CH. III. ARISTARCHUS AND EUCLID, 21 



the south pole will be in summer and the north pole in 

 winter, while at the fourth and last point you have the 

 northern spring and the southern autumn. This was what 

 Aristarchus discovered, namely, that the changing seasons 

 are entirely caused by the earth having its axis (or the line 

 from pole to pole) obliqtie 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 were caused by the 

 earth 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 

 you will read 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. We must not pass through the third 

 century before Christ without mentioning Euclid, the great 

 mathematician and geometer, who collected together the 

 problems 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 sunbeam 

 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 



