n] 



The Annual Mo^on of the 



NORTH POLE 



months the sun is on the whole lower down in the sky than 

 in silmmer, and that in particular its midday height is less. 



ii. The sun's path on the celestial sphere is therefore 

 oblique to the equator, lying partly on one side of it and 

 partly on the other. A good deal of careful observation 

 of the kind we have been describing must, however, have 

 been necessary before it was ascertained that the sun'/, 

 annual path on the celestial sphere (see fig. 4) is a great 

 circle (that is, a circle having its centre at the centre of 

 the sphere). This great circle is now called the ecliptic 

 (because eclipses take place only when the moon is in 

 or near it), and the angle at which it cuts the equator is 

 called the obliquity of the ecliptic. The Chinese claim to 

 have measured the obliquity in 1 100 B.C., and to have found 

 the remarkably accurate value 23 52' (cf. chapter n., 35). 

 The truth of this statement may reasonably be doubted, but 

 on the other hand the statement of some late Greek writers 

 that either Pythagoras or Anaximander (6th century EC.) was 

 the first to discover the 

 obliquity of the ecliptic is 

 almost certainly wrong. It 

 must have been known with 

 reasonable accuracy to both 

 Chaldaeans and Egyptians 

 long before. 



When the sun crosses the 

 equator the day is equal to 

 the night, and the times 

 when this occurs are con- 

 sequently known as the 

 equinoxes, the vernal equi- 

 nox occurring when the sun 

 crosses the equator from 

 south to north (about March 

 2ist), and the autumnal 



equinox when it crosses back (about September 23rd). 

 The points on the celestial sphere where the sun crosses 

 the equator (A, c in fig. 4), i.e. where ecliptic and equator 

 cross one another, are called the equinoctial points, 

 occasionally also the equinoxes. 



After the vernal equinox the eu.i in its path along the 



SOUTH POLE 



FIG. 4. The equator and the 

 ecliptic./' 



