The first ornaments of the Alexandrian school were A ristillus and Timocharis, who flourished under Ptolemy Soter, about 300 years before Christ. They were chiefly employed in observations on the planets, and in deter mining the position of the stars with regard to the equinoxes and the equator. From the former, Ptolemy deduced his theory of the planetary motions ; and Hip parchus was enabled by the latter to determine the pre ession of the equinoctial points.
The next astronomer of the Alexandrian school was Aristarchus of Samos. He observed the summer solstice in the year 281 A. C. He determined the diameter of the sun to be the 720th part of the whole circumference ; and embracing the Pythagorean doctrine of the earth's motion, he concluded, from the positions of the stars, when the earth was in the opposite points of its orbit, that their distance was immeasurably greater than that of the sun, or to use his own beautiful comparison, that the distance of the stars, was, to the distance of the earth .from the sun, as the centre of a circle is to its circum ference. The genius of Aristarchus, however, is still more conspicuous in the method which he employed for the distance of the sun. By this method, which we have already explained in the article ARTS TARCHUS, he found the sun's distance to be 18 or 20 times greater than that of the moon ; a result, the inac curacy of which arises, not from the method, but from the error of the instruments by which the angles were measured.
Eratosthenes, an Athenian philosopher, born at Cvreue, A. C. 276, was invited to Alexandria by Ptolemy Euer getes, and appointed keeper of the royal library. By means of instruments which Ptolemy erected in the Museum at Alexandria, Eratosthenes found the obliquity of the ecliptic 23° 51' 20". He imagined the diameter of the sun to be 27 times greater than that of the earth ; and if we can trust to the translation of Plutarch by Xilander, he made the distance of the moon 780000 stadia, and that of the sell, 804000000. Eratosthenes, however, is chiefly distinguished for his ingenious method of ascertaining the diameter of the earth, by measuring the celestial arch between the zeniths of two places situated in the same meridian. At Sienna, one of the most southern cities of ancient Egypt, he remarked a deep well, which, at the time of the summer solstice, was completely enlightened by the sun, and hence he concluded that the sun was then in the zenith of this place. On the same day, at the solstice, he found, by means of a concave hemisphere, with a vertical stile equal to the radius of concavity, that the arch intercepted between the bottom of the stile and the extremity of its shadow, or, what is the same thing, that the distance of the sun from the zenith of Alexandria was 7° 12'. The
celestial arch, therefore, intercepted between the zeniths of the two cities, being equal to a 50th part of a great circle, and the distance between Alexandria and Sienna being 5000 stadia, it followed that the circumference of the earth was 5000x50=25000 stadia ; a stadium being equal to 85 toises, 3 feet, 17 inches. Aristotle, Cleome des, Possidonius, and Ptolemy, make the circumference of the earth 400,300,240,180 stadia respectively; but it would appear, by comparing the real distances of several places with the distances as determined by the ancients, that all these four measures are the same quantity ex pressed in different stadia ; the stadium of Ptolemy be ing 684.76 feet, that of Possidonius 513.570, that of Cleomedes 410.856, and that of Aristotle 308.142 feet.
Conon of Samos, who flourished about the year 260, made several observations on the rising and setting of the fixed stars, and collected all the eclipses that had been preserved by the Egyptians. He is said to have added Berenice's Hair to the number of the constella tions, in consequence of that queen, who was the wife of Ptolemy Soter, having fulfilled a vow of suspending her hair, remarkable for its beauty, in the temple of Venus, when her husband returned in triumph from the war in Asia. Archimedes, the friend and contemporary of Conon, will shine with greater lustre in the history of geometry and mechanics ; but he appears to have made observations on the solstices, and to have constructed a sphere in which the motions of the sun, moon, and five planets, were all represented with their proper velocity. Apollonius of Pergxa, another celebrated geometer, lived about the same time, and deserves to be noticed in the history of astronomy, for his ingenious method of explaining the stations and retrogradations of the planets. He supposed the planets to describe a small circle call ed the epicycle, while the centre of this circle was car ried round the earth in a greater circle, called the de ferent circle. Apollonius seems also to have invented the method of projections, and to have been the first who attempted to effect an alliance between geometry and as tronotny.