.62 A Short History of Astronomy [CH. n. 



motions by means of epicycles, but whether these attempts 

 marked any advance on what had been done by Apollonius 

 and Hipparchus is uncertain. 



It is interesting also to find in Pliny (A.D. 23-79) tne 

 well-known modern argument for the spherical form of the 

 earth, that when a ship sails away the masts, etc., remain 

 visible after the hull has disappeared from view. 



A new measurement of the circumference of the earth by 

 Posidonius (born about the end of Hipparchus's life) may 

 also be noticed ; he adopted a method similar to that of 

 Eratosthenes ( 36), and arrived at two different results. 

 The later estimate, to which he seems to have attached 

 most weight, was 180,000 stadia, a result which was about 

 as much below the truth as that of Eratosthenes was 

 above it. 



/ 46. The last great name in Greek astronomy is that 

 , . /yfjf Claudius Ptolemaeus, commonly known as Ptolemy ', of 

 whose life nothing is known except that he lived in 

 Alexandria about the middle of the 2nd century A.D. 

 His reputation rests chiefly on his great astronomical 

 treatise, known as the Almagest* which is the source 

 from which by far the greater part of our knowledge of 

 Greek astronomy is derived, and which may be fairly 

 regarded as the astronomical Bible of the Middle Ages. 

 Several other minor astronomical and astrological treatises 

 are attributed to him, some of which are probably not 

 genuine, and he was also the author of an important work 

 on geography, and possibly of a treatise on Optics, which 

 is, however, not certainly authentic and maybe of Arabian 

 origin. The Optics discusses, among other topics, the 

 refraction or bending of light, by the atmosphere on the 

 earth : it is pointed out that the light of a star or other 

 heavenly body s, on entering our atmosphere (at A) and on 

 penetrating to the lower and denser portions of it, must 

 be gradually bent or refracted, the result being that the 



* The chief MS. bears the title fieydXr] (rvvraZts, or great composi- 

 tion ^though the author refers to his book elsewhere as fj.adr)fj.aTiKr) 

 o-Wats (mathematical composition). The Arabian translators, either 

 through admiration or carelessness, converted jmeydXr), great, into 

 HeyLffT-rj, greatest, and hence it became known by the Arabs as 

 Al Magisti, whence the Latin Almagestum and our Almagest. 



