30 SCIENCE OF THE GREEKS. FT. I. 



cult to be explained here. Hipparchus was supposed to 

 have made a catalogue of 1080 stars, but recent researches 

 seem to prove that these were made later by Ptolemy. He 

 also calculated accurately when eclipses of the sun or the 

 moon would take place. But his great discovery was that 

 called the ' Precession of the Equinoxes.' This is a very 

 complicated movement not easy to understand ; 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 movement 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 posi- 

 tion that the north pole is in the dark, or the northern 

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

 the sun to the centre of the earth will not meet the equator 

 but a point to the south of it. But now pass the ball on to 

 the next point when neither pole is in shade, and when it is 

 equal day and equal night over the globe (our spring equi- 

 nox), a line now drawn from the sun will fall directly upon 



