CH. iv. CIRCUMFERENCE OF THE EARTH. 29 



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 unimport- 

 ant. He now measured 

 



the distance between Alex- 

 andria and Syene, and found 

 it to be 5,000 stadia, or 

 about 625 miles, and mul- 

 tiplying this by so he eot T F , IC ' 2 * 



* J Diagram showing how Eratosthenes mea- 



625 X 50 = 31,250 miles sured the circumference of the earth. 



,111- c A, Gnomon at Alexandria. B, Gnomon at 



as the whole circumference Syene . CD> Length of shadow of gnomon . 



Of the earth, measured D E ' Distance from Alexandria to Syene. 



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 mountain- 

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

 continents, and many other geographical problems. 



Hipparchus, 160 B.C. 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- 

 nomers who lived before the Christian era. He collected 

 and examined all the discoveries made by the earlier obser- 

 vers, and made many new observations; but astronomy had 

 now become so complicated that the problems are too dim- 



