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Transcriber's Note
The punctuation and spelling from the original text have been faithfully
preserved. Only obvious typographical errors have been corrected.
There are several mathematical formulas within the text. They are
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PIONEERS OF SCIENCE
[Illustration]
[Illustration: NEWTON
_From the picture by Kneller, 1689, now at Cambridge_]
PIONEERS OF SCIENCE
BY
OLIVER LODGE, F.R.S.
PROFESSOR OF PHYSICS IN VICTORIA UNIVERSITY COLLEGE, LIVERPOOL
_WITH PORTRAITS AND OTHER ILLUSTRATIONS_
London
MACMILLAN AND CO.
AND NEW YORK
1893
RICHARD CLAY AND SONS, LIMITED,
LONDON AND BUNGAY.
PREFACE
This book takes its origin in a course of lectures on the history and
progress of Astronomy arranged for me in the year 1887 by three of my
colleagues (A.C.B., J.M., G.H.R.), one of whom gave the course its name.
The lectures having been found interesting, it was natural to write them
out in full and publish.
If I may claim for them any merit, I should say it consists in their
simple statement and explanation of scientific facts and laws. The
biographical details are compiled from all readily available sources,
there is no novelty or originality about them; though it is hoped that
there may be some vividness. I have simply tried to present a living
figure of each Pioneer in turn, and to trace his influence on the
progress of thought.
I am indebted to many biographers and writers, among others to Mr.
E.J.C. Morton, whose excellent set of lives published by the S.P.C.K.
saved me much trouble in the early part of the course.
As we approach recent times the subject grows more complex, and the men
more nearly contemporaries; hence the biographical aspect diminishes and
the scientific treatment becomes fuller, but in no case has it been
allowed to become technical and generally unreadable.
To the friends (C.C.C., F.W.H.M., E.F.R.) who with great kindness have
revised the proofs, and have indicated places where the facts could be
made more readily intelligible by a clearer statement, I express my
genuine gratitude.
UNIVERSITY COLLEGE, LIVERPOOL,
_November, 1892_.
CONTENTS
_PART I_
LECTURE I
PAGE
COPERNICUS AND THE MOTION OF THE EARTH 2
LECTURE II
TYCHO BRAHE AND THE EARLIEST OBSERVATORY 32
LECTURE III
KEPLER AND THE LAWS OF PLANETARY MOTION 56
LECTURE IV
GALILEO AND THE INVENTION OF THE TELESCOPE 80
LECTURE V
GALILEO AND THE INQUISITION 108
LECTURE VI
DESCARTES AND HIS THEORY OF VORTICES 136
LECTURE VII
SIR ISAAC NEWTON 159
LECTURE VIII
NEWTON AND THE LAW OF GRAVITATION 180
LECTURE IX
NEWTON'S "PRINCIPIA" 203
_PART II_
LECTURE X
ROEMER AND BRADLEY AND THE VELOCITY OF LIGHT 232
LECTURE XI
LAGRANGE AND LAPLACE--THE STABILITY OF THE SOLAR SYSTEM,
AND THE NEBULAR HYPOTHESIS 254
LECTURE XII
HERSCHEL AND THE MOTION OF THE FIXED STARS 273
LECTURE XIII
THE DISCOVERY OF THE ASTEROIDS 294
LECTURE XIV
BESSEL--THE DISTANCES OF THE STARS, AND THE DISCOVERY OF
STELLAR PLANETS 304
LECTURE XV
THE DISCOVERY OF NEPTUNE 317
LECTURE XVI
COMETS AND METEORS 331
LECTURE XVII
THE TIDES 353
LECTURE XVIII
THE TIDES, AND PLANETARY EVOLUTION 379
ILLUSTRATIONS
FIG. PAGE
1. ARCHIMEDES 8
2. LEONARDO DA VINCI 10
3. COPERNICUS 12
4. HOMERIC COSMOGONY 15
5. EGYPTIAN SYMBOL OF THE UNIVERSE 16
6. HINDOO EARTH 17
7. ORDER OF ANCIENT PLANETS CORRESPONDING TO THE DAYS OF
THE WEEK 19
8. PTOLEMAIC SYSTEM 20
9. SPECIMENS OF APPARENT PATHS OF VENUS AND OF MARS
AMONG THE STARS 21
10. APPARENT EPICYCLIC ORBITS OF JUPITER AND SATURN 22
11. EGYPTIAN SYSTEM 24
12. TRUE ORBITS OF EARTH AND JUPITER 25
13. ORBITS OF MERCURY AND EARTH 25
14. COPERNICAN SYSTEM AS FREQUENTLY REPRESENTED 26
15. SLOW MOVEMENT OF THE NORTH POLE IN A CIRCLE AMONG
THE STARS 29
16. TYCHONIC SYSTEM, SHOWING THE SUN WITH ALL THE PLANETS
REVOLVING ROUND THE EARTH 38
17. PORTRAIT OF TYCHO 41
18. EARLY OUT-DOOR QUADRANT OF TYCHO 43
19. MAP OF DENMARK, SHOWING THE ISLAND OF HUEN 45
20. URANIBURG 46
21. ASTROLABE 47
22. TYCHO'S LARGE SEXTANT 48
23. THE QUADRANT IN URANIBURG 49
24. TYCHO'S FORM OF TRANSIT CIRCLE 50
25. A MODERN TRANSIT CIRCLE 51
26. ORBITS OF SOME OF THE PLANETS DRAWN TO SCALE 60
27. MANY-SIDED POLYGON OR APPROXIMATE CIRCLE ENVELOPED
BY STRAIGHT LINES 61
28. KEPLER'S IDEA OF THE REGULAR SOLIDS 62
29. DIAGRAM OF EQUANT 67
30. EXCENTRIC CIRCLE SUPPOSED TO BE DIVIDED INTO EQUAL AREAS 68
31. MODE OF DRAWING AN ELLIPSE 70
32. KEPLER'S DIAGRAM PROVING EQUABLE DESCRIPTION OF AREAS
FOR AN ELLIPSE 71
33. DIAGRAM OF A PLANET'S VELOCITY IN DIFFERENT PARTS OF ITS ORBIT 72
34. PORTRAIT OF KEPLER 76
35. CURVE DESCRIBED BY A PROJECTILE 82
36. TWO FORMS OF PULSILOGY 87
37. TOWER OF PISA 91
38. VIEW OF THE HALF-MOON IN SMALL TELESCOPE 97
39. PORTION OF THE LUNAR SURFACE MORE HIGHLY MAGNIFIED 98
40. ANOTHER PORTION OF THE LUNAR SURFACE 99
41. LUNAR LANDSCAPE SHOWING EARTH 100
42. GALILEO'S METHOD OF ESTIMATING THE HEIGHT OF LUNAR MOUNTAIN 101
43. SOME CLUSTERS AND NEBULAE 102
44. STAGES OF THE DISCOVERY OF JUPITER'S SATELLITES 103
45. ECLIPSES OF JUPITER'S SATELLITES 105
46. OLD DRAWINGS OF SATURN BY DIFFERENT OBSERVERS, WITH
THE IMPERFECT INSTRUMENTS OF THAT DAY 111
47. PHASES OF VENUS 112
48. SUNSPOTS AS SEEN WITH LOW POWER 113
49. A PORTION OF THE SUN'S DISK AS SEEN IN A POWERFUL MODERN
TELESCOPE 114
50. SATURN AND HIS RINGS 115
51. MAP OF ITALY 118
52. PORTRAIT OF GALILEO 126
53. PORTRAIT OF DESCARTES 148
54. DESCARTES'S EYE DIAGRAM 151
55. DESCARTES'S DIAGRAM OF VORTICES FROM HIS "PRINCIPIA" 152
56. MANOR-HOUSE OF WOOLSTHORPE 162
57. PROJECTILE DIAGRAM 170
58. } { 171
59. } DIAGRAMS ILLUSTRATIVE OF THOSE NEAR THE BEGINNING { 174
60. } OF NEWTON'S "PRINCIPIA" { 175
61-2. } { 175
63. PRISMATIC DISPERSION 182
64. A SINGLE CONSTITUENT OF WHITE LIGHT IS CAPABLE OF NO
MORE DISPERSION 183
65. PARALLEL BEAM PASSING THROUGH A LENS 184
66. NEWTON'S TELESCOPE 186
67. THE SEXTANT, AS NOW MADE 187
68. NEWTON WHEN YOUNG 196
69. SIR ISAAC NEWTON 200
70. ANOTHER "PRINCIPIA" DIAGRAM 207
71. WELL-KNOWN MODEL EXHIBITING THE OBLATE SPHEROIDAL
FORM AS A CONSEQUENCE OF SPINNING ABOUT A CENTRAL
AXIS 219
72. JUPITER 221
73. DIAGRAM OF EYE LOOKING AT A LIGHT REFLECTED IN A DISTANT
MIRROR THROUGH THE TEETH OF A REVOLVING WHEEL 238
74. FIZEAU'S WHEEL, SHOWING THE APPEARANCE OF DISTANT
IMAGE SEEN THROUGH ITS TEETH 239
75. ECLIPSES OF ONE OF JUPITER'S SATELLITES 241
76. A TRANSIT INSTRUMENT FOR THE BRITISH ASTRONOMICAL EXPEDITION,
1874 243
77. DIAGRAM OF EQUATORIALLY MOUNTED TELESCOPE 245
78. ABERRATION DIAGRAM 250
79. SHOWING THE THREE CONJUNCTION PLACES IN THE ORBITS OF
JUPITER AND SATURN 259
80. LORD ROSSE'S DRAWING OF THE SPIRAL NEBULA IN CANES
VENATICI 269
81. SATURN 271
82. PRINCIPLE OF NEWTONIAN REFLECTOR 278
83. HERSCHEL'S 40-FOOT TELESCOPE 283
84. WILLIAM HERSCHEL 285
85. CAROLINE HERSCHEL 287
86. DOUBLE STARS 288
87. OLD DRAWING OF THE CLUSTER IN HERCULES 290
88. OLD DRAWING OF THE ANDROMEDA NEBULA 291
89. THE GREAT NEBULA IN ORION 292
90. PLANETARY ORBITS TO SCALE 297
91. DIAGRAM ILLUSTRATING PARALLAX 307
92. THE KOeNIGSBERG HELIOMETER 312
93. PERTURBATIONS OF URANUS 320
94. URANUS' AND NEPTUNE'S RELATIVE POSITIONS 325
95. METEORITE 333
96. METEOR STREAM CROSSING FIELD OF TELESCOPE 334
97. DIAGRAM OF DIRECTION OF EARTH'S ORBITAL MOTION 335
98. PARABOLIC AND ELLIPTIC ORBITS 340
99. ORBIT OF HALLEY'S COMET 341
100. VARIOUS APPEARANCES OF HALLEY'S COMET WHEN LAST SEEN 342
101. HEAD OF DONATI'S COMET OF 1858 343
102. COMET 344
103. ENCKE'S COMET 345
104. BIELA'S COMET AS LAST SEEN IN TWO PORTIONS 346
105. RADIANT POINT PERSPECTIVE 348
106. PRESENT ORBIT OF NOVEMBER METEORS 349
107. ORBIT OF NOVEMBER METEORS BEFORE AND AFTER ENCOUNTER
WITH URANUS 351
108. THE MERSEY 355
109. CO-TIDAL LINES, SHOWING THE WAY THE TIDAL WAVE
REACHES THE BRITISH ISLES FROM THE ATLANTIC 359
110. WHIRLING EARTH MODEL 364
111. EARTH AND MOON MODEL 365
112. EARTH AND MOON (EARTH'S ROTATION NEGLECTED) 366
113. MAPS SHOWING HOW COMPARATIVELY FREE FROM LAND OBSTRUCTION
THE OCEAN IN THE SOUTHERN HEMISPHERE IS 369
114. SPRING AND NEAP TIDES 370
115. TIDAL CLOCK 371
116. SIR WILLIAM THOMSON (LORD KELVIN) 373
117. TIDE-GAUGE FOR RECORDING LOCAL TIDES 375
118. HARMONIC ANALYZER 375
119. TIDE-PREDICTER 376
120. WEEKLY SHEET OF CURVES 377
PIONEERS OF SCIENCE
PART I
_FROM DUSK TO DAYLIGHT_
DATES AND SUMMARY OF FACTS FOR LECTURE I
_Physical Science of the Ancients._ Thales 640 B.C., Anaximander 610
B.C., PYTHAGORAS 600 B.C., Anaxagoras 500 B.C., Eudoxus 400 B.C.,
ARISTOTLE 384 B.C., Aristarchus 300 B.C., ARCHIMEDES 287 B.C.,
Eratosthenes 276 B.C., HIPPARCHUS 160 B.C., Ptolemy 100 A.D.
_Science of the Middle Ages._ Cultivated only among the Arabs; largely
in the forms of astrology, alchemy, and algebra.
_Return of Science to Europe._ Roger Bacon 1240, Leonardo da Vinci 1480,
(Printing 1455), Columbus 1492, Copernicus 1543.
_A sketch of Copernik's life and work._ Born 1473 at Thorn in Poland.
Studied mathematics at Bologna. Became an ecclesiastic. Lived at
Frauenburg near mouth of Vistula. Substituted for the apparent motion of
the heavens the real motion of the earth. Published tables of planetary
motions. Motion still supposed to be in epicycles. Worked out his ideas
for 36 years, and finally dedicated his work to the Pope. Died just as
his book was printed, aged 72, a century before the birth of Newton. A
colossal statue by Thorwaldsen erected at Warsaw in 1830.
PIONEERS OF SCIENCE
LECTURE I
COPERNICUS AND THE MOTION OF THE EARTH
The ordinary run of men live among phenomena of which they know nothing
and care less. They see bodies fall to the earth, they hear sounds, they
kindle fires, they see the heavens roll above them, but of the causes
and inner working of the whole they are ignorant, and with their
ignorance they are content.
"Understand the structure of a soap-bubble?" said a cultivated literary
man whom I know; "I wouldn't cross the street to know it!"
And if this is a prevalent attitude now, what must have been the
attitude in ancient times, when mankind was emerging from savagery, and
when history seems composed of harassments by wars abroad and
revolutions at home? In the most violently disturbed times indeed, those
with which ordinary history is mainly occupied, science is quite
impossible. It needs as its condition, in order to flourish, a fairly
quiet, untroubled state, or else a cloister or university removed from
the din and bustle of the political and commercial world. In such places
it has taken its rise, and in such peaceful places and quiet times true
science will continue to be cultivated.
The great bulk of mankind must always remain, I suppose, more or less
careless of scientific research and scientific result, except in so far
as it affects their modes of locomotion, their health and pleasure, or
their purse.
But among a people hurried and busy and preoccupied, some in the pursuit
of riches, some in the pursuit of pleasure, and some, the majority, in
the struggle for existence, there arise in every generation, here and
there, one or two great souls--men who seem of another age and country,
who look upon the bustle and feverish activity and are not infected by
it, who watch others achieving prizes of riches and pleasure and are not
disturbed, who look on the world and the universe they are born in with
quite other eyes. To them it appears not as a bazaar to buy and to sell
in; not as a ladder to scramble up (or down) helter-skelter without
knowing whither or why; but as a fact--a great and mysterious fact--to
be pondered over, studied, and perchance in some small measure
understood. By the multitude these men were sneered at as eccentric or
feared as supernatural. Their calm, clear, contemplative attitude seemed
either insane or diabolic; and accordingly they have been pitied as
enthusiasts or killed as blasphemers. One of these great souls may have
been a prophet or preacher, and have called to his generation to bethink
them of why and what they were, to struggle less and meditate more, to
search for things of true value and not for dross. Another has been a
poet or musician, and has uttered in words or in song thoughts dimly
possible to many men, but by them unutterable and left inarticulate.
Another has been influenced still more _directly_ by the universe around
him, has felt at times overpowered by the mystery and solemnity of it
all, and has been impelled by a force stronger than himself to study it,
patiently, slowly, diligently; content if he could gather a few crumbs
of the great harvest of knowledge, happy if he could grasp some great
generalization or wide-embracing law, and so in some small measure enter
into the mind and thought of the Designer of all this wondrous frame of
things.
These last have been the men of science, the great and heaven-born men
of science; and they are few. In our own day, amid the throng of
inventions, there are a multitude of small men using the name of science
but working for their own ends, jostling and scrambling just as they
would jostle and scramble in any other trade or profession. These may be
workers, they may and do advance knowledge, but they are never pioneers.
Not to them is it given to open out great tracts of unexplored
territory, or to view the promised land as from a mountain-top. Of them
we shall not speak; we will concern ourselves only with the greatest,
the epoch-making men, to whose life and work we and all who come after
them owe so much. Such a man was Thales. Such was Archimedes,
Hipparchus, Copernicus. Such pre-eminently was Newton.
Now I am not going to attempt a history of science. Such a work in ten
lectures would be absurd. I intend to pick out a few salient names here
and there, and to study these in some detail, rather than by attempting
to deal with too many to lose individuality and distinctness.
We know so little of the great names of antiquity, that they are for
this purpose scarcely suitable. In some departments the science of the
Greeks was remarkable, though it is completely overshadowed by their
philosophy; yet it was largely based on what has proved to be a wrong
method of procedure, viz the introspective and conjectural, rather than
the inductive and experimental methods. They investigated Nature by
studying their own minds, by considering the meanings of words, rather
than by studying things and recording phenomena. This wrong (though by
no means, on the face of it, absurd) method was not pursued exclusively,
else would their science have been valueless, but the influence it had
was such as materially to detract from the value of their speculations
and discoveries. For when truth and falsehood are inextricably woven
into a statement, the truth is as hopelessly hidden as if it had never
been stated, for we have no criterion to distinguish the false from the
true.
[Illustration: FIG. 1.--Archimedes.]
Besides this, however, many of their discoveries were ultimately lost to
the world, some, as at Alexandria, by fire--the bigoted work of a
Mohammedan conqueror--some by irruption of barbarians; and all were
buried so long and so completely by the night of the dark ages, that
they had to be rediscovered almost as absolutely and completely as
though they had never been. Some of the names of antiquity we shall have
occasion to refer to; so I have arranged some of them in chronological
order on page 4, and as a representative one I may specially emphasize
Archimedes, one of the greatest men of science there has ever been, and
the father of physics.
The only effective link between the old and the new science is afforded
by the Arabs. The dark ages come as an utter gap in the scientific
history of Europe, and for more than a thousand years there was not a
scientific man of note except in Arabia; and with the Arabs knowledge
was so mixed up with magic and enchantment that one cannot contemplate
it with any degree of satisfaction, and little real progress was made.
In some of the _Waverley Novels_ you can realize the state of matters in
these times; and you know how the only approach to science is through
some Arab sorcerer or astrologer, maintained usually by a monarch, and
consulted upon all great occasions, as the oracles were of old.
In the thirteenth century, however, a really great scientific man
appeared, who may be said to herald the dawn of modern science in
Europe. This man was Roger Bacon. He cannot be said to do more than
herald it, however, for we must wait two hundred years for the next name
of great magnitude; moreover he was isolated, and so far in advance of
his time that he left no followers. His own work suffered from the
prevailing ignorance, for he was persecuted and imprisoned, not for the
commonplace and natural reason that he frightened the Church, but merely
because he was eccentric in his habits and knew too much.
The man I spoke of as coming two hundred years later is Leonardo da
Vinci. True he is best known as an artist, but if you read his works you
will come to the conclusion that he was the most scientific artist who
ever lived. He teaches the laws of perspective (then new), of light and
shade, of colour, of the equilibrium of bodies, and of a multitude of
other matters where science touches on art--not always quite correctly
according to modern ideas, but in beautiful and precise language. For
clear and conscious power, for wide-embracing knowledge and skill,
Leonardo is one of the most remarkable men that ever lived.
About this time the tremendous invention of printing was achieved, and
Columbus unwittingly discovered the New World. The middle of the next
century must be taken as the real dawn of modern science; for the year
1543 marks the publication of the life-work of Copernicus.
[Illustration: FIG. 2.--Leonardo da Vinci.]
Nicolas Copernik was his proper name. Copernicus is merely the Latinized
form of it, according to the then prevailing fashion. He was born at
Thorn, in Polish Prussia, in 1473. His father is believed to have been a
German. He graduated at Cracow as doctor in arts and medicine, and was
destined for the ecclesiastical profession. The details of his life are
few; it seems to have been quiet and uneventful, and we know very little
about it. He was instructed in astronomy at Cracow, and learnt
mathematics at Bologna. Thence he went to Rome, where he was made
Professor of Mathematics; and soon afterwards he went into orders. On
his return home, he took charge of the principal church in his native
place, and became a canon. At Frauenburg, near the mouth of the Vistula,
he lived the remainder of his life. We find him reporting on coinage for
the Government, but otherwise he does not appear as having entered into
the life of the times.
He was a quiet, scholarly monk of studious habits, and with a reputation
which drew to him several earnest students, who received _viva voce_
instruction from him; so, in study and meditation, his life passed.
He compiled tables of the planetary motions which were far more correct
than any which had hitherto appeared, and which remained serviceable for
long afterwards. The Ptolemaic system of the heavens, which had been the
orthodox system all through the Christian era, he endeavoured to improve
and simplify by the hypothesis that the sun was the centre of the system
instead of the earth; and the first consequences of this change he
worked out for many years, producing in the end a great book: his one
life-work. This famous work, "De Revolutionibus Orbium Coelestium,"
embodied all his painstaking calculations, applied his new system to
each of the bodies in the solar system in succession, and treated
besides of much other recondite matter. Towards the close of his life it
was put into type. He can scarcely be said to have lived to see it
appear, for he was stricken with paralysis before its completion; but a
printed copy was brought to his bedside and put into his hands, so that
he might just feel it before he died.
[Illustration: FIG. 3.--Copernicus.]
That Copernicus was a giant in intellect or power--such as had lived in
the past, and were destined to live in the near future--I see no reason
whatever to believe. He was just a quiet, earnest, patient, and
God-fearing man, a deep student, an unbiassed thinker, although with no
specially brilliant or striking gifts; yet to him it was given to effect
such a revolution in the whole course of man's thoughts as is difficult
to parallel.
You know what the outcome of his work was. It proved--he did not merely
speculate, he proved--that the earth is a planet like the others, and
that it revolves round the sun.
Yes, it can be summed up in a sentence, but what a revelation it
contains. If you have never made an effort to grasp the full
significance of this discovery you will not appreciate it. The doctrine
is very familiar to us now, we have heard it, I suppose, since we were
four years old, but can you realize it? I know it was a long time before
I could. Think of the solid earth, with trees and houses, cities and
countries, mountains and seas--think of the vast tracts of land in Asia,
Africa, and America--and then picture the whole mass spinning like a
top, and rushing along its annual course round the sun at the rate of
nineteen miles every second.
Were we not accustomed to it, the idea would be staggering. No wonder it
was received with incredulity. But the difficulties of the conception
are not only physical, they are still more felt from the speculative and
theological points of view. With this last, indeed, the reconcilement
cannot be considered complete even yet. Theologians do not, indeed, now
_deny_ the fact of the earth's subordination in the scheme of the
universe, but many of them ignore it and pass it by. So soon as the
Church awoke to a perception of the tremendous and revolutionary import
of the new doctrines, it was bound to resist them or be false to its
traditions. For the whole tenor of men's thought must have been changed
had they accepted it. If the earth were not the central and
all-important body in the universe, if the sun and planets and stars
were not attendant and subsidiary lights, but were other worlds larger
and perhaps superior to ours, where was man's place in the universe?
and where were the doctrines they had maintained as irrefragable? I by
no means assert that the new doctrines were really utterly
irreconcilable with the more essential parts of the old dogmas, if only
theologians had had patience and genius enough to consider the matter
calmly. I suppose that in that case they might have reached the amount
of reconciliation at present attained, and not only have left scientific
truth in peace to spread as it could, but might perhaps themselves have
joined the band of earnest students and workers, as so many of the
higher Catholic clergy do at the present day.
But this was too much to expect. Such a revelation was not to be
accepted in a day or in a century--the easiest plan was to treat it as a
heresy, and try to crush it out.
Not in Copernik's life, however, did they perceive the dangerous
tendency of the doctrine--partly because it was buried in a ponderous
and learned treatise not likely to be easily understood; partly,
perhaps, because its propounder was himself an ecclesiastic; mainly
because he was a patient and judicious man, not given to loud or
intolerant assertion, but content to state his views in quiet
conversation, and to let them gently spread for thirty years before he
published them. And, when he did publish them, he used the happy device
of dedicating his great book to the Pope, and a cardinal bore the
expense of printing it. Thus did the Roman Church stand sponsor to a
system of truth against which it was destined in the next century to
hurl its anathemas, and to inflict on its conspicuous adherents torture,
imprisonment, and death.
To realize the change of thought, the utterly new view of the universe,
which the Copernican theory introduced, we must go back to preceding
ages, and try to recall the views which had been held as probable
concerning the form of the earth and the motion of the heavenly bodies.
[Illustration: FIG. 4.--Homeric Cosmogony.]
The earliest recorded notion of the earth is the very natural one that
it is a flat area floating in an illimitable ocean. The sun was a god
who drove his chariot across the heavens once a day; and Anaxagoras was
threatened with death and punished with banishment for teaching that the
sun was only a ball of fire, and that it might perhaps be as big as the
country of Greece. The obvious difficulty as to how the sun got back to
the east again every morning was got over--not by the conjecture that he
went back in the dark, nor by the idea that there was a fresh sun every
day; though, indeed, it was once believed that the moon was created once
a month, and periodically cut up into stars--but by the doctrine that in
the northern part of the earth was a high range of mountains, and that
the sun travelled round on the surface of the sea behind these.
Sometimes, indeed, you find a representation of the sun being rowed
round in a boat. Later on it was perceived to be necessary that the sun
should be able to travel beneath the earth, and so the earth was
supposed to be supported on pillars or on roots, or to be a dome-shaped
body floating in air--much like Dean Swift's island of Laputa. The
elephant and tortoise of the Hindu earth are, no doubt, emblematic or
typical, not literal.
[Illustration: FIG. 5.--Egyptian Symbol of the Universe.
The earth a figure with leaves, the heaven a figure with stars, the
principle of equilibrium and support, the boats of the rising and
setting sun.]
Aristotle, however, taught that the earth must be a sphere, and used all
the orthodox arguments of the present children's geography-books about
the way you see ships at sea, and about lunar eclipses.
To imagine a possible antipodes must, however, have been a tremendous
difficulty in the way of this conception of a sphere, and I scarcely
suppose that any one can at that time have contemplated the possibility
of such upside-down regions being inhabited. I find that intelligent
children invariably feel the greatest difficulty in realizing the
existence of inhabitants on the opposite side of the earth. Stupid
children, like stupid persons in general, will of course believe
anything they are told, and much good may the belief do them; but the
kind of difficulties felt by intelligent and thoughtful children are
most instructive, since it is quite certain that the early philosophers
must have encountered and overcome those very same difficulties by their
own genius.
[Illustration: FIG. 6.--Hindoo Earth.]
However, somehow or other the conception of a spherical earth was
gradually grasped, and the heavenly bodies were perceived all to revolve
round it: some moving regularly, as the stars, all fixed together into
one spherical shell or firmament; some moving irregularly and apparently
anomalously--these irregular bodies were therefore called planets [or
wanderers]. Seven of them were known, viz. Moon, Mercury, Venus, Sun,
Mars, Jupiter, Saturn, and there is little doubt that this number seven,
so suggested, is the origin of the seven days of the week.
The above order of the ancient planets is that of their supposed
distance from the earth. Not always, however, are they thus quoted
by the ancients: sometimes the sun is supposed nearer than Mercury
or Venus. It has always been known that the moon was the nearest of
the heavenly bodies; and some rough notion of its distance was
current. Mars, Jupiter, and Saturn were placed in that order
because that is the order of their apparent motions, and it was
natural to suppose that the slowest moving bodies were the furthest
off.
The order of the days of the week shows what astrologers considered
to be the order of the planets; on their system of each successive
hour of the day being ruled over by the successive planets taken in
order. The diagram (fig. 7) shows that if the Sun rule the first
hour of a certain day (thereby giving its name to the day) Venus
will rule the second hour, Mercury the third, and so on; the Sun
will thus be found to rule the eighth, fifteenth, and twenty-second
hour of that day, Venus the twenty-third, and Mercury the
twenty-fourth hour; so the Moon will rule the first hour of the
next day, which will therefore be Monday. On the same principle
(numbering round the hours successively, with the arrows) the first
hour of the next day will be found to be ruled by Mars, or by the
Saxon deity corresponding thereto; the first hour of the day after,
by Mercury (_Mercredi_), and so on (following the straight lines of
the pattern).
The order of the planets round the circle counter-clockwise, _i.e._
the direction of their proper motions, is that quoted above in the
text.
To explain the motion of the planets and reduce them to any sort of law
was a work of tremendous difficulty. The greatest astronomer of ancient
times was Hipparchus, and to him the system known as the Ptolemaic
system is no doubt largely due. But it was delivered to the world mainly
by Ptolemy, and goes by his name. This was a fine piece of work, and a
great advance on anything that had gone before; for although it is of
course saturated with error, still it is based on a large substratum of
truth. Its superiority to all the previously mentioned systems is
obvious. And it really did in its more developed form describe the
observed motions of the planets.
Each planet was, in the early stages of this system, as taught, say, by
Eudoxus, supposed to be set in a crystal sphere, which revolved so as to
carry the planet with it. The sphere had to be of crystal to account for
the visibility of other planets and the stars through it. Outside the
seven planetary spheres, arranged one inside the other, was a still
larger one in which were set the stars. This was believed to turn all
the others, and was called the _primum mobile_. The whole system was
supposed to produce, in its revolution, for the few privileged to hear
the music of the spheres, a sound as of some magnificent harmony.
[Illustration: FIG. 7.--Order of ancient planets corresponding to the
days of the week.]
The enthusiastic disciples of Pythagoras believed that their master was
privileged to hear this noble chant; and far be it from us to doubt
that the rapt and absorbing pleasure of contemplating the harmony of
nature, to a man so eminently great as Pythagoras, must be truly and
adequately represented by some such poetic conception.
[Illustration: FIG. 8.--Ptolemaic system.]
The precise kind of motion supposed to be communicated from the _primum
mobile_ to the other spheres so as to produce the observed motions of
the planets was modified and improved by various philosophers until it
developed into the epicyclic train of Hipparchus and of Ptolemy.
It is very instructive to observe a planet (say Mars or Jupiter) night
after night and plot down its place with reference to the fixed stars
on a celestial globe or star-map. Or, instead of direct observation by
alignment with known stars, it is easier to look out its right ascension
and declination in _Whitaker's Almanac_, and plot those down. If this be
done for a year or two, it will be found that the motion of the planet
is by no means regular, but that though on the whole it advances it
sometimes is stationary and sometimes goes back.[1]
[Illustration: FIG. 9.--Specimens of Apparent paths of Venus and of Mars
among the stars.]
[Illustration: FIG. 10.--Apparent epicyclic orbits of Jupiter and
Saturn; the Earth being supposed fixed at the centre, with the Sun
revolving in a small circle. A loop is made by each planet every year.]
These "stations" and "retrogressions" of the planets were well known to
the ancients. It was not to be supposed for a moment that the crystal
spheres were subject to any irregularity, neither was uniform circular
motion to be readily abandoned; so it was surmised that the main sphere
carried, not the planet itself, but the centre or axis of a subordinate
sphere, and that the planet was carried by this. The minor sphere could
be allowed to revolve at a different uniform pace from the main sphere,
and so a curve of some complexity could be obtained.
A curve described in space by a point of a circle or sphere, which
itself is carried along at the same time, is some kind of cycloid; if
the centre of the tracing circle travels along a straight line, we get
the ordinary cycloid, the curve traced in air by a nail on a
coach-wheel; but if the centre of the tracing circle be carried round
another circle the curve described is called an epicycloid. By such
curves the planetary stations and retrogressions could be explained. A
large sphere would have to revolve once for a "year" of the particular
planet, carrying with it a subsidiary sphere in which the planet was
fixed; this latter sphere revolving once for a "year" of the earth. The
actual looped curve thus described is depicted for Jupiter and Saturn in
the annexed diagram (fig. 10.)
It was long ago perceived that real material spheres were
unnecessary; such spheres indeed, though possibly transparent to
light, would be impermeable to comets: any other epicyclic gearing
would serve, and as a mere description of the motion it is simpler
to think of a system of jointed bars, one long arm carrying a
shorter arm, the two revolving at different rates, and the end of
the short one carrying the planet. This does all that is needful
for the first approximation to a planet's motion. In so far as the
motion cannot be thus truly stated, the short arm may be supposed
to carry another, and that another, and so on, so that the
resultant motion of the planet is compounded of a large number of
circular motions of different periods; by this device any required
amount of complexity could be attained. We shall return to this at
greater length in Lecture III.
The main features of the motion, as shown in the diagram, required
only two arms for their expression; one arm revolving with the
average motion of the planet, and the other revolving with the
apparent motion of the sun, and always pointing in the same
direction as the single arm supposed to carry the sun. This last
fact is of course because the motion to be represented does not
really belong to the planet at all, but to the earth, and so all
the main epicyclic motions for the superior planets were the same.
As for the inferior planets (Mercury and Venus) they only appear
to oscillate like the bob of a pendulum about the sun, and so it is
very obvious that they must be really revolving round it. An
ancient Egyptian system perceived this truth; but the Ptolemaic
system imagined them to revolve round the earth like the rest, with
an artificial system of epicycles to prevent their ever getting far
away from the neighbourhood of the sun.
It is easy now to see how the Copernican system explains the main
features of planetary motion, the stations and retrogressions,
quite naturally and without any complexity.
[Illustration: FIG. 11.--Egyptian system.]
Let the outer circle represent the orbit of Jupiter, and the inner
circle the orbit of the earth, which is moving faster than Jupiter
(since Jupiter takes 4332 days to make one revolution); then
remember that the apparent position of Jupiter is referred to the
infinitely distant fixed stars and refer to fig. 12.
Let E_1, E_2, &c., be successive positions of the earth; J_1,
J_2, &c., corresponding positions of Jupiter. Produce the lines
E_1 J_1, E_2 J_2, &c., to an enormously greater circle
outside, and it will be seen that the termination of these lines,
representing apparent positions of Jupiter among the stars,
advances while the earth goes from E_1 to E_3; is almost
stationary from somewhere about E_3 to E_4; and recedes from
E_4 to E_5; so that evidently the recessions of Jupiter are
only apparent, and are due to the orbital motion of the earth. The
apparent complications in the path of Jupiter, shown in Fig. 10,
are seen to be caused simply by the motion of the earth, and to be
thus completely and easily explained.
[Illustration: FIG. 12.--True orbits of Earth and Jupiter.]
The same thing for an inferior planet, say Mercury, is even still
more easily seen (_vide_ figure 13).
The motion of Mercury is direct from M'' to M''', retrograde from
M''' to M'', and stationary at M'' and M'''. It appears to
oscillate, taking 72.5 days for its direct swing, and 43.5 for its
return swing.
[Illustration: FIG. 13.--Orbit of Mercury and Earth.]
On this system no artificiality is required to prevent Mercury's
ever getting far from the sun: the radius of its orbit limits its
real and apparent excursions. Even if the earth were stationary,
the motions of Mercury and Venus would not be _essentially_
modified, but the stations and retrogressions of the superior
planets, Mars, Jupiter, &c., would wholly cease.
The complexity of the old mode of regarding apparent motion may be
illustrated by the case of a traveller in a railway train unaware
of his own motion. It is as though trees, hedges, distant objects,
were all flying past him and contorting themselves as you may see
the furrows of a ploughed field do when travelling, while you
yourself seem stationary amidst it all. How great a simplicity
would be introduced by the hypothesis that, after all, these things
might be stationary and one's self moving.
[Illustration: FIG. 14.--Copernican system as frequently represented.
But the cometary orbit is a much later addition, and no attempt is made
to show the relative distances of the planets.]
Now you are not to suppose that the system of Copernicus swept away the
entire doctrine of epicycles; that doctrine can hardly be said to be
swept away even now. As a description of a planet's motion it is not
incorrect, though it is geometrically cumbrous. If you describe the
motion of a railway train by stating that every point on the rim of each
wheel describes a cycloid with reference to the earth, and a circle with
reference to the train, and that the motion of the train is compounded
of these cycloidal and circular motions, you will not be saying what is
false, only what is cumbrous.
The Ptolemaic system demanded large epicycles, depending on the motion
of the earth, these are what Copernicus overthrew; but to express the
minuter details of the motion smaller epicycles remained, and grew more
and more complex as observations increased in accuracy, until a greater
man than either Copernicus or Ptolemy, viz. Kepler, replaced them all by
a simple ellipse.
One point I must not omit from this brief notice of the work of
Copernicus. Hipparchus had, by most sagacious interpretation of certain
observations of his, discovered a remarkable phenomenon called the
precession of the equinoxes. It was a discovery of the first magnitude,
and such as would raise to great fame the man who should have made it in
any period of the world's history, even the present. It is scarcely
expressible in popular language, and without some technical terms; but I
can try.
The plane of the earth's orbit produced into the sky gives the apparent
path of the sun throughout a year. This path is known as the ecliptic,
because eclipses only happen when the moon is in it. The sun keeps to it
accurately, but the planets wander somewhat above and below it (fig. 9),
and the moon wanders a good deal. It is manifest, however, in order that
there may be an eclipse of any kind, that a straight line must be able
to be drawn through earth and moon and sun (not necessarily through
their centres of course), and this is impossible unless some parts of
the three bodies are in one plane, viz. the ecliptic, or something very
near it. The ecliptic is a great circle of the sphere, and is usually
drawn on both celestial and terrestrial globes.
The earth's equator also produced into the sky, where it may still be
called the equator (sometimes it is awkwardly called "the equinoctial"),
gives another great circle inclined to the ecliptic and cutting it at
two opposite points, labelled respectively [Aries symbol] and [Libra
symbol], and together called "the equinoxes." The reason for the name is
that when the sun is in that part of the ecliptic it is temporarily also
on the equator, and hence is symmetrically situated with respect to the
earth's axis of rotation, and consequently day and night are equal all
over the earth.
Well, Hipparchus found, by plotting the position of the sun for a long
time,[2] that these points of intersection, or equinoxes, were not
stationary from century to century, but slowly moved among the stars,
moving as it were to meet the sun, so that he gets back to one of these
points again 20 minutes 23-1/4 seconds before it has really completed a
revolution, _i.e._ before the true year is fairly over. This slow
movement forward of the goal-post is called precession--the precession
of the equinoxes. (One result of it is to shorten our years by about 20
minutes each; for the shortened period has to be called a year, because
it is on the position of the sun with respect to the earth's axis that
our seasons depend.) Copernicus perceived that, assuming the motion of
the earth, a clearer account of this motion could be given. The ordinary
approximate statement concerning the earth's axis is that it remains
parallel to itself, _i.e._ has a fixed direction as the earth moves
round the sun. But if, instead of being thus fixed, it be supposed to
have a slow movement of revolution, so that it traces out a cone in the
course of about 26,000 years, then, since the equator of course goes
with it, the motion of its intersection with the fixed ecliptic is so
far accounted for. That is to say, the precession of the equinoxes is
seen to be dependent on, and caused by, a slow conical movement of the
earth's axis.
The prolongation of each end of the earth's axis into the sky, or the
celestial north and south poles, will thus slowly trace out an
approximate circle among the stars; and the course of the north pole
during historic time is exhibited in the annexed diagram.
It is now situated near one of the stars of the Lesser Bear, which we
therefore call the Pole star; but not always was it so, nor will it be
so in the future. The position of the north pole 4000 years ago is shown
in the figure; and a revolution will be completed in something like
26,000 years.[3]
[Illustration: FIG. 15.--Slow movement of the north pole in a circle
among the stars. (Copied from Sir R. Ball.)]
This perception of the conical motion of the earth's axis was a
beautiful generalization of Copernik's, whereby a multitude of facts
were grouped into a single phenomenon. Of course he did not explain the
motion of the axis itself. He stated the fact that it so moved, and I do
not suppose it ever struck him to seek for an explanation.
An explanation was given later, and that a most complete one; but the
idea even of seeking for it is a brilliant and striking one: the
achievement of the explanation by a single individual in the way it
actually was accomplished is one of the most astounding things in the
history of science; and were it not that the same individual
accomplished a dozen other things, equally and some still more
extraordinary, we should rank that man as one of the greatest
astronomers that ever lived.
As it is, he is Sir Isaac Newton.
We are to remember, then, as the life-work of Copernicus, that he placed
the sun in its true place as the centre of the solar system, instead of
the earth; that he greatly simplified the theory of planetary motion by
this step, and also by the simpler epicyclic chain which now sufficed,
and which he worked out mathematically; that he exhibited the precession
of the equinoxes (discovered by Hipparchus) as due to a conical motion
of the earth's axis; and that, by means of his simpler theory and more
exact planetary tables, he reduced to some sort of order the confused
chaos of the Ptolemaic system, whose accumulation of complexity and of
outstanding errors threatened to render astronomy impossible by the mere
burden of its detail.
There are many imperfections in his system, it is true; but his great
merit is that he dared to look at the facts of Nature with his own eyes,
unhampered by the prejudice of centuries. A system venerable with age,
and supported by great names, was universally believed, and had been
believed for centuries. To doubt this system, and to seek after another
and better one, at a time when all men's minds were governed by
tradition and authority, and when to doubt was sin--this required a
great mind and a high character. Such a mind and such a character had
this monk of Frauenburg. And it is interesting to notice that the
so-called religious scruples of smaller and less truly religious men did
not affect Copernicus; it was no dread of consequences to one form of
truth that led him to delay the publication of the other form of truth
specially revealed to him. In his dedication he says:--
"If there be some babblers who, though ignorant of all mathematics, take
upon them to judge of these things, and dare to blame and cavil at my
work, because of some passage of Scripture which they have wrested to
their own purpose, I regard them not, and will not scruple to hold their
judgment in contempt."
I will conclude with the words of one of his biographers (Mr. E.J.C.
Morton):--
"Copernicus cannot be said to have flooded with light the dark places of
nature--in the way that one stupendous mind subsequently did--but still,
as we look back through the long vista of the history of science, the
dim Titanic figure of the old monk seems to rear itself out of the dull
flats around it, pierces with its head the mists that overshadow them,
and catches the first gleam of the rising sun,
"'... like some iron peak, by the Creator
Fired with the red glow of the rushing morn.'"
DATES AND SUMMARY OF FACTS FOR LECTURE II
Copernicus lived from 1473 to 1543, and was contemporary with Paracelsus
and Raphael.
Tycho Brahe from 1546 to 1601.
Kepler from 1571 to 1630.
Galileo from 1564 to 1642.
Gilbert from 1540 to 1603.
Francis Bacon from 1561 to 1626.
Descartes from 1596 to 1650.
_A sketch of Tycho Brahe's life and work._ Tycho was a Danish noble,
born on his ancestral estate at Knudstorp, near Helsinborg, in 1546.
Adopted by his uncle, and sent to the University of Copenhagen to study
law. Attracted to astronomy by the occurrence of an eclipse on its
predicted day, August 21st, 1560. Began to construct astronomical
instruments, especially a quadrant and a sextant. Observed at Augsburg
and Wittenberg. Studied alchemy, but was recalled to astronomy by the
appearance of a new star. Overcame his aristocratic prejudices, and
delivered a course of lectures at Copenhagen, at the request of the
king. After this he married a peasant girl. Again travelled and observed
in Germany. In 1576 was sent for to Denmark by Frederick II., and
established in the island of Huen, with an endowment enabling him to
devote his life to astronomy. Built Uraniburg, furnished it with
splendid instruments, and became the founder of accurate instrumental
astronomy. His theories were poor, but his observations were admirable.
In 1592 Frederick died, and five years later, Tycho was impoverished and
practically banished. After wandering till 1599, he was invited to
Prague by the Emperor Rudolf, and there received John Kepler among other
pupils. But the sentence of exile was too severe, and he died in 1601,
aged 54 years.
A man of strong character, untiring energy, and devotion to accuracy,
his influence on astronomy has been immense.
LECTURE II
TYCHO BRAHE AND THE EARLIEST OBSERVATORY
We have seen how Copernicus placed the earth in its true position in the
solar system, making it merely one of a number of other worlds revolving
about a central luminary. And observe that there are two phenomena to be
thus accounted for and explained: first, the diurnal revolution of the
heavens; second, the annual motion of the sun among the stars.