The mass of evidence seems in favor of the plains of Chaldea, being the primal seat of observative A. The risings and settings of the heavenly bodies and eclipses were sub jects of observation and notation by their priests at a very remote period. Simplicius and Porphyry mention that Aristotle had transmitted to him from Babylon, by order of Alexander the great, a catalogue of eclipses observed during 1903 years preceding the conquest of that city by the Macedonians. Ptolemy gives six of the eclipses from this catalogue, but the earliest does not extend further back than 720 B.c. The probability therefore is that the statement of Simplicius, as to their early date, is an exag,geration. In these observations, the time is only given in hours, and the part of the diameter eclipsed within a quarter; but rough as they are, they are the earliest reliable observa tions extant; and a comparison of them with modern observations, led Halley to the dis covery of the doctrine of the moon's acceleration—that is, that she now moves round the earth with greater velocity than formerly. It is remarkably illustrative of their habit of diligent observation, that the Chaldeans were acquainted with the cycle of 6585+ days, during which the moon makes about 223 synodical revolutions, and experiences the same number of- eclipses, alike, too, in order and magnitude, comparing cycle with cycle. The clepsydra as a clock, the gnomon for determining the solstices, and a hemi spherical dial for ascertaining the positions of the sun, were used by the Chaldeans, and they have the credit of the invention of the zodiac and the duodecimal division of the day.
The Egyptians, it is supposed, were the first instructors of the Greeks in A. They do not, however, appear to have observed much for themselves. The meaning of what data they have left behind them can be guessed at only in a few instances. No mention is made by Ptolemy of the idea ascribed to them, that the planets Mercury and Venus moved round the sun; the probability therefore is, Ptolemy not being likely to overlook such a novel theory, that they entertained no such notion at the time of his visit, but that it is an after-thought of more recent ages. From the accuracy with which some of the pyramids face the cardinal points, there is a supposition that t ley must have been erected for astronomical purposes; but if it be true, as is stated, that Thales taught the Egyp tians how to find the height of the pyramids by the shadow, and that the latter informed Herodotus that the sun had twice been seen to rise in the west, the conclusion is that the A. of the ancient Egyptians was very meager and absurd.
Up to this time, A. is little else than tradition. The Greeks have the honor of elevat ing it into a reliable history, and to the dignity of a science. Thales (640 nc), the founder of the Ionic school, laid the foundation of Greek A. He it was who first prop agated the theory of the earth's sphericity. The sphere lie divided into five zones. He %predicted the year of a great solar eclipse, but this it is now supposed he must have casually succeeded in doing—the Greeks at this time having no observations of their own to guide them—by means of the Chaldean saros, or period of 18 years and 10 days, which gives a regular recurrence of eclipses. He made the Greeks, who, prior to his
time, were content to navigate their vessels by the Great Bear—a rough approximation to the north—acquainted with the lesser constellation of that name, a much better guide for the mariner. His system, however, contained a good deal of absurdity. Among other things, he held that the stars were composed of tire, and that the earth was the center of the universe. The successors of Thales held opinions which in many respects are wonderfully in accordance with modern ideas. Anaximander, it is said, held that the • earth moved about its own axis, and that the moon's light was reflected from the sun. To him is also attributed, on somewhat slender authority, the belief in the grand idea of the plurality of worlds. Anaxagoras, who transferred the Ionic school from Miletus to Athens, is said to have offered a conjecture that, like the earth, the moon had habitations, hills, and valleys.
Pythagoras who was the next astronomer of eminence, was very far in ad vance of his predecessors. He promulgated, on grounds fanciful enough, the theory, the truth of which, however, has been since established, that the sun is the center of the planetary world, and that the earth circulates round it. Pythagoras also first taught that the morning and evening star were in reality one and the same planet. But the views of Pythagoras met with little or no support from his successors until the time of Copernicus. Between Pythagoras and the advent of the Alexandrian school, nearly a couple of centuries later, the most prominent names in astronomical annals are those of Meton (432 D.c.), who introduced the luni-solar cycle, as already intimated, and, in con junction with Euctemon, observed a solstice at Athens in the year 424 B.C. ; Callippus (330 p.c.), who improved the Metonic cycle; Eudoxus of Cnidus (370 B.c.), who brought into Greece the year of 365+ days, and wrote some works on A.; and Nicetas of Syra cuse, who is reported to have taught the diurnal motion of the earth on its axis.
To the Alexandrian school, owing its existence to the munificent Ptolemies, we are indebted for the first systematic observations in A. Hitherto the truths of A. rested on no better evidence than the conjectures of sagacious minds, and these being opposed to the testimony of the senses, met with but little acceptance from the world. The Alexandrian school originated a connected series of observations relative to the consti tution of the universe. The positions of the fixed stars were determined, the paths of the planets carefully traced, and the solar and lunar inequalities more accurately ascer tained. Angular distances were calculated with instruments suitable to the purpose by trigonometrical methods, and ultimately the school of Alexandria presented to the world the first system of theoretical astronomy that had ever comprehended an entire plan of the celestial motions. The system we know to be false, and inferior to the Pythagorean notions; but it had the merit of being founded upon a long and patient observation of phenomena, a principle which finally brought about its own destruction, while the pre vious theories were the results of pure hypothesis.