CH. iv. PRECESSION OF THE EQUINOXES. 31 



the equator, so that the sun's path meets the equator at 

 this point, which is called the equinoctial point. Pass on 

 till the south pole of your ball is in the dark, the sun will 

 now fall directly on a point north of the equator (making our 

 summer solstice). Pass on again to the point of equal day 

 and equal night, and the sun again falls direct on the equa- 

 tor, causing our autumn equinox. Now, if the earth did not 

 make that small circle in space like the top, the sun would 

 always touch the equator at exactly those same points of 

 the earth's orbit or path round the sun ; but the effect of 

 that movement is to pull the equator slightly back, so that 

 the points where the ecliptic and the equator cut each 

 other are 50 J seconds more to the west every year, and in 

 this way the equinoxes would travel all round the orbit from 

 east to west in about 26,000 years were it not that a gradual 

 change in the direction of the major axis of the earth's 

 orbit, known as " the revolution of the apsides," shortens 

 the period and reduces it to 21,000 years. Hipparchus 

 discovered this precession (or going forward) of the equi- 

 noxes ; though he did not know, what Newton afterwards 

 discovered, that it is caused by the sun and moon pulling 

 at the mass of extra matter which is gathered round the 

 equator. 



