ASTRONOMY, 



sun; and uot as perishing meteors, form- 

 ed in the atmosphere, as they were 

 thought to be in after times. From this 

 time to the foundation of the school of 

 Alexandria, the history of astronomy 

 among the Greeks offers nothing remark- 

 able, except some attempts of Eudoxus to 

 explain the celestial phenomena : and 

 the celebrated cycle of 19 years, which 

 had been imagined by Meton, in order 

 to conciliate the solar and lunar motions. 

 This is the most accurate period, for a 

 short interval of time, that could have 

 been devised, for embracing an exact 

 number of revolutions of these two lumi- 

 naries ; and is so simple and useful, that 

 when Meton proposed it to the Greeks, 

 assembled at the Olympic games, as the 

 basis of their calendar, it was received 

 with great approbation, and unanimously 

 adopted by all their colonies. In the 

 school of Alexandria, we see, for the first 

 time, a combined system of observations, 

 made with instruments proper for mea- 

 suring angles, and calculated trigonome- 

 trically. Astronomy accordingly took a 

 new form, which succeeding ages have 

 only brought to greater perfection. The 

 position of the stars began at this time 

 to be determined , they traced the course 

 of the planets with great care ; and the 

 inequalities of the solar and lunar motions 

 became better known. It was, in short, 

 in this celebrated school, that a new sys- 

 tem of astronomy arose, which embraced 

 the whole of the celestial motions ; and 

 though inferior to that of Pythagoras, and 

 even false in theory, it afforded the means, 

 by the numerous observations which it 

 furnished, of detecting its own fallacy, and 

 of enabling astronomers in later times to 

 discover the true system of nature. It 

 was from their observations of the princi- 

 pal zodiacal stars, that Hipparchus was 

 led to discover the precision of the equi- 

 noxes ; and Ptolemy also founded upon 

 them his theory of the motions of the pla- 

 nets. Next after these was Aristarchus, 

 of Samos, who made the most delicate 

 elements of the science the objects of his 

 i-escixrcn. Among other things of this 

 kind, he attempted to determine the mag- 

 nit ide and distance of the sun; and 

 though, as may be supposed, the results 

 he obtained were considerably wide of the 

 truth, the methods he employed to re- 

 solve these difficult problems do great 

 honour to hi:, .cenirs. The celebrity of his 

 successor, Eratosthenes, urisrs, chiefly 

 from his attempt to measure the e;n!'i, 

 and his observations on the obliquity ot' 

 the ecliptic. Having remarked, at Syene, 



a well, which was enlightened to its boi- 

 torn by the sun, on the day of the summer 

 solstice, he observed the meridian height 

 of the sun on the same day at Alexandria, 

 and found that the celestial arc continued 

 between the two places was the 5<3th 

 part of the whole circumference ; and as 

 their distance was estimated at 500 stadia, 

 he fixed the length of a great circle of 

 the earth at 250,000 : but as the length 

 of the stadium employed by this astrono- 

 mer is not known, we cannot appreciate 

 the exactness ofhismeasurement. Among 

 others who cultivated and improved this 

 science we may also mention the cele- 

 brated Archimedes, who constructed a 

 kind of planetarium, or orrery, for re- 

 presenting the principal phenomena of 

 heavenly bodies. But of all the astrono- 

 mers of antiquity, Hipparchus of Bithynia 

 is the one, who, by the number and pre- 

 cision of his observations, as well as by 

 the important result which he derived 

 from them, is the most entitled to our 

 esteem. He flourished at Alexandria 

 about the year 162 before the Christian 

 aera; and began his astronomical labours 

 by attempting to determine, with more 

 exactness than had hitherto been done, 

 the length of the tropical year, which he- 

 fixed at 365 days, 5 hours, and 55 minutes, 

 being nearly 4| minutes too great. Like 

 most of his predecessors, he founded his 

 system upon an uniform circular motion 

 of the sun ; but, instead of placing the 

 earth in the centre of the solar orbit, he 

 removed it to the distance of J-_th part 

 of the radius, and fixed the apogee to the 

 sixth degree of Gemini. By means of 

 these data, he formed the first solar tables 

 of which any mention is made in the his- 

 tory of astronomy ; and though defective, 

 and even erroneous in principle, they are 

 a durable monument of his genius, which 

 three centuries afterwards were respect- 

 ed by Ptolemy, without his presuming to 

 alter them. The great astronomer next 

 considered the motions of the moon, and 

 endeavoured to measure the exact time of 

 her revolution, by a comparison of an- 

 cient eclipses. He also determined the 

 eccentricity and inclination of her orbit, 

 as well as the motion of her nodes and 

 apogee ; and calculated all the eclipses 

 that were to happen for 600 years to 

 come. 



Between the time of Hipparchus and 

 Ptolemy, the chief observers of a^y note 

 are Agrippa, Mcnelaus, and Theon; the 

 two latter of which are belter known as 

 geometricians tL an astronomers. We re 7 

 mark, however, in this interval, the re- 



