AST 



AST 



wliicli lie brought with him into Italy, to which country he 

 was obhjred to retire ofi account oi the dcfpi)tifin whicli then 

 prevailed at Athens. Here he firlt taujrlit the true fv (lem of 

 the world, which, many centuries after, was revived by Co- 

 pernicus ; but hid his dodrines from the vulgar, in iuiitation 

 of the Egyptian priells rvho had been his inftrnctors. It 

 was even thought,- in this fchool, that the planets were in- 

 habited bodies, like the earth ; and that the ilars, which 

 are diffeminated through infinite fpace, are funs, and the 

 centres of other planctaiy fyftems. They alfo confidcred 

 the comets as pei-manent bodies, moving roi.md the fun ; and 

 not as pcrifhing meteors, foniied in the atmofphere, ai they 

 were thoucjht to be in after times 



From this time to the foundation of the fchool of Alex- 

 andria, the hiftory of aftronomv among the Greeks offers 

 nothing remarkable, except fome attempts of Eudoxus to 

 explain the celeftial phenomena ; and the celebrated cvcle 

 of 19 years, which had been imagined by Metun, in order 

 to conciliate the folar and lunar motions. Tliis is the moll 

 accurate period, for a ihort interval of time, that could 

 have been dcvifed for embracing an exaft nunsber of revo- 

 lutions of thele two luminaries ; and is fo iimplc and ufe- 

 ful, that, when Meton propofed it to the Greeks, aflVmbled 

 at the Olympic games, as tlie bafis of their calendar, it was 

 received with great approbation, and unanimoufly adopted 

 by all their colonies. 



In the fchool of Alexandria, wc fee, for the firft time, 

 a combined fyftem ot obfervations, made with inllruments 

 proper for meafuring angles, and calculated trigonometri- 

 cally. Allrouomy, accordingly, took a new form, which 

 fucceeding ages have only brought to greater perfection. 

 The pofitiou of the ftars began at this time to be deter- 

 mined ; they traced the courfe of the planets with greater 

 care ; and the inequalities of the folar and lunar motions 

 became better known. It was, in fliort, in this celebrated 

 fchool, that a new fyftem of aftronomv arofc, which cm- 

 braced the whole of the celeftial motions ; and though in- 

 ferior to that of Pythagoras, and even falfe in theory, it 

 afforded the means, by the numerous obfervations which it 

 furniftied, of detecting its own fallacy, and of enabling 

 aftronomers in later times to dilcovcr the true fyftem of 

 nature. 



* Ariilvllus and Timocharis were the firft obfervers in this 

 rlfing inllitution. They flonrlftied about the year 290 before 

 Chrift ; and by their afiiduous labours, were the means of 

 greatly improving this fcience. It v,as from their obferva- 

 tions of the principal zodiacal ftars, that Hipparchus was 

 led to difcover the prcceffion of the equinoxes; and Ptolemy 

 alfo founded upon them his theory of the motions of the 

 planets. 



Next after thefe, was Aiiftarchus of Samos, who made 

 the moft delicate elements of the fcience the ohjetls ot his 

 refearch. Among other things of this kind, lie attempted 

 to determine the magnitude and diftance of the fun ; and 

 though, as may be fuppofed, the refults he obtained were 

 conliderably wide of the truth, the metliods he employed 

 to refolve thele difficult problems, do great honour to his 

 genius. He alfo endeavoured to revive the opinion of the 

 Pythagorean fchool, \\ ith refpeft to the motion of the 

 earth ; but as his writings npcn this fubiecl have not been 

 preferved, we are ignorant to ^vhat ])oint he had advanced, 

 by this means, in his explication of the celeftial pha;- 

 nomena. 



The celebrity of his fucceftbr Eratofthenes, aril'es chiefly 

 from his attempt to meafnre the earth, and his obfervations 

 on the obliquity of the ecliptic. Having remarked at Sy- 

 ene, a well which was enlightened to lU bottom by the 



fun, on the day of the fummer folftice, he obfcrved tlie 

 meridian height of the fun on the fame day at Alexandria; 

 and found that the celeftial arc, contained between the two 

 places, was the 50th part of the whole circumference j 

 and as their diftance was eftimated at 500 ftadia, he fixed 

 tlie length of a great circle of the earth at 250,000 ; but 

 as the length of the ftadium employed by this aftronomer it 

 not known, wc cannot appreciate the exaclnefs of hii 

 meafurement. 



Among others who cultivated and improved this fcience, 

 wc may alfo mention the celebrated Archimedes, who coii- 

 ftrucEled a kind of planetarium or orrery, for repi'efenting 

 the principal phxnomcna of the heavenly bodies. But of 

 all the aftronomers of antiquity, Hipparchus of liithvnia is 

 the one, who, by the number and precifion of !iis obferva- 

 tions, as well as by the important refults which he derived 

 from them, is the moft entitled to our efteem. He flourilhed 

 at -Alexandria about the year 162 before the Chriftian atra; 

 and began his aftionomical labours by attempting to duter- 

 mine, with more exadlnefs than had hitherto been done, 

 the length of the tropical year, which he fixed at 365 

 days, 5 hours, and ^^ minutes, being near d.i minutes too 

 great. Like moft of his predeceffors, he founded hit 

 fyftem upon an uniform circular motion of the fun ; but 

 inftead of placing the earth in the centre of the folar orbit, 

 he removed it to the diftance of ,';th part of the radius, 

 and fixed the apogee to the fixth degree of Gemini. By 

 means ot thefe data, lie formed the firft folar tables of 

 which any mention is made iu the hiftory of aftronomy ; and 

 though defeftive and even eiToneous in principle, they are a 

 durable monument of his genius, which three centuries 

 afterwards were refpefted by Ptolemy, without his pre- 

 fuming to alter them. 



The great aftronomer next confiJered the motions of the 

 moon, and endeavoured to m.eafure the exacl time of her 

 revolution, by a compavifon of ancient eclipfes. He alfo 

 determined tlie eccentricity and inclination of her orbit, a» 

 well as the motion of her nodes and apogee ; and calculated 

 all the eclipfes that were to happen for 600 years to come. 

 We are, befidcs, indebted to him for the important dif- 

 covery of the preccflion of the equinoxes (fee PRtcts- 

 sion), which was the fruit of the long and difficult enler- 

 prife he undertook of making a catalogue of tiie fixed ftars, 

 with their latitudes, longitudes, and apparent magnitudes. 



Geographyis alio indebted to Hipparchus for the metliod 

 of fixing the fituation of places upon the earth, by means 

 of their latitude and longitude ; in obtaining tlie latter of 

 which, he appears to have been the firft who employed 

 eclipfes of the mocm ; and as thefe refearches required 

 numerous calculations, they gave birth, under his hands, 

 to fpherical trigonometi-y. Many of his principal works 

 perilhed with the library of Alexandria; but his catalogue 

 of the ftars, and feveral of his obfervations, have been pre- 

 ferved by Ptolemy in his Almageft. 



Between the time of Hipparchus and Ptolemy, the chief 

 obfervers of any note arc Agrippa, Menelaus, and Theon ; 

 the two latter of which are better known as geometricians 

 than aftronomers. We remark, however, in this interval, 

 the reformation of the calendar by Julius C.cfar, and a 

 more exact knowledge of the flux and reflux of the ocean 

 (fecTiDFs). Pofidonius, a celebrated ftoic philofopher, 

 who lived about eighty ) ears before Chrift, appears to have 

 been the full who obferved the relation of thele pha-nomeiw 

 with the motions of the moon; and of wli'ch Pliny, the 

 naturalift, has given a defcription, remarkable lor its ac- 

 curacy. 



Ptolemy, the worthy fucceflbr of Hipparchus, was born 

 Y 2 a*. 



