CH. in. EUCLID AND ARCHIMEDES. 21 



from pole to pole), oblique to its path round the sun, called 

 the ecliptic. This is called the obliquity of the ecliptic. 



Aristarchus appears also to have been the first Greek 

 who understood that day and night are caused by the earthf 

 turning round on its axis every day. If the Greeks had! 

 understood his teaching, especially about the earth moving! 

 round the sun, they would have made much more progress 

 in astronomy. But no one believed him, and more than 

 1700 years passed away before Copernicus, of whom we 

 shall speak in Chapter IX., discovered this great truth over 

 again. This Greek theory of the earth moving round the 

 sun is often called the Pythagorean system, for it was 

 thought that Pythagoras taught it ; but we have seen that, 

 though Pythagoras knew that the earth moves, he did not 

 believe that it went round the sun. 



Euclid, 300 B.C. We must not pass through the third 

 century before Christ without mentioning Euclid, the great 

 mathematician and geometer, who collected together the 

 propositions in the * Elements of Euclid,' known to every 

 schoolboy. He was born at Alexandria about 300 B.C. 

 His works are too difficult for us to examine, and the only 

 discovery of his we can mention is, that light travels in 

 straight lines called ' rays.' Thus, if you look at a sun- 

 beam shining across a dusty room, you can see the light 

 reflected in a straight line along the particles of dust, and if 

 you let sunlight through a hole in the shutter into a dark 

 room, it will light up a spot on the wall or floor exactly 

 opposite to the sun ; the middle of the sun, the middle of 

 the hole in the shutter, and the middle of the spot of light, 

 will all be in a straight line. 



Archimedes, 287 B.C. Another famous geometer, 

 Archimedes of Syracuse, born 287 B.C., lived about the 

 same time as Euclid. He studied for many years at 



