Part 1 (2/2)

So Hipparchus, about 150 B.C., and Ptolemy a little later, were able to use the observations of Chaldaean 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 Chaldaeans 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 diminis.h.i.+ng 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, wis.h.i.+ng 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 a.s.syrians 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 ma.s.s 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 s.h.i.+ning 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 s.p.a.ce 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 a.s.sign the author with certainty. Probably many of them were independently taken by Chinese, Indian, Persian, Tartar, Egyptian, Babylonian, a.s.syrian, 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.

[Ill.u.s.tration: CHALDaeAN 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 CHALDaeANS.

The last section must have made clear the difficulties the way of a.s.signing 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 princ.i.p.al 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, Chaldaeans, a.s.syrians, 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.

<script>