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Title: History of Astronomy
Author: George Forbes
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*** START OF THE PROJECT GUTENBERG EBOOK HISTORY OF ASTRONOMY ***
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HISTORY OF ASTRONOMY
BY
GEORGE FORBES,
M.A., F.R.S., M. INST. C. E.,
(FORMERLY PROFESSOR OF NATURAL PHILOSOPHY, ANDERSON’S
COLLEGE, GLASGOW)
AUTHOR OF “THE TRANSIT OF VENUS,” RENDU’S “THEORY OF THE
GLACIERS OF SAVOY,” ETC., ETC.
CONTENTS
PREFACE
BOOK I. THE GEOMETRICAL PERIOD
1. PRIMITIVE ASTRONOMY AND ASTROLOGY
2. ANCIENT ASTRONOMY—CHINESE AND CHALDÆANS
3. ANCIENT GREEK ASTRONOMY
4. THE REIGN OF EPICYCLES—FROM PTOLEMY TO COPERNICUS
BOOK II. THE DYNAMICAL PERIOD
5. DISCOVERY OF THE TRUE SOLAR SYSTEM—TYCHO BRAHE—KEPLER
6. GALILEO AND THE TELESCOPE—NOTIONS OF GRAVITY BY HORROCKS,
ETC.
7. SIR ISAAC NEWTON—LAW OF UNIVERSAL GRAVITATION
8. NEWTON’S SUCCESSORS—HALLEY, EULER, LAGRANGE, LAPLACE, ETC.
9. DISCOVERY OF NEW PLANETS—HERSCHEL, PIAZZI, ADAMS, AND LE
VERRIER
BOOK III. OBSERVATION
10. INSTRUMENTS OF PRECISION—SIZE OF THE SOLAR SYSTEM
11. HISTORY OF THE TELESCOPE—SPECTROSCOPE
BOOK IV. THE PHYSICAL PERIOD
12. THE SUN
13. THE MOON AND PLANETS
14. COMETS AND METEORS
15. THE STARS AND NEBULÆ
INDEX
PREFAC
E
An attempt has been made in these pages to trace the evolution of intellectual thought in
the progress of astronomical discovery, and, by recognising the different points of view
of the different ages, to give due credit even to the ancients. No one can expect, in a
history of astronomy of limited size, to find a treatise on “practical” or on “theoretical
astronomy,” nor a complete “descriptive astronomy,” and still less a book on “speculative
astronomy.” Something of each of these is essential, however, for tracing the progress of
thought and knowledge which it is the object of this History to describe.
The progress of human knowledge is measured by the increased habit of looking at facts
from new points of view, as much as by the accumulation of facts. The mental capacity of
one age does not seem to differ from that of other ages; but it is the imagination of new
points of view that gives a wider scope to that capacity. And this is cumulative, and
therefore progressive. Aristotle viewed the solar system as a geometrical problem; Kepler
and Newton converted the point of view into a dynamical one. Aristotle’s mental capacity
to understand the meaning of facts or to criticise a train of reasoning may have been equal
to that of Kepler or Newton, but the point of view was different.
Then, again, new points of view are provided by the invention of new methods in that
system of logic which we call mathematics. All that mathematics can do is to assure us
that a statement A is equivalent to statements B, C, D, or is one of the facts expressed by
the statements B, C, D; so that we may know, if B, C, and D are true, then A is true. To
many people our inability to understand all that is contained in statements B, C, and D,
without the cumbrous process of a mathematical demonstration, proves the feebleness of
the human mind as a logical machine. For it required the new point of view imagined by
Newton’s analysis to enable people to see that, so far as planetary orbits are concerned,
Kepler’s three laws (B, C, D) were identical with Newton’s law of gravitation (A). No
one recognises more than the mathematical astronomer this feebleness of the human
intellect, and no one is more conscious of the limitations of the logical process called
mathematics, which even now has not solved directly the problem of only three bodies.
These reflections, arising from the writing of this History, go to explain the invariable
humility of the great mathematical astronomers. Newton’s comparison of himself to the
child on the seashore applies to them all. As each new discovery opens up, it may be,
boundless oceans for investigation, for wonder, and for admiration, the great
astronomers, refusing to accept mere hypotheses as true, have founded upon these
discoveries a science as exact in its observation of facts as in theories. So it is that these
men, who have built up the most sure and most solid of all the sciences, refuse to invite
others to join them in vain speculation. The writer has, therefore, in this short History,
tried to follow that great master, Airy, whose pupil he was, and the key to whose
character was exactness and accuracy; and he recognises that Science is impotent except
in her own limited sphere.
It has been necessary to curtail many parts of the History in the attempt—perhaps a
hopeless one—to lay before the reader in a limited space enough about each age to
illustrate its tone and spirit, the ideals of the workers, the gradual addition of new points
of view and of new means of investigation.
It would, indeed, be a pleasure to entertain the hope that these pages might, among new
recruits, arouse an interest in the greatest of all the sciences, or that those who have
handled the theoretical or practical side might be led by them to read in the original some
of the classics of astronomy. Many students have much compassion for the schoolboy of
to-day, who is not allowed the luxury of learning the art of reasoning from him who still
remains pre-eminently its greatest exponent, Euclid. These students pity also the man of
to-morrow, who is not to be allowed to read, in the original Latin of the brilliant Kepler,
how he was able—by observations taken from a moving platform, the earth, of the
directions of a moving object, Mars—to deduce the exact shape of the path of each of
these planets, and their actual positions on these paths at any time. Kepler’s masterpiece
is one of the most interesting books that was ever written, combining wit, imagination,
ingenuity, and certainty.
Lastly, it must be noted that, as a History of England cannot deal with the present
Parliament, so also the unfinished researches and untested hypotheses of many well-
known astronomers of to-day cannot be included among the records of the History of
Astronomy. The writer regrets the necessity that thus arises of leaving without mention
the names of many who are now making history in astronomical work.
G. F.
August 1st, 1909.
BOOK I. THE GEOMETRICAL PERIOD
1. PRIMITIVE ASTRONOMY AND ASTROLOGY.
The growth of intelligence in the human race has its counterpart in that of the individual,
especially in the earliest stages. Intellectual activity and the development of reasoning
powers are in both cases based upon the accumulation of experiences, and on the
comparison, classification, arrangement, and nomenclature of these experiences. During
the infancy of each the succession of events can be watched, but there can be no à priori
anticipations. Experience alone, in both cases, leads to the idea of cause and effect as a
principle that seems to dominate our present universe, as a rule for predicting the course
of events, and as a guide to the choice of a course of action. This idea of cause and effect
is the most potent factor in developing the history of the human race, as of the individual.
In no realm of nature is the principle of cause and effect more conspicuous than in
astronomy; and we fall into the habit of thinking of its laws as not only being
unchangeable in our universe, but necessary to the conception of any universe that might
have been substituted in its place. The first inhabitants of the world were compelled to
accommodate their acts to the daily and annual alternations of light and darkness and of
heat and cold, as much as to the irregular changes of weather, attacks of disease, and the
fortune of war. They soon came to regard the influence of the sun, in connection with
light and heat, as a cause. This led to a search for other signs in the heavens. If the
appearance of a comet was sometimes noted simultaneously with the death of a great
ruler, or an eclipse with a scourge of plague, these might well be looked upon as causes
in the same sense that the veering or backing of the wind is regarded as a cause of fine or
foul weather.
For these reasons we find that the earnest men of all ages have recorded the occurrence of
comets, eclipses, new stars, meteor showers, and remarkable conjunctions of the planets,
as well as plagues and famines, floods and droughts, wars and the deaths of great rulers.
Sometimes they thought they could trace connections which might lead them to say that a
comet presaged famine, or an eclipse war.
Even if these men were sometimes led to evolve laws of cause and effect which now
seem to us absurd, let us be tolerant, and gratefully acknowledge that these astrologers,
when they suggested such “working hypotheses,” were laying the foundations of
observation and deduction.
If the ancient Chaldæans gave to the planetary conjunctions an influence over terrestrial
events, let us remember that in our own time people have searched for connection
between terrestrial conditions and periods of unusual prevalence of sun spots; while De la
Rue, Loewy, and Balfour Stewart[1] thought they found a connection between sun-spot
displays and the planetary positions. Thus we find scientific men, even in our own time,
responsible for the belief that storms in the Indian Ocean, the fertility of German vines,
famines in India, and high or low Nile-floods in Egypt follow the planetary positions.
And, again, the desire to foretell the weather is so laudable that we cannot blame the
ancient Greeks for announcing the influence of the moon with as much confidence as it is
affirmed in Lord Wolseley’s Soldier’s Pocket Book.
Even if the scientific spirit of observation and deduction (astronomy) has sometimes led
to erroneous systems for predicting terrestrial events (astrology), we owe to the old
astronomer and astrologer alike the deepest gratitude for their diligence in recording
astronomical events. For, out of the scanty records which have survived the destructive
acts of fire and flood, of monarchs and mobs, we have found much that has helped to a
fuller knowledge of the heavenly motions than was possible without these records.
So Hipparchus, about 150 B.C., and Ptolemy a little later, were able to use the
observations of Chaldæan astrologers, as well as those of Alexandrian astronomers, and
to make some discoveries which have helped the progress of astronomy in all ages. So,
also, Mr. Cowell[2] has examined the marks made on the baked bricks used by the
Chaldæans for recording the eclipses of 1062 B.C. and 762 B.C.; and has thereby been
enabled, in the last few years, to correct the lunar tables of Hansen, and to find a more
accurate value for the secular acceleration of the moon’s longitude and the node of her
orbit than any that could be obtained from modern observations made with instruments of
the highest precision.
So again, Mr. Hind [3] was enabled to trace back the period during which Halley’s comet
has been a member of the solar system, and to identify it in the Chinese observations of
comets as far back as 12 B.C. Cowell and Cromellin extended the date to 240 B.C. In the
same way the comet 1861.i. has been traced back in the Chinese records to 617 A.D. [4]
The theoretical views founded on Newton’s great law of universal gravitation led to the
conclusion that the inclination of the earth’s equator to the plane of her orbit (the
obliquity of the ecliptic) has been diminishing slowly since prehistoric times; and this
fact has been confirmed by Egyptian and Chinese observations on the length of the
shadow of a vertical pillar, made thousands of years before the Christian era, in summer
and winter.
There are other reasons why we must be tolerant of the crude notions of the ancients. The
historian, wishing to give credit wherever it may be due, is met by two difficulties.
Firstly, only a few records of very ancient astronomy are extant, and the authenticity of
many of these is open to doubt. Secondly, it is very difficult to divest ourselves of present
knowledge, and to appreciate the originality of thought required to make the first
beginnings.
With regard to the first point, we are generally dependent upon histories written long
after the events. The astronomy of Egyptians, Babylonians, and Assyrians is known to us
mainly through the Greek historians, and for information about the Chinese we rely upon
the researches of travellers and missionaries in comparatively recent times. The testimony
of the Greek writers has fortunately been confirmed, and we now have in addition a mass
of facts translated from the original sculptures, papyri, and inscribed bricks, dating back
thousands of years.
In attempting to appraise the efforts of the beginners we must remember that it was
natural to look upon the earth (as all the first astronomers did) as a circular plane,
surrounded and bounded by the heaven, which was a solid vault, or hemisphere, with its
concavity turned downwards. The stars seemed to be fixed on this vault; the moon, and
later the planets, were seen to crawl over it. It was a great step to look on the vault as a
hollow sphere carrying the sun too. It must have been difficult to believe that at midday
the stars are shining as brightly in the blue sky as they do at night. It must have been
difficult to explain how the sun, having set in the west, could get back to rise in the east
without being seen if it was always the same sun. It was a great step to suppose the earth
to be spherical, and to ascribe the diurnal motions to its rotation. Probably the greatest
step ever made in astronomical theory was the placing of the sun, moon, and planets at
different distances from the earth instead of having them stuck on the vault of heaven. It
was a transition from “flatland” to a space of three dimensions.
Great progress was made when systematic observations began, such as following the
motion of the moon and planets among the stars, and the inferred motion of the sun
among the stars, by observing their heliacal risings—i.e., the times of year when a star
would first be seen to rise at sunrise, and when it could last be seen to rise at sunset. The
grouping of the stars into constellations and recording their places was a useful
observation. The theoretical prediction of eclipses of the sun and moon, and of the
motions of the planets among the stars, became later the highest goal in astronomy.
To not one of the above important steps in the progress of astronomy can we assign the
author with certainty. Probably many of them were independently taken by Chinese,
Indian, Persian, Tartar, Egyptian, Babylonian, Assyrian, Phoenician, and Greek
astronomers. And we have not a particle of information about the discoveries, which may
have been great, by other peoples—by the Druids, the Mexicans, and the Peruvians, for
example.
We do know this, that all nations required to have a calendar. The solar year, the lunar
month, and the day were the units, and it is owing to their incommensurability that we
find so many calendars proposed and in use at different times. The only object to be
attained by comparing the chronologies of ancient races is to fix the actual dates of
observations recorded, and this is not a part of a history of astronomy.
In conclusion, let us bear in mind the limited point of view of the ancients when we try to
estimate their merit. Let us remember that the first astronomy was of two dimensions; the
second astronomy was of three dimensions, but still purely geometrical. Since Kepler’s
day we have had a dynamical astronomy.
FOOTNOTES:
[1] Trans. R. S. E., xxiii. 1864, p. 499, On Sun Spots, etc., by B. Stewart. Also Trans. R.
S. 1860-70. Also Prof. Ernest Brown, in R. A. S. Monthly Notices, 1900.
[2] R. A. S. Monthly Notices, Sup.; 1905.
[3] R. A. S. Monthly Notices, vol. x., p. 65.
[4] R. S. E. Proc., vol. x., 1880.
2. ANCIENT ASTRONOMY—THE CHINESE AND
CHALDÆANS.
The last section must have made clear the difficulties the way of assigning to the ancient
nations their proper place in the development of primitive notions about astronomy. The
fact that some alleged observations date back to a period before the Chinese had invented
the art of writing leads immediately to the question how far tradition can be trusted.
Our first detailed knowledge was gathered in the far East by travellers, and by the Jesuit
priests, and was published in the eighteenth century. The Asiatic Society of Bengal
contributed translations of Brahmin literature. The two principal sources of knowledge
about Chinese astronomy were supplied, first by Father Souciet, who in 1729 published
Observations Astronomical, Geographical, Chronological, and Physical, drawn from
ancient Chinese books; and later by Father Moyriac-de-Mailla, who in 1777-1785
published Annals of the Chinese Empire, translated from Tong-Kien-Kang-Mou.
Bailly, in his Astronomie Ancienne (1781), drew, from these and other sources, the
conclusion that all we know of the astronomical learning of the Chinese, Indians,
Chaldæans, Assyrians, and Egyptians is but the remnant of a far more complete
astronomy of which no trace can be found.
Delambre, in his Histoire de l’Astronomie Ancienne (1817), ridicules the opinion of
Bailly, and considers that the progress made by all of these nations is insignificant.
It will be well now to give an idea of some of the astronomy of the ancients not yet
entirely discredited. China and Babylon may be taken as typical examples.
China.—It would appear that Fohi, the first emperor, reigned about 2952 B.C., and
shortly afterwards Yu-Chi made a sphere to represent the motions of the celestial bodies.
It is also mentioned, in the book called Chu-King, supposed to have been written in 2205
B.C., that a similar sphere was made in the time of Yao (2357 B.C.).[1] It is said that the
Emperor Chueni (2513 B.C.) saw five planets in conjunction the same day that the sun
and moon were in conjunction. This is discussed by Father Martin (MSS. of De Lisle);
also by M. Desvignolles (Mem. Acad. Berlin, vol. iii., p. 193), and by M. Kirsch (ditto,
vol. v., p. 19), who both found that Mars, Jupiter, Saturn, and Mercury were all between
the eleventh and eighteenth degrees of Pisces, all visible together in the evening on
February 28th 2446 B.C., while on the same day the sun and moon were in conjunction at
9 a.m., and that on March 1st the moon was in conjunction with the other four planets.
But this needs confirmation.
Yao, referred to above, gave instructions to his astronomers to determine the positions of
the solstices and equinoxes, and they reported the names of the stars in the places
occupied by the sun at these seasons, and in 2285 B.C. he gave them further orders. If
this account be true, it shows a knowledge that the vault of heaven is a complete sphere,
and that stars are shining at mid-day, although eclipsed by the sun’s brightness.
It is also asserted, in the book called Chu-King, that in the time of Yao the year was
known to have 365¼ days, and that he adopted 365 days and added an intercalary day
every four years (as in the Julian Calendar). This may be true or not, but the ancient
Chinese certainly seem to have divided the circle into 365 degrees. To learn the length of
the year needed only patient observation—a characteristic of the Chinese; but many
younger nations got into a terrible mess with their calendar from ignorance of the year’s
length.
It is stated that in 2159 B.C. the royal astronomers Hi and Ho failed to predict an eclipse.
It probably created great terror, for they were executed in punishment for their neglect. If
this account be true, it means that in the twenty-second century B.C. some rule for
calculating eclipses was in use. Here, again, patient observation would easily lead to the
detection of the eighteen-year cycle known to the Chaldeans as the Saros. It consists of
235 lunations, and in that time the pole of the moon’s orbit revolves just once round the
pole of the ecliptic, and for this reason the eclipses in one cycle are repeated with very
slight modification in the next cycle, and so on for many centuries.
It may be that the neglect of their duties by Hi and Ho, and their punishment, influenced
Chinese astronomy; or that the succeeding records have not been available to later
scholars; but the fact remains that—although at long intervals observations were made of
eclipses, comets, and falling stars, and of the position of the solstices, and of the obliquity
of the ecliptic—records become rare, until 776 B.C., when eclipses began to be recorded
once more with some approach to continuity. Shortly afterwards notices of comets were
added. Biot gave a list of these, and Mr. John Williams, in 1871, published Observations
of Comets from 611 B.C. to 1640 A.D., Extracted from the Chinese Annals.
With regard to those centuries concerning which we have no astronomical Chinese
records, it is fair to state that it is recorded that some centuries before the Christian era, in
the reign of Tsin-Chi-Hoang, all the classical and scientific books that could be found
were ordered to be destroyed. If true, our loss therefrom is as great as from the burning of
the Alexandrian library by the Caliph Omar. He burnt all the books because he held that
they must be either consistent or inconsistent with the Koran, and in the one case they
were superfluous, in the other case objectionable.
Chaldæans.—Until the last half century historians were accustomed to look back upon
the Greeks, who led the world from the fifth to the third century B.C., as the pioneers of
art, literature, and science. But the excavations and researches of later years make us
more ready to grant that in science as in art the Greeks only developed what they derived
from the Egyptians, Babylonians, and Assyrians. The Greek historians said as much, in
fact; and modern commentators used to attribute the assertion to undue modesty. Since,
however, the records of the libraries have been unearthed it has been recognised that the
Babylonians were in no way inferior in the matter of original scientific investigation to
other races of the same era.
The Chaldæans, being the most ancient Babylonians, held the same station and dignity in
the State as did the priests in Egypt, and spent all their time in the study of philosophy
and astronomy, and the arts of divination and astrology. They held that the world of
which we have a conception is an eternal world without any beginning or ending, in
which all things are ordered by rules supported by a divine providence, and that the
heavenly bodies do not move by chance, nor by their own will, but by the determinate
will and appointment of the gods. They recorded these movements, but mainly in the
hope of tracing the will of the gods in mundane affairs. Ptolemy (about 130 A.D.) made
use of Babylonian eclipses in the eighth century B.C. for improving his solar and lunar
tables.
Fragments of a library at Agade have been preserved at Nineveh, from which we learn
that the star-charts were even then divided into constellations, which were known by the
names which they bear to this day, and that the signs of the zodiac were used for
determining the courses of the sun, moon, and of the five planets Mercury, Venus, Mars,
Jupiter, and Saturn.
We have records of observations carried on under Asshurbanapal, who sent astronomers
to different parts to study celestial phenomena. Here is one:—
To the Director of Observations,—My Lord, his humble servant Nabushum-iddin, Great
Astronomer of Nineveh, writes thus: “May Nabu and Marduk be propitious to the
Director of these Observations, my Lord. The fifteenth day we observed the Node of the
moon, and the moon was eclipsed.”
The Phoenicians are supposed to have used the stars for navigation, but there are no
records. The Egyptian priests tried to keep such astronomical knowledge as they
possessed to themselves. It is probable that they had arbitrary rules for predicting
eclipses. All that was known to the Greeks about Egyptian science is to be found in the
writings of Diodorus Siculus. But confirmatory and more authentic facts have been
derived from late explorations. Thus we learn from E. B. Knobel[2] about the Jewish
calendar dates, on records of land sales in Aramaic papyri at Assuan, translated by
Professor A. H. Sayce and A. E. Cowley, (1) that the lunar cycle of nineteen years was
used by the Jews in the fifth century B.C. [the present reformed Jewish calendar dating
from the fourth century A.D.], a date a “little more than a century after the grandfathers
and great-grandfathers of those whose business is recorded had fled into Egypt with
Jeremiah” (Sayce); and (2) that the order of intercalation at that time was not dissimilar to
that in use at the present day.
Then again, Knobel reminds us of “the most interesting discovery a few years ago by
Father Strassmeier of a Babylonian tablet recording a partial lunar eclipse at Babylon in
the seventh year of Cambyses, on the fourteenth day of the Jewish month Tammuz.”
Ptolemy, in the Almagest (Suntaxis), says it occurred in the seventh year of Cambyses,
on the night of the seventeenth and eighteenth of the Egyptian month Phamenoth. Pingré
and Oppolzer fix the date July 16th, 533 B.C. Thus are the relations of the chronologies
of Jews and Egyptians established by these explorations.
FOOTNOTES:
[1] These ancient dates are uncertain.
[2] R. A. S. Monthly Notices, vol. lxviii., No. 5, March, 1908.
3. ANCIENT GREEK ASTRONOMY.
We have our information about the earliest Greek astronomy from Herodotus (born 480
B.C.). He put the traditions into writing. Thales (639-546 B.C.) is said to have predicted
an eclipse, which caused much alarm, and ended the battle between the Medes and
Lydians. Airy fixed the date May 28th, 585 B.C. But other modern astronomers give
different dates. Thales went to Egypt to study science, and learnt from its priests the
length of the year (which was kept a profound secret!), and the signs of the zodiac, and
the positions of the solstices. He held that the sun, moon, and stars are not mere spots on
the heavenly vault, but solids; that the moon derives her light from the sun, and that this
fact explains her phases; that an eclipse of the moon happens when the earth cuts off the
sun’s light from her. He supposed the earth to be flat, and to float upon water. He
determined the ratio of the sun’s diameter to its orbit, and apparently made out the
diameter correctly as half a degree. He left nothing in writing.
His successors, Anaximander (610-547 B.C.) and Anaximenes (550-475 B.C.), held
absurd notions about the sun, moon, and stars, while Heraclitus (540-500 B.C.) supposed
that the stars were lighted each night like lamps, and the sun each morning. Parmenides
supposed the earth to be a sphere.
Pythagoras (569-470 B.C.) visited Egypt to study science. He deduced his system, in
which the earth revolves in an orbit, from fantastic first principles, of which the following
are examples: “The circular motion is the most perfect motion,” “Fire is more worthy
than earth,” “Ten is the perfect number.” He wrote nothing, but is supposed to have said
that the earth, moon, five planets, and fixed stars all revolve round the sun, which itself
revolves round an imaginary central fire called the Antichthon. Copernicus in the
sixteenth century claimed Pythagoras as the founder of the system which he, Copernicus,
revived.
Anaxagoras (born 499 B.C.) studied astronomy in Egypt. He explained the return of the
sun to the east each morning by its going under the flat earth in the night. He held that in
a solar eclipse the moon hides the sun, and in a lunar eclipse the moon enters the earth’s
shadow—both excellent opinions. But he entertained absurd ideas of the vortical motion
of the heavens whisking stones into the sky, there to be ignited by the fiery firmament to
form stars. He was prosecuted for this unsettling opinion, and for maintaining that the
moon is an inhabited earth. He was defended by Pericles (432 B.C.).
Solon dabbled, like many others, in reforms of the calendar. The common year of the
Greeks originally had 360 days—twelve months of thirty days. Solon’s year was 354
days. It is obvious that these erroneous years would, before long, remove the summer to
January and the winter to July. To prevent this it was customary at regular intervals to
intercalate days or months. Meton (432 B.C.) introduced a reform based on the nineteen-
year cycle. This is not the same as the Egyptian and Chaldean eclipse cycle called Saros
of 223 lunations, or a little over eighteen years. The Metonic cycle is 235 lunations or
nineteen years, after which period the sun and moon occupy the same position relative to
the stars. It is still used for fixing the date of Easter, the number of the year in Melon’s
cycle being the golden number of our prayer-books. Melon’s system divided the 235
lunations into months of thirty days and omitted every sixty-third day. Of the nineteen
years, twelve had twelve months and seven had thirteen months.
Callippus (330 B.C.) used a cycle four times as long, 940 lunations, but one day short of
Melon’s seventy-six years. This was more correct.
Eudoxus (406-350 B.C.) is said to have travelled with Plato in Egypt. He made
astronomical observations in Asia Minor, Sicily, and Italy, and described the starry
heavens divided into constellations. His name is connected with a planetary theory which
as generally stated sounds most fanciful. He imagined the fixed stars to be on a vault of
heaven; and the sun, moon, and planets to be upon similar vaults or spheres, twenty-six
revolving spheres in all, the motion of each planet being resolved into its components,
and a separate sphere being assigned for each component motion. Callippus (330 B.C.)
increased the number to thirty-three. It is now generally accepted that the real existence
of these spheres was not suggested, but the idea was only a mathematical conception to
facilitate the construction of tables for predicting the places of the heavenly bodies.
Aristotle (384-322 B.C.) summed up the state of astronomical knowledge in his time, and
held the earth to be fixed in the centre of the world.
Nicetas, Heraclides, and Ecphantes supposed the earth to revolve on its axis, but to have
no orbital motion.
The short epitome so far given illustrates the extraordinary deductive methods adopted by
the ancient Greeks. But they went much farther in the same direction. They seem to have
been in great difficulty to explain how the earth is supported, just as were those who
invented the myth of Atlas, or the Indians with the tortoise. Thales thought that the flat
earth floated on water. Anaxagoras thought that, being flat, it would be buoyed up and
supported on the air like a kite. Democritus thought it remained fixed, like the donkey
between two bundles of hay, because it was equidistant from all parts of the containing
sphere, and there was no reason why it should incline one way rather than another.
Empedocles attributed its state of rest to centrifugal force by the rapid circular movement
of the heavens, as water is stationary in a pail when whirled round by a string.
Democritus further supposed that the inclination of the flat earth to the ecliptic was due to
the greater weight of the southern parts owing to the exuberant vegetation.
For further references to similar efforts of imagination the reader is referred to Sir George
Cornwall Lewis’s Historical Survey of the Astronomy of the Ancients; London, 1862. His
list of authorities is very complete, but some of his conclusions are doubtful. At p. 113 of
that work he records the real opinions of Socrates as set forth by Xenophon; and the
reader will, perhaps, sympathise with Socrates in his views on contemporary astronomy:
—
With regard to astronomy he [Socrates] considered a knowledge of it desirable to the
extent of determining the day of the year or month, and the hour of the night, ... but as to
learning the courses of the stars, to be occupied with the planets, and to inquire about
their distances from the earth, and their orbits, and the causes of their motions, he
strongly objected to such a waste of valuable time. He dwelt on the contradictions and
conflicting opinions of the physical philosophers, ... and, in fine, he held that the
speculators on the universe and on the laws of the heavenly bodies were no better than
madmen (Xen. Mem, i. 1, 11-15).
Plato (born 429 B.C.), the pupil of Socrates, the fellow-student of Euclid, and a follower
of Pythagoras, studied science in his travels in Egypt and elsewhere. He was held in so
great reverence by all learned men that a problem which he set to the astronomers was the
keynote to all astronomical investigation from this date till the time of Kepler in the
sixteenth century. He proposed to astronomers the problem of representing the courses of
the planets by circular and uniform motions.
Systematic observation among the Greeks began with the rise of the Alexandrian school.
Aristillus and Timocharis set up instruments and fixed the positions of the zodiacal stars,
near to which all the planets in their orbits pass, thus facilitating the determination of
planetary motions. Aristarchus (320-250 B.C.) showed that the sun must be at least
nineteen times as far off as the moon, which is far short of the mark. He also found the
sun’s diameter, correctly, to be half a degree. Eratosthenes (276-196 B.C.) measured the
inclination to the equator of the sun’s apparent path in the heavens—i.e., he measured the
obliquity of the ecliptic, making it 23° 51’, confirming our knowledge of its continuous
diminution during historical times. He measured an arc of meridian, from Alexandria to
Syene (Assuan), and found the difference of latitude by the length of a shadow at noon,
summer solstice. He deduced the diameter of the earth, 250,000 stadia. Unfortunately, we
do not know the length of the stadium he used.
Hipparchus (190-120 B.C.) may be regarded as the founder of observational astronomy.
He measured the obliquity of the ecliptic, and agreed with Eratosthenes. He altered the
length of the tropical year from 365 days, 6 hours to 365 days, 5 hours, 53 minutes—still
four minutes too much. He measured the equation of time and the irregular motion of the
sun; and allowed for this in his calculations by supposing that the centre, about which the
sun moves uniformly, is situated a little distance from the fixed earth. He called this point
the excentric. The line from the earth to the “excentric” was called the line of apses. A
circle having this centre was called the equant, and he supposed that a radius drawn to the
sun from the excentric passes over equal arcs on the equant in equal times. He then
computed tables for predicting the place of the sun.
He proceeded in the same way to compute Lunar tables. Making use of Chaldæan
eclipses, he was able to get an accurate value of the moon’s mean motion. [Halley, in
1693, compared this value with his own measurements, and so discovered the
acceleration of the moon’s mean motion. This was conclusively established, but could not
be explained by the Newtonian theory for quite a long time.] He determined the plane of
the moon’s orbit and its inclination to the ecliptic. The motion of this plane round the
pole of the ecliptic once in eighteen years complicated the problem. He located the
moon’s excentric as he had done the sun’s. He also discovered some of the minor
irregularities of the moon’s motion, due, as Newton’s theory proves, to the disturbing
action of the sun’s attraction.
In the year 134 B.C. Hipparchus observed a new star. This upset every notion about the
permanence of the fixed stars. He then set to work to catalogue all the principal stars so
as to know if any others appeared or disappeared. Here his experiences resembled those
of several later astronomers, who, when in search of some special object, have been
rewarded by a discovery in a totally different direction. On comparing his star positions
with those of Timocharis and Aristillus he found no stars that had appeared or
disappeared in the interval of 150 years; but he found that all the stars seemed to have
changed their places with reference to that point in the heavens where the ecliptic is 90°
from the poles of the earth—i.e., the equinox. He found that this could be explained by a
motion of the equinox in the direction of the apparent diurnal motion of the stars. This
discovery of precession of the equinoxes, which takes place at the rate of 52".1 every
year, was necessary for the progress of accurate astronomical observations. It is due to a
steady revolution of the earth’s pole round the pole of the ecliptic once in 26,000 years in
the opposite direction to the planetary revolutions.
Hipparchus was also the inventor of trigonometry, both plane and spherical. He explained
the method of using eclipses for determining the longitude.
In connection with Hipparchus’ great discovery it may be mentioned that modern
astronomers have often attempted to fix dates in history by the effects of precession of
the equinoxes. (1) At about the date when the Great Pyramid may have been built γ
Draconis was near to the pole, and must have been used as the pole-star. In the north face
of the Great Pyramid is the entrance to an inclined passage, and six of the nine pyramids
at Gizeh possess the same feature; all the passages being inclined at an angle between 26°
and 27° to the horizon and in the plane of the meridian. It also appears that 4,000 years
ago—i.e., about 2100 B.C.—an observer at the lower end of the passage would be able to
see γ Draconis, the then pole-star, at its lower culmination.[1] It has been suggested that
the passage was made for this purpose. On other grounds the date assigned to the Great
Pyramid is 2123 B.C.
(2) The Chaldæans gave names to constellations now invisible from Babylon which
would have been visible in 2000 B.C., at which date it is claimed that these people were
studying astronomy.
(3) In the Odyssey, Calypso directs Odysseus, in accordance with Phoenician rules for
navigating the Mediterranean, to keep the Great Bear “ever on the left as he traversed the
deep” when sailing from the pillars of Hercules (Gibraltar) to Corfu. Yet such a course
taken now would land the traveller in Africa. Odysseus is said in his voyage in springtime
to have seen the Pleiades and Arcturus setting late, which seemed to early commentators
a proof of Homer’s inaccuracy. Likewise Homer, both in the Odyssey [2] (v. 272-5) and
in the Iliad (xviii. 489), asserts that the Great Bear never set in those latitudes. Now it has
been found that the precession of the equinoxes explains all these puzzles; shows that in
springtime on the Mediterranean the Bear was just above the horizon, near the sea but not
touching it, between 750 B.C. and 1000 B.C.; and fixes the date of the poems, thus
confirming other evidence, and establishing Homer’s character for accuracy. [3]
(4) The orientation of Egyptian temples and Druidical stones is such that possibly they
were so placed as to assist in the observation of the heliacal risings [4] of certain stars. If
the star were known, this would give an approximate date. Up to the present the results of
these investigations are far from being conclusive.
Ptolemy (130 A.D.) wrote the Suntaxis, or Almagest, which includes a cyclopedia of
astronomy, containing a summary of knowledge at that date. We have no evidence
beyond his own statement that he was a practical observer. He theorised on the planetary
motions, and held that the earth is fixed in the centre of the universe. He adopted the
excentric and equant of Hipparchus to explain the unequal motions of the sun and moon.
He adopted the epicycles and deferents which had been used by Apollonius and others to
explain the retrograde motions of the planets. We, who know that the earth revolves
round the sun once in a year, can understand that the apparent motion of a planet is only
its motion relative to the earth. If, then, we suppose the earth fixed and the sun to revolve
round it once a year, and the planets each in its own period, it is only necessary to impose
upon each of these an additional annual motion to enable us to represent truly the
apparent motions. This way of looking at the apparent motions shows why each planet,
when nearest to the earth, seems to move for a time in a retrograde direction. The
attempts of Ptolemy and others of his time to explain the retrograde motion in this way
were only approximate. Let us suppose each planet to have a bar with one end centred at
the earth. If at the other end of the bar one end of a shorter bar is pivotted, having the
planet at its other end, then the planet is given an annual motion in the secondary circle
(the epicycle), whose centre revolves round the earth on the primary circle (the deferent),
at a uniform rate round the excentric. Ptolemy supposed the centres of the epicycles of
Mercury and Venus to be on a bar passing through the sun, and to be between the earth
and the sun. The centres of the epicycles of Mars, Jupiter, and Saturn were supposed to
be further away than the sun. Mercury and Venus were supposed to revolve in their
epicycles in their own periodic times and in the deferent round the earth in a year. The
major planets were supposed to revolve in the deferent round the earth in their own
periodic times, and in their epicycles once in a year.
It did not occur to Ptolemy to place the centres of the epicycles of Mercury and Venus at
the sun, and to extend the same system to the major planets. Something of this sort had
been proposed by the Egyptians (we are told by Cicero and others), and was accepted by
Tycho Brahe; and was as true a representation of the relative motions in the solar system
as when we suppose the sun to be fixed and the earth to revolve.
The cumbrous system advocated by Ptolemy answered its purpose, enabling him to
predict astronomical events approximately. He improved the lunar theory considerably,
and discovered minor inequalities which could be allowed for by the addition of new
epicycles. We may look upon these epicycles of Apollonius, and the excentric of
Hipparchus, as the responses of these astronomers to the demand of Plato for uniform
circular motions. Their use became more and more confirmed, until the seventeenth
century, when the accurate observations of Tycho Brahe enabled Kepler to abolish these
purely geometrical makeshifts, and to substitute a system in which the sun became
physically its controller.
FOOTNOTES:
[1] Phil. Mag., vol. xxiv., pp. 481-4.
[2]
Plaeiadas t’ esoronte kai opse duonta bootaen
‘Arkton th’ aen kai amaxan epiklaesin kaleousin,
‘Ae t’ autou strephetai kai t’ Oriona dokeuei,
Oin d’ammoros esti loetron Okeanoio.
“The Pleiades and Boötes that setteth late, and the Bear, which they likewise call the
Wain, which turneth ever in one place, and keepeth watch upon Orion, and alone hath no
part in the baths of the ocean.”
[3] See Pearson in the Camb. Phil. Soc. Proc., vol. iv., pt. ii., p. 93, on whose authority
the above statements are made.
[4] See p. 6 for definition.
4. THE REIGN OF EPICYCLES—FROM PTOLEMY
TO COPERNICUS.
After Ptolemy had published his book there seemed to be nothing more to do for the solar
system except to go on observing and finding more and more accurate values for the
constants involved--viz., the periods of revolution, the diameter of the deferent,[1] and its
ratio to that of the epicycle,[2] the distance of the excentric[3] from the centre of the
deferent, and the position of the line of apses,[4] besides the inclination and position of
the plane of the planet’s orbit. The only object ever aimed at in those days was to prepare
tables for predicting the places of the planets. It was not a mechanical problem; there was
no notion of a governing law of forces.
From this time onwards all interest in astronomy seemed, in Europe at least, to sink to a
low ebb. When the Caliph Omar, in the middle of the seventh century, burnt the library of
Alexandria, which had been the centre of intellectual progress, that centre migrated to
Baghdad, and the Arabs became the leaders of science and philosophy. In astronomy they
made careful observations. In the middle of the ninth century Albategnius, a Syrian
prince, improved the value of excentricity of the sun’s orbit, observed the motion of the
moon’s apse, and thought he detected a smaller progression of the sun’s apse. His tables
were much more accurate than Ptolemy’s. Abul Wefa, in the tenth century, seems to have
discovered the moon’s “variation.” Meanwhile the Moors were leaders of science in the
west, and Arzachel of Toledo improved the solar tables very much. Ulugh Begh,
grandson of the great Tamerlane the Tartar, built a fine observatory at Samarcand in the
fifteenth century, and made a great catalogue of stars, the first since the time of
Hipparchus.
At the close of the fifteenth century King Alphonso of Spain employed computers to
produce the Alphonsine Tables (1488 A.D.), Purbach translated Ptolemy’s book, and
observations were carried out in Germany by Müller, known as Regiomontanus, and
Waltherus.
Nicolai Copernicus, a Sclav, was born in 1473 at Thorn, in Polish Prussia. He studied at
Cracow and in Italy. He was a priest, and settled at Frauenberg. He did not undertake
continuous observations, but devoted himself to simplifying the planetary systems and
devising means for more accurately predicting the positions of the sun, moon, and
planets. He had no idea of framing a solar system on a dynamical basis. His great object
was to increase the accuracy of the calculations and the tables. The results of his
cogitations were printed just before his death in an interesting book, De Revolutionibus
Orbium Celestium. It is only by careful reading of this book that the true position of
Copernicus can be realised. He noticed that Nicetas and others had ascribed the apparent
diurnal rotation of the heavens to a real daily rotation of the earth about its axis, in the
opposite direction to the apparent motion of the stars. Also in the writings of Martianus
Capella he learnt that the Egyptians had supposed Mercury and Venus to revolve round
the sun, and to be carried with him in his annual motion round the earth. He noticed that
the same supposition, if extended to Mars, Jupiter, and Saturn, would explain easily why
they, and especially Mars, seem so much brighter in opposition. For Mars would then be
a great deal nearer to the earth than at other times. It would also explain the retrograde
motion of planets when in opposition.
We must here notice that at this stage Copernicus was actually confronted with the
system accepted later by Tycho Brahe, with the earth fixed. But he now recalled and
accepted the views of Pythagoras and others, according to which the sun is fixed and the
earth revolves; and it must be noted that, geometrically, there is no difference of any sort
between the Egyptian or Tychonic system and that of Pythagoras as revived by
Copernicus, except that on the latter theory the stars ought to seem to move when the
earth changes its position—a test which failed completely with the rough means of
observation then available. The radical defect of all solar systems previous to the time of
Kepler (1609 A.D.) was the slavish yielding to Plato’s dictum demanding uniform
circular motion for the planets, and the consequent evolution of the epicycle, which was
fatal to any conception of a dynamical theory.
Copernicus could not sever himself from this obnoxious tradition.[5] It is true that neither
the Pythagorean nor the Egypto-Tychonic system required epicycles for explaining
retrograde motion, as the Ptolemaic theory did. Furthermore, either system could use the
excentric of Hipparchus to explain the irregular motion known as the equation of the
centre. But Copernicus remarked that he could also use an epicycle for this purpose, or
that he could use both an excentric and an epicycle for each planet, and so bring theory
still closer into accord with observation. And this he proceeded to do.[6] Moreover,
observers had found irregularities in the moon’s motion, due, as we now know, to the
disturbing attraction of the sun. To correct for these irregularities Copernicus introduced
epicycle on epicycle in the lunar orbit.
This is in its main features the system propounded by Copernicus. But attention must, to
state the case fully, be drawn to two points to be found in his first and sixth books
respectively. The first point relates to the seasons, and it shows a strange ignorance of the
laws of rotating bodies. To use the words of Delambre,[7] in drawing attention to the
strange conception,
he imagined that the earth, revolving round the sun, ought always to show to it the same
face; the contrary phenomena surprised him: to explain them he invented a third motion,
and added it to the two real motions (rotation and orbital revolution). By this third motion
the earth, he held, made a revolution on itself and on the poles of the ecliptic once a
year ... Copernicus did not know that motion in a straight line is the natural motion, and
that motion in a curve is the resultant of several movements. He believed, with Aristotle,
that circular motion was the natural one.
Copernicus made this rotation of the earth’s axis about the pole of the ecliptic retrograde
(i.e., opposite to the orbital revolution), and by making it perform more than one
complete revolution in a year, the added part being 1/26000 of the whole, he was able to
include the precession of the equinoxes in his explanation of the seasons. His explanation
of the seasons is given on leaf 10 of his book (the pages of this book are not all
numbered, only alternate pages, or leaves).
In his sixth book he discusses the inclination of the planetary orbits to the ecliptic. In
regard to this the theory of Copernicus is unique; and it will be best to explain this in the
words of Grant in his great work.[8] He says:—
Copernicus, as we have already remarked, did not attack the principle of the epicyclical
theory: he merely sought to make it more simple by placing the centre of the earth’s orbit
in the centre of the universe. This was the point to which the motions of the planets were
referred, for the planes of their orbits were made to pass through it, and their points of
least and greatest velocities were also determined with reference to it. By this
arrangement the sun was situate mathematically near the centre of the planetary system,
but he did not appear to have any physical connexion with the planets as the centre of
their motions.
According to Copernicus’ sixth book, the planes of the planetary orbits do not pass
through the sun, and the lines of apses do not pass through to the sun.
Such was the theory advanced by Copernicus: The earth moves in an epicycle, on a
deferent whose centre is a little distance from the sun. The planets move in a similar way
on epicycles, but their deferents have no geometrical or physical relation to the sun. The
moon moves on an epicycle centred on a second epicycle, itself centred on a deferent,
excentric to the earth. The earth’s axis rotates about the pole of the ecliptic, making one
revolution and a twenty-six thousandth part of a revolution in the sidereal year, in the
opposite direction to its orbital motion.
In view of this fanciful structure it must be noted, in fairness to Copernicus, that he
repeatedly states that the reader is not obliged to accept his system as showing the real
motions; that it does not matter whether they be true, even approximately, or not, so long
as they enable us to compute tables from which the places of the planets among the stars
can be predicted.[9] He says that whoever is not satisfied with this explanation must be
contented by being told that “mathematics are for mathematicians” (Mathematicis
mathematica scribuntur).
At the same time he expresses his conviction over and over again that the earth is in
motion. It is with him a pious belief, just as it was with Pythagoras and his school and
with Aristarchus. “But” (as Dreyer says in his most interesting book, Tycho Brahe)
“proofs of the physical truth of his system Copernicus had given none, and could give
none,” any more than Pythagoras or Aristarchus.
There was nothing so startlingly simple in his system as to lead the cautious astronomer
to accept it, as there was in the later Keplerian system; and the absence of parallax in the
stars seemed to condemn his system, which had no physical basis to recommend it, and
no simplification at all over the Egypto-Tychonic system, to which Copernicus himself
drew attention. It has been necessary to devote perhaps undue space to the interesting
work of Copernicus, because by a curious chance his name has become so widely known.
He has been spoken of very generally as the founder of the solar system that is now
accepted. This seems unfair, and on reading over what has been written about him at
different times it will be noticed that the astronomers—those who have evidently read his
great book—are very cautious in the words with which they eulogise him, and refrain
from attributing to him the foundation of our solar system, which is entirely due to
Kepler. It is only the more popular writers who give the idea that a revolution had been
effected when Pythagoras’ system was revived, and when Copernicus supported his view
that the earth moves and is not fixed.
It may be easy to explain the association of the name of Copernicus with the Keplerian
system. But the time has long passed when the historian can support in any way this
popular error, which was started not by astronomers acquainted with Kepler’s work, but
by those who desired to put the Church in the wrong by extolling Copernicus.
Copernicus dreaded much the abuse he expected to receive from philosophers for
opposing the authority of Aristotle, who had declared that the earth was fixed. So he
sought and obtained the support of the Church, dedicating his great work to Pope Paul III.
in a lengthy explanatory epistle. The Bishop of Cracow set up a memorial tablet in his
honour.
Copernicus was the most refined exponent, and almost the last representative, of the
Epicyclical School. As has been already stated, his successor, Tycho Brahe, supported the
same use of epicycles and excentrics as Copernicus, though he held the earth to be fixed.
But Tycho Brahe was eminently a practical observer, and took little part in theory; and
his observations formed so essential a portion of the system of Kepler that it is only fair
to include his name among these who laid the foundations of the solar system which we
accept to-day.
In now taking leave of the system of epicycles let it be remarked that it has been held up
to ridicule more than it deserves. On reading Airy’s account of epicycles, in the
beautifully clear language of his Six Lectures on Astronomy, the impression is made that
the jointed bars there spoken of for describing the circles were supposed to be real. This
is no more the case than that the spheres of Eudoxus and Callippus were supposed to be
real. Both were introduced only to illustrate the mathematical conception upon which the
solar, planetary, and lunar tables were constructed. The epicycles represented nothing
more nor less than the first terms in the Fourier series, which in the last century has
become a basis of such calculations, both in astronomy and physics generally.
FOOTNOTES:
[1] For definition see p. 22.
