CH. IV. CIRCUMFERENCE OF THE EARTH, 29 



Let the large circle represent the earth ; b the gnomon 

 at Syene, and a the gnomon at Alexandria. The length of 

 the shadow c D of the gnomon a, will bear the same pro- 

 portion to the circumference of the small circle (drawn from 

 the top of the gnomon as Fig. 2. 



a centre), that the distance 

 from Alexandria to Syene 

 (d to e) does to the whole 

 circumference of the globe. 

 This is true only if the rays 

 from the sun to Alexandria 

 and to Syene are parallel 

 (or run at equal distances). 

 They are not really quite 

 parallel because they meet 

 in the sun, but Eratosthenes 

 knew that the sun was at 



<:nr>i nr> pnnrmniiQ rlictnnrA Diagram showing how Eratosthenes mea- 

 SUCn an enormous aiStance j.,,red the circumference of the earth. 



+T,«4- 4-1,^;.- ^^-^^^^^X. 4.^ ^^^T, A, Gnomon at Alexandria, b, Gnomon at 

 that their approach to each Syene. cd, Length ofshadoW of gnomon, 



other was quite Unimpor- ^ e> D-tance from Alexandria to Syene. 



tant. He now measured the distance between Alexandria 

 and Syene and found it to be 5,000 stadia, or about 625 

 miles, and multiplying this by 50 he got 625 x 50 = 31,250 

 miles as the whole circumference of the earth, measured 

 round from pole to pole. This result is not quite correct, 

 but as nearly as could be expected from a first rough 

 attempt. Eratosthenes also studied the direction of moun- 

 tain-chains, the way in which clouds carry rain, the shape of 

 the continents, and many other geographical problems. 



Hipparchus, 160.— Nearly one hundred years after 

 Eratosthenes, the great astronomer Hipparchus was bom, 

 160 B.C. Hipparchus was the most famous of all the astro- 



