need of postulating with Laplace an excessively high temperature of
+the original nebula. The chief objection to Laplace's hypothesis by
+modern theorists is that the detachment of rings, though possible, would
+likely be a rare occurrence; protuberances or lumps on the equatorial
+exterior of a swiftly revolving mass would be more likely, and it is
+much easier to see how such masses would ultimately become planets than
+it is to follow the disruption of a possible ring and the necessary
+steps of the process by which it would condense into a final planet. The
+continued progress of research in many departments of astronomy has had
+important bearing on the nebular hypothesis, and we may rest assured
+that this hypothesis in somewhat modified form can hardly fail of
+ultimate acceptance, though not in every essential as its great
+originator left it.
+
+Lord Rosse's discovery of spiral nebulae, followed up by Keeler's
+photographic search for these bodies, revealing their actual existence
+in the heavens by the hundreds of thousands, has led to another
+criticism of the Laplacian theory. Could Laplace have known of the
+existence of these objects in such vast numbers, his hypothesis would no
+doubt have been suitably modified to account for their formation and
+development. It is generally considered that the ring of Saturn
+suggested to Laplace the ring feature in his scheme of origin of planets
+and satellites; so far as we know, the Saturnian ring is unique, the
+only object of its kind in the heavens. Whereas, next to the star
+itself, the spiral nebula is the type object which occurs most
+frequently. A theory, therefore, which will satisfactorily account for
+the origin and development of spiral nebulae must command recognition as
+of great importance in the cosmogony.
+
+Such a theory has been set forth by Chamberlin and Moulton in their
+planetesimal hypothesis, according to which the genesis of spiral nebulae
+happens when two giant suns approach each other so closely that
+tide-producing effects take place on a vast scale. These suns need not
+be luminous; they may perhaps belong to the class of dark or
+extinguished suns. The evidences of the existence of such in vast
+numbers throughout the universe is thought to be well established.
+
+Now, on close approach, what happens? There will be huge tides, and the
+nearer the bodies come to each other, the vaster the scale on which
+tides will be formed. If the bodies are liquid or gaseous, they will be
+distorted by the force of gravitation, and the figure of both bodies
+will become ellipsoidal; and at last under greater stress, the
+restraining shell of both bodies will burst asunder on opposite sides in
+streams of matter from the interior. In this manner the arms of the
+spiral are formed.
+
+As Chamberlin puts it: "If, with these potent forces thus nearly
+balanced, the sun closely approaches another sun, or body of like
+magnitude ... the gravity which restrains this enormous elastic power
+will be reduced along the line of mutual attraction. At the same time
+the pressure transverse to this line of relief will be increased. Such
+localized relief and intensified pressure must bring into action
+corresponding portions of the sun's elastic potency, resulting in
+protuberances of corresponding mass and high velocity."
+
+Only a fraction of one per cent of the sun's mass ejected in this
+fashion would be sufficient to generate the entire planetary system.
+Nuclei or knots in the arms of the spiral gradually grew by accretion,
+the four interior knots forming Mercury, Venus, the Earth, and Mars. The
+earth knot was a double one, which developed into the earth-moon system.
+The absence of a dominating nucleus beyond Mars accounts for the zone of
+the asteroids remaining in some sense in the original planetesimal
+condition. The vaster nuclei beyond Mars gradually condensed into
+Jupiter, Saturn, Uranus, and Neptune; and lesser nuclei related to the
+larger ones form the systems of moons or satellites.
+
+The orbits of the planetesimals and the planetary and satellite nuclei
+would be very eccentric, forming a confusion of ellipses with frequently
+crossing paths. Collisions would occur, and the nuclei would inevitably
+grow by accretion. Each planet, then, would clear up the planetesimals
+of its zone; and Moulton shows that this process would give rise to
+axial revolution of the planet in the same direction as its orbital
+revolution. The eccentricities would finally disappear, and the entire
+mass would revolve in a nearly circular orbit.
+
+Rotation twists the streams into the spiral form, and the huge amounts
+of wreckage from the near-collision are thrown into eddies. The
+fragments or particles (planetesimals) which have given the name to the
+theory, begin their motion round their central sun in elliptical paths
+as required by gravitation. The form of the spiral is preserved by the
+orbital motion of its particles. There is a gradual gathering together
+of the planetesimals at points or nodes of intersection, and these
+become aggregations of matter, nuclei that will perhaps become planets,
+though more likely other stars. The appulse or near approach is but one
+of the methods by which the spiral nebulae may have come into existence.
+The planetesimal hypothesis would seem to account for the formation of
+many of these objects as we see them in the sky, though perhaps it is
+hardly competent to replace entirely the Laplacian hypothesis of the
+formation of the solar system, which would appear to be a special case
+by itself.
+
+It will be observed that while the Laplacian hypothesis is concerned in
+the main with the progressive development of the solar system, and
+systems of a like order surrounding other stellar centers, whose
+existence is highly probable, the origin and development of the stellar
+universe is a vaster problem which can only be undertaken and completed
+in its broadest bearings when the structure of the stellar universe has
+been ascertained.
+
+Darwin's important investigations in 1877-1878 on tidal friction may be
+here related. Before his day acceptance of the ring-theory of
+development of the moon from the earth had scarcely been questioned; but
+his recondite mathematical researches on the tidal reaction between a
+central yielding mass and a body revolving round it brought to light the
+unsuspected effect of tides raised upon both bodies by their mutual
+attraction. The type of tides here meant is not the usual rise and fall
+of the waters of the ocean, but primeval tides in the plastic material
+of which the earth in its early history was composed. The Newtonian law
+of gravitation afforded a complete explanation of the rise and fall of
+the waters of the oceans, but as applied to the motions of planets and
+satellites by the Lagrangian formulae, it presupposed that all these
+bodies are rigid and unyielding. However, mutual tides of phenomenal
+height in their early plastic substances must have been a necessary
+consequence of the action of the Newtonian law, and they gradually drew
+upon the earth's rotational moment of momentum.
+
+In its very early history, before there was any moon to produce tides,
+the earth rotated much more rapidly, that is, the day was very much
+shorter than now, probably about five or six hours long. And with the
+rapid whirling, it was not a Laplacian ring that was detached, but a
+huge globular mass was separated from the plastic earth's equator.
+Darwin shows that the gravitative interaction of the two bodies
+immediately began to raise tides of extraordinary height in both,
+therefore tending to slow down the rotational periods of both bodies.
+Action and reaction being equal, the reaction at once began driving the
+moon away from the earth and thereby lengthening its period of
+revolution. So small was the mass of the moon and so near was it to the
+earth, that its relative rotational energy was in time completely used
+up, and the moon has ever since turned her constant face toward us.
+Tides of sun and moon in the plastic earth, acting through the ages,
+slowed down the earth's rotation to its present period, or the length of
+the day.
+
+Moulton, however, has investigated the tidal theory of the origin of the
+moon in the light of the planetesimal hypothesis, concluding that the
+moon never was part of the earth and separated therefrom by too rapid
+rotation of the earth, but that the distance of the two bodies has
+always been the same as now. The more massive earth has in its
+development throughout time robbed the less massive moon in the gradual
+process of accretion. So the moon has never acquired either an ocean or
+atmosphere, and this view is acceptable to geologists who have studied
+the sheer lunar surface, Shaler of Harvard among the first, and laid the
+foundations for a separate science of selenology.
+
+Tidal friction has also been operant in producing sun-raised tides upon
+the early plastic substances which composed the planets: more powerfully
+in the case of planets nearer the sun; less rapidly if the planet's mass
+is large; also less completely if the planet has solidified earlier on
+account of its small dimensions. So Darwin would account for the
+present rotation periods of all the planets: both Mercury and Venus
+powerfully acted on by the sun on account of their nearness to him, and
+their rotational energy completely exhausted, so that they now and for
+all time turn a constant face toward him, as the moon does to the earth;
+earth and possibly Mars even yet undergoing a very slight lengthening of
+their day; Jupiter and Saturn, also Uranus and probably Neptune, still
+exhibiting relatively swift axial rotation, because of their great mass
+and great original moment of momentum, and also by reason of their vast
+distances from the central tide-raising body, the sun.
+
+By applying to stellar systems the principles developed by Darwin, See
+accounted for the fact, to which he was the first to direct attention,
+that the great eccentricity of the binary orbits is a necessary result
+of the secular action of tidal friction. The double stars, then, were
+double nebulae, originally single, but separated by a process allied to
+that known as "fission" in protozoans. Indeed, Poincare proved
+mathematically that a swiftly revolving nebula, in consequence of
+contraction, first undergoes distortion into a pear-shaped or hour-glass
+figure, the two masses ultimately separating entirely; and the
+observations of the Herschels, Lord Rosse and others, with the recent
+photographic plates at the Lick and Mount Wilson observatories, afford
+immediate confirmation in a multitude of double nebulae, widely scattered
+throughout the nebular regions of the heavens.
+
+Jeans of Cambridge, England, among the most recent of mathematical
+investigators of the cosmogony, balances the advantages and
+disadvantages of the differing cosmogonic systems as follows, in his
+"Problems of Cosmogony and Stellar Dynamics": "Some hundreds of millions
+of years ago all the stars within our Galactic universe formed a single
+mass of excessively tenuous gas in slow rotation. As imagined by
+Laplace, this mass contracted owing to loss of energy by radiation, and
+so increased its angular velocity until it assumed a lenticular
+shape.... After this, further contraction was a sheer mathematical
+impossibility and the system had to expand. The mechanism of expansion
+was provided by matter being thrown off from the sharp edge of the
+lenticular figure, the lenticular center now forming the nucleus, and
+the thrown-off matter forming the arms, of a spiral nebula of the normal
+type. The long filaments of matter which constituted the arms, being
+gravitationally unstable, first formed into chains of condensation about
+nuclei, and ultimately formed detached masses of gas. With continued
+shrinkage, the temperature of these masses increased until they attained
+to incandescence, and shone as luminous stars. At the same time their
+velocity of rotation increased until a large proportion of them broke up
+by fission into binary systems. The majority of the stars broke away
+from their neighbors and so formed a cluster of irregularly moving
+stars--our present Galactic universe, in which the flattened shape of
+the original nebula may still be traced in the concentration about the
+Galactic plane, while the original motion along the nebular arms still
+persists in the form of 'star-streaming.' In some cases a pair or small
+group of stars failed to get clear of one another's gravitational
+attractions and remain describing orbits about one another as wide
+binaries or multiple stars. The stars which were formed last, the
+present B-type stars, have been unusually immune from disturbance by
+their neighbors, partly because they were born when adjacent stars had
+almost ceased to interfere with one another, partly because their
+exceptionally large mass minimized the effect of such interference as
+may have occurred; consequently they remain moving in the plane in which
+they were formed, many of them still constituting closely associated
+groups of stars--the moving star clusters.
+
+"At intervals it must have happened that two stars passed relatively
+near to one another in their motion through the universe. We conjecture
+that something like 300 million years ago our sun experienced an
+encounter of this kind, a large star passing within a distance of about
+the sun's diameter from its surface. The effect of this, as we have
+seen, would be the ejection of a stream of gas toward the passing star.
+At this epoch the sun is supposed to have been dark and cold, its
+density being so low that its radius was perhaps comparable with the
+present radius of Neptune's orbit. The ejected stream of matter,
+becoming still colder by radiation, may have condensed into liquid near
+its ends and perhaps partially also near its middle. Such a jet of
+matter would be longitudinally unstable and would condense into detached
+nuclei which would ultimately form planets."
+
+
+
+
+CHAPTER LXI
+
+COSMOGONY IN TRANSITION
+
+
+We have seen how Wright in 1750 initiated a theory of evolution, not
+only of the solar system, but of all the stars and nebulae as well; how
+Kant in 1752 by elaborating this theory sought to develop the details of
+evolution of the solar system on the basis of the Newtonian law, though
+weakened, as we know, by serious errors in applying physical laws; how
+Laplace in 1796 put forward his nebular hypothesis of origin and
+development of the solar system, by contraction from an original gaseous
+nebula in accord with the Newtonian law; how Sir William Herschel in
+1810 saw in all nebulae merely the stuff that stars are made of; how Lord
+Rosse in 1845 discovered spiral nebulae; how Helmholtz in 1854 put
+forward his contraction theory of maintenance of the solar heat,
+seemingly reinforcing the Laplacian theory; how Lane in 1870 proved that
+a contracting gaseous star might rise in temperature; how Roche in 1873
+in attempting to modify the Laplacian hypothesis, pointed out the
+conditions under which a satellite would be broken up by tidal strains;
+how Darwin in 1879 showed that the theory of tidal evolution of
+non-rigid bodies might account for the formation of the moon, and binary
+stars might originate by fission; how Keeler in 1900 discovered the vast
+numbers of spiral nebulae; how Chamberlin and Moulton in 1903 put
+forward the planetesimal hypothesis of formation of the spiral nebulae,
+showing also how that hypothesis might account for the evolution of the
+solar system; and how Jeans in 1916 advocated the median ground in
+evolution of the arms of the spiral nebulae, showing that they will break
+up into nuclei, if sufficiently massive.
+
+In all these theories, truth and error, or lack of complete knowledge,
+appear to be intermingled in varying proportions. Is it not early yet to
+say, either that any one of them must be abandoned as totally wrong, or
+on the other hand that any one of them, or indeed any single hypothesis,
+can explain all the evolutionary processes of the universe?
+
+Clearly the great problems cannot all be solved by the kinetic theory of
+gases and the law of gravitation alone. Recent physical researches into
+sub-atomic energy and the structure and properties of matter, appear to
+point in the direction where we must next look for more light on such
+questions as the origin and maintenance of the sun's heat, the complex
+phenomena of variable stars and the progressive evolution of the myriad
+bodies of the stellar universe. Because we have actually seen one star
+turn into a nebula we should not jump to the conclusion that all nebulae
+are formed from stars, even if this might seem a direct inference from
+the high radial velocities of planetary nebulae.
+
+Quite as obviously many of the spiral nebulae are in a stage of
+transition into local universes of stars--even more obvious from the
+marvelous photographs in our day than the evolution of stars from nebulae
+of all types was to Herschel in his day.
+
+The physicist must further investigate such questions as the building up
+of heavy atomic elements by gravitative condensation of such lighter
+ones as compose the nebulae; and laboratory investigation must elucidate
+further the process of development of energy from atomic disintegration
+under very high pressures. This leads to a reclassification of the stars
+on a temperature basis.
+
+Equally important is the inquiry into the mechanism of radiative
+equilibrium in sun and stars. Not impossibly the process of the earth's
+upper atmosphere in maintaining a terrestrial equilibrium may afford
+some clue. What this physical mechanism may be is very incompletely
+known, but it is now open to further research through recent progress of
+aeronautics, which will afford the investigator a "ceiling" of 50,000
+feet and probably more. Beneath this level, perhaps even below 40,000
+feet, lie all the strata, including the inversion layer, where the sun's
+heat is conserved and an equilibrium maintained.
+
+Even ten years ago, had an astronomer been asked about the physical
+condition of the interior of the stars, he would have replied that
+information of this character could only be had on visiting the stars
+themselves--and perhaps not even then. But at the Cardiff meeting of the
+British Association in 1920, Eddington, the president of Section A,
+delivered an address on the internal constitution of the stars. He cites
+the recent investigations of Russell and others on truly gaseous stars,
+like Aldebaran, Arcturus, Antares and Canopus, which are in a diffuse
+state and are the most powerful light-givers, and thus are to be
+distinguished from the denser stars like our Sun. The term _giants_ is
+applied to the former, and _dwarfs_ to the latter, in accord with
+Russell's theory.
+
+As density increases through contraction, these terms represent the
+progressive stages, from earlier to later, in a star's history. A red or
+M-type star begins its history as a giant of comparatively low
+temperature. Contracting, according to Lane's law, its temperature must
+rise until its density becomes such that it no longer behaves as a
+perfect gas. Much depends on the star's mass; but after its maximum
+temperature is attained, the star, which has shrunk to the proportions
+of a dwarf, goes on cooling and contracts still further.
+
+Each temperature-level is reached and passed twice, once during the
+ascending stage and once again in descending--once as a giant, and once
+as a dwarf. Thus there are vast differences in luminosity: the huge
+giant, having a far larger surface than the shrunken dwarf, radiates an
+amount of light correspondingly greater.
+
+The physicist recognizes heat in two forms--the energy of motion of
+material atoms, and the energy of ether waves. In hot bodies with which
+we are familiar, the second form is quite insignificant; but in the
+giant stars, the two forms are present in about equal proportions. The
+super-heated conditions of the interior of the stars can only be
+estimated in millions of degrees; and the problem is not one of
+convection currents, as formerly thought, bringing hot masses to the
+surface from the highly heated interior, but how can the heat of the
+interior be barred against leakage and reduced to the relatively small
+radiation emitted by the stars. "Smaller stars have to manufacture the
+radiant heat which they emit, living from hand to mouth; the giant stars
+merely leak radiant heat from their store."
+
+So a radioactive type of equilibrium must be established, rather than a
+convective one. Laboratory investigations of the very short waves are
+now in progress, bearing on the transparency of stellar material to the
+radiation traversing it; and the penetrating power of the star's
+radiation is much like that of X-rays. The opacity is remarkably high,
+explaining why the star is so nearly "heat-tight."
+
+Opacity being constant, the total radiation of a giant star depends on
+its mass only, and is quite independent of its temperature or state of
+diffuseness. So that the total radiation of a star which is measured
+roughly by its luminosity, may readily remain constant during the entire
+'giant' stage of its history. As Russell originally pointed out, giant
+stars of every spectral type have nearly the same luminosity. From the
+range of luminosity of the giant stars, then, we may infer their range
+of masses: they come out much alike, agreeing well with results obtained
+by double-star investigation.
+
+These studies of radiation and internal condition of the stars again
+bring up the question of the original source of that supply of radiant
+energy continually squandered by all self-luminous bodies. The giant
+stars are especially prodigal, and radiate at least a hundredfold faster
+than the sun.
+
+"A star is drawing on some vast reservoir of energy," says Eddington,
+"by means unknown to us. This reservoir can scarcely be other than the
+sub-atomic energy which, it is known, exists abundantly in all matter;
+we sometimes dream that man will one day learn how to release it and use
+it for his service. The store is well-nigh inexhaustible, if only it
+could be tapped. There is sufficient in the sun to maintain its output
+of heat for fifteen billion years."
+
+
+ * * * * *
+
+
+Transcriber's Notes:
+
+
+Obvious punctuation errors repaired. Hyphenation and spelling was
+standardized by using the most prevalent form. The oe ligature was
+converted to the letters "oe". Whole and fractional parts of numbers
+are displayed as follows: 365-1/4.
+
+
+ Page Correction
+ ==== ===================
+ 20 Aa => Aya
+ 39 Ulugh Begh => Ulugh Beg
+ 46 Instaurata Mecanica => Instauratae Mechanica
+ 58 Oscillatorium Horologium => Horologium Oscillatorium
+ 225 seceded => succeeded
+ 226 areoplane => aeroplane
+ 320 Plate 2 - Vulpeculae => Vulpecula
+
+
+Text Emphasis
+
+ _Text_ - Italic
+
+ =Text= - Bold
+
+
+
+
+
+
+
+End of the Project Gutenberg EBook of Astronomy, by David Todd
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