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[Illustration: SIR ISAAC NEWTON (From the bust by Roubiliac In Trinity
College, Cambridge.)]

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



PREFACE


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.

[Illustration: CHALDÆAN BAKED BRICK OR TABLET, _Obverse and reverse
sides_, Containing record of solar eclipse, 1062 B.C., used lately by
Cowell for rendering the lunar theory more accurate than was possible
by finest modern observations. (British Museum collection,
No. 35908.)]

[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 3651/4 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.

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