Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/815

] Samos, the friend of Archimedes, collected the records of eclipses, which had been observed by the ancient Egyptians ; and Callimachus ascribes to him the constellation of Berenice s hair. Archimedes also claims a high rank among the cultivators of astronomy. His celebrated planetarium, which represented the motions of the sun, moon, planets, and starry sphere, has been a frequent theme of the praises of the poets:— Jura poli, rerumque fidem, legesque dooruin, Ecce Syracosius transtulit arte scnex.&quot; —Claudian, Epigr. 18.

of Perga solved the important problem of the stations and retrogradations of the planets by means of epicycles and deferents; and he is entitled to the glory of having formed the alliance between geometry and astronomy, which has been productive of the greatest advantages to both.

Astronomy, which had as yet only consisted of a knowledge of isolated facts, acquired a systematic form, and almost a new existence, from the genius of, perhaps the greatest of all ancient philosophers in the sciences which are not purely speculative. This illustrious founder of astronomical science was born at Nicæa in Bithynia, and observed at Rhodes. Flamsteed and Cassini, probably misled by some ambiguous expressions of Ptolemy, have related that his observations were made at Alexandria ; and this opinion seems generally to have been adopted by historians. The question has been examined carefully, and at considerable length, by Delambre (Astronomie Ancienne), who comes to the conclusion that there is no reason whatever to infer that Hipparchus ever saw Alexandria, Ptolemy, in reporting the observations of Hipparchus, supposed Rhodes and Alexandria to be situated on the same meridian, and consequently does not find it necessary to mention the place at which the observa tions were made. Hipparchus commenced his brilliant career by verifying the determination of the obliquity of the ecliptic made by Eratosthenes. He next directed his attention to the length of the tropical year. By comparing one of his own observations of the summer solstice with a similar one made by Aristarchus 140 years before, he found that the anciently received value of 365^ days was too great by seven minutes. This leaves the tropical year a value still too great ; but it is probable that the error arose from the inaccuracy of the observation of Aristarchus ; for the observations of Hipparchus, compared with those of the moderns, make the length of the tropical year amount to 365 days, 5 hours, and 49 minutes, which is only 12 seconds greater than the truth. By a careful observation of the solstices and equinoxes, he discovered that the year is not divided by these points into four equal parts, the sun occupying 944 days in passing from the vernal equinox to the summer solstice, and only 92 The attention of Hipparchus was next directed to the motions of the moon ; and on this subject his researches were attended with equal success. From the comparison of a great number of the most circumstantial and accurate observations of eclipses recorded by the Chaldeans, he was enabled to determine the period of the moon s revolution relatively to the stars, to the sun, to her nodes, and to her apogee. These determinations are among the most valuable results of ancient astronomy, inasmuch as they corroborate one of the finest theoretical deductions the acceleration of the mean lunar motion and thus furnish one of the most delicate tests of the truth of Newton s law of gravitation. It was indeed, by a comparison of the observations of Hipparchus with those of the Arabian and modern astro nomers that Dr Halley was led to the discovery of that important phenomenon. Hipparchus also determined the eccentricity of the lunar orbit, and its inclination to the plane of the ecliptic ; and the values which he assigned to these elements, making allowance for the evection and the inequalities of the moon s motion in latitude, are within a few minutes the same as those which are now observed. lie had also an idea of the second inequality of the moon s motion, namely, the evection, and made all the necessary preparations for a discovery which was reserved for Ptolemy. He likewise approximated to the parallax of the moon, which he attempted to deduce from that of the sun, by determining the length of the frustum cut off from the cone of the terrestrial shadow by the moon, when she tra verses it in her eclipses. From the parallax he concluded that the greatest and least distances of the rnoon are re spectively equal to 78 and 67 semi-diameters of the earth, and that the distance of the sun is equal to 1300 of the same semi-diameters. The first of these determinations- exceeds the truth, the second falls greatly short of it, thc- distance of the sun being nearly equal to 24,000 terrestrial semi-diameters. It may, however, be remarked that Ptolemy, who undertook to correct Hipparchus with regard to the parallax, deviated still farther from the truth. The apparition of a new star in the time of Hipparchus induced him to undertake the formation of a catalogue of all the stars visible above his horizon, to fix their relative- positions, and mark their configurations, in order that pos terity might have the means of observing any changes- which might in future take place in the state of the heavens. This arduous undertaking was rewarded by the important discovery of the precession of the equinoxes, one of the fundamental elements of astronomy. By comparing hi* own observations with those of Aristillus and Timocharis r he found that the first point of Aries, which, in the time- of these astronomers, or 150 years before, corresponded with the vernal equinox, had advanced two degrees, according to the order of the signs, or at the rate of 48&amp;gt; seconds a year. This determination is not very far from 