THE ALEXANDRIAN PERIOD 85 



Conon and Dositheus, the friends of Archimedes, 

 were especially notable observers. Conon, in par- 

 ticular, discovered a group of stars which he called 

 " Berenice's Hair' in honour of the wife of Ptolemy 

 Euergetes. 



Taking as a basis the celestial map of Eudoxus, 

 Aratus of Soli wrote a descriptive poem on the starry 

 heavens, which, although possessing no great literary 

 qualities, made an enormous sensation. It had several 

 Roman commentators, amongst them Cicero, and, 

 with its illustrations of antique figures, enjoyed great 

 fame in the Middle Ages. 



However, the greatest astronomer of antiquity was 

 incontestably Hipparchus, who was born at Nicaea in 

 Bithynia and spent the greater part of his life at Rhodes. 

 One of his observations on the star r\ Canis Majoris 

 enables us (as Delambre has shown) to fix the date of his 

 work at about the year 120 B.C. His scientific activity 

 was prodigious. In his youth, he composed a Com- 

 mentary on the Phenomena of Aratus and Eudoxus, 

 which is unfortunately the only one of his writings 

 now extant. He constructed several instruments, 

 amongst others a dioptra for measuring the apparent 

 diameter of the sun by a much simpler method than 

 that of Archimedes. His apparatus is composed of a 

 graduated scale on one end of which is a sight and on 

 which slides a cursor. To take an angular measure- 

 ment the cursor is moved until the eye looking through 

 the sight sees it cover the magnitude to be measured, 

 such as, for example, the diameter of the sun. This 

 instrument with few modifications became that known 

 as Jacob's staff, or cross-staff. Hipparchus also made 

 use of two instruments to which he gave the name of 

 astrolabe. " The first, or spherical, astrolabe was com- 

 posed of several metallic circles, some fixed, others 

 movable. The first circle of all was the meridian ; it 

 was suspended from a fixed point, or better still, sup- 



