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 

 such an enormous distance 

 that their approach to each 

 other was quite unimpor- 

 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 born, 

 1 60 B.C. Hipparchus was the most famous of all the astro- 



Diagram showing how Eratosthenes mea- 

 sured the circumference of the earth. 



A, Gnomon at Alexandria. B, Gnomon at 

 Syene. c D, Length of shadow of gnomon. 

 D E, Distance from Alexandria to Syene. 



