454 LECTURE XLVIII. / 



comets were only eccentric planets.* Some time after this, the lunar 

 period of Meto was publicly made known at the Olympic games, and was 

 universally adopted as the basis of the calendar. (Plate XXXVIII. Fig. 

 527.) 



The next occurrence which deserves to be noticed, with respect to astro- 

 nomy, is the foundation of the school of Alexandria, which waa the first 

 source of accurate and continued observations. Upon the death of Alex- 

 ander and the subsequent division of his empire, the province of Egypt 

 fell to the lot of Ptolemy Soter ; a prince whose love of science, and whose 

 munificence towards its professors, attracted to his capital a great number 

 of learned men from various parts of Greece. His son, Ptolemy Phila- 

 delphus, continued and increased the benefits conferred on them by his 

 father, and built the magnificent edifice which contained, together with 

 the celebrated library, collected by Demetrius Phalereus, an observatory, 

 furnished with the necessary books and instruments.t The first astro- 

 nomers, who were appointed to occupy this building, were Aristyllus and 

 Timocharis ; they flourished about 300 years before Christ, and observed 

 with accuracy the places of the principal stars of the zodiac. Aristarchus 

 of Samos was the next : he imagined a method of finding the sun's 

 distance, by observing the portion of the moon's disc that is enlightened, 

 when she is precisely in the quadrature, or 90 distant from the sun ; and 

 although he failed in his attempt to determine the sun's distance with 

 accuracy, yet he showed that it was much greater than could at that time 

 have been otherwise imagined ; and he asserted that the earth was but as 

 a point in comparison with the magnitude of the universe. His esti- 

 mation of the distance of the sun is made by Archimedes the basis of a 

 calculation of the number of grains of sand that would be contained in the 

 whole heavenly sphere, intended as an illustration of the powers of 

 numerical reckoning, and of the utility of a decimal system of notation, 

 which was the foundation of the modern arithmetic. || 



Eratosthenes, the successor of Aristarchus,^" is known by his observation 

 of the obliquity of the ecliptic, and his measurement of a certain portion 

 of the earth's circumference ; the whole of which he determined to be 

 250,000 stadia ; but the length of his stadium is uncertain. Ptolemy, 

 calculating perhaps from the same measures, or from some others still 

 more ancient, calls it 180,000 ; which, if the stadium is determined from 

 the Nilometer at Cairo, and from the base of the pyramid, is within one 

 thousandth part of the truth, the length of the base of the pyramid being 

 equal to 400 Egyptian cubits, or to 729 feet 10 inches English. 



Hipparchus of Bithynia flourished at Alexandria about the year 140 

 before Christ. Employing the observations of Timocharis, and comparing 

 them with his own, he discovered the precession of the equinoxes. He 

 also observed that the summer was 9 days longer than the winter, and that 



* Aristotle, Meteor. 1. 1, c. 6. t Strabo, Geog. 1. 13. 



I Ptolemy, Almagest. 1. 6, c. 3. 



Arist. Sam. de Magnit. et Dist. Solis et Lunse, 4to, Pis. 1572. Wallis's Op. 

 vol. iii. 



|| Archimedes, Arenarius, ed. Paris, 1615, p. 449. 

 ^ Cleomedes, Cyc. Th. 1. 1, c. 10, in Arati Op. Oxf. 1672, fol. 



