40 A Short History of Astronomy ecu. 11 



being known to be 5,000 stadia, Eratosthenes thus arrived 

 at 250,000 stadia as an estimate of the circumference 

 of the earth, a number altered into 252,000 in order to 

 give an exact number of stadia (700) for each degree on the 

 earth. It is evident that the data employed were rough, 

 though the principle of the method is perfectly sound ; 

 it is, however, difficult to estimate the correctness of the 

 result on account of the uncertainty as to the value of 

 the stadium used. If, as seems probable, it was the 

 common Olympic stadium, the result is about 20 per cent. 

 too great, but according Jo another interpretation * the 

 result is less than i per cent, in error (cf. chapter x., 221). 



Another measurement due to Eratosthenes was 'that 

 of the obliquity of the ecliptic, which he estimated at 

 f of a right angle, or 23 51', the error in which is only 

 about 7'. 



37. An immense advance in astronomy was made by 

 ffipparchuSy whom all competent critics have agreed to 

 rank far above any other astronomer of the ancient world, 

 and who must stand side by side with the greatest astro- 

 nomers of all time. Unfortunately only one unimportant 

 book of his has been preserved, and our knowledge of 

 his work is derived almost entirely from the writings of his 

 great admirer and disciple Ptolemy, who lived nearly three 

 centuries later ( 46 seqq.\ We have also scarcely any 

 information about his life. He was born either at Nicaea 

 in Bithynia or in Rhodes, in? which island he erected an 

 observatory and did most of his work. There is no 

 evidence that he belonged to the Alexandrine school, 

 though he probably visited Alexandria and may have made 

 some observations there. Ptolemy mentions observations 

 made by him in 146 B.C., 126 B.C., and at many inter- 

 mediate dates, as well as a rather doubtful one of 161 B.C. 

 The period of his greatest activity must therefore have been 

 about the middle of the 2nd century B.C. 



Apart from individual astronomical discoveries, his chief 

 services to astronomy may be put under four heads. He 

 invented or greatly developed a special branch of mathe- 



* That of M. Paul Tannery : Recherches sur VHistoire de V Astro- 

 nomic Ancienne, chap. v. 



