30 SCIENCE OF THE GREEKS. px. L 



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 diffi- 

 cult to be explained here. Hipparchus made a catalogue of 

 i,o8o stars, and showed how they are grouped with regard to 

 the ecliptic He also calculated accurately when eclipses of 

 the sun or the moon would take place. But his great disco- 

 very was that called the * Precession of the Equinoxes.' This 

 is a very complicated movement which you can only under- 

 stand by reading works on Astronomy ; but I will try to give 

 a rough idea of it, in order that you may always connect it 

 with the name of Hipparchus. 



We have seen that the earth has two movements — one, 

 turning on its own axis causing day and night ; the other, 

 travelling round the sun, causing the seasons of the year. 

 But besides these it has a third curious movement, just like 

 a spinning-top when it is going to fall. Look at a top a 

 little while before it falls, and you will see that, because it is 

 leaning sideways, the top of it makes a small circle in the 

 air. Now our earth, because it is pulled at the equator by 

 the sun, moon, and planets, makes just such a small circle 

 in space ; so that, instead of the north pole pointing quite 

 straight to the polar star, it makes a little circuit in the 

 sky, with the polar star in the centre. The pole moves 

 very slowly, taking twenty-one thousand years to go all 

 round this circle. To understand the effect of this move- 

 ment we must examine more closely what the equinoxes 

 are. Take your ball again and make it go round the lamp 

 with its axis inclined (see p. 20). When you have it in such 

 a position that the north pole is in the dark, or the northern 

 winter solstice, you will find that a straight line drawn from 



