48 A Short History of Astronomy [CH. n. 



moon's motion is, however, necessary before discussing his 

 theory. 



We have already spoken (chapter i., 16) of the lunar 

 month as the period during which the moon returns to the 

 same position with respect to the sun ; more precisely this 

 period (about .29! days) is spoken of as a lunation or 

 vjsynodic monthy; as, however, the sun moves eastward on 

 the celestial sphere like the moon but more slowly, the 

 moon returns to the same position with respect to the 

 stars in a somewhat shorter time ; this periqd (about 27 

 days 8 hours) is known as thevsidereal month.) Mgain, the 

 moon's path on the celestial sphere is slightly/inclined to 

 the ecliptic, and may be regarded approximately as a great 

 circle cutting the ecliptic in two nodes, at an angle which 

 Hipparchus was probably the first to fix definitely at 

 about 5. Moreover, the moon's path is always changing 

 in such a way that, the inclination to the ecliptic remaining 

 nearly constant (but cf. chapter v., in), the nodes move 

 slowly backwards (from east to west) along the ecliptic, 

 performing a complete revolution in about 19 years. It is 

 therefore convenient to give a special name, Vthe draconitic 

 month,^jto the period (about 27 days 5 hours) during which 

 the moon returns to the same position with respect to the 

 nodes. 



Again, the motion of the moon, like that of the sun, is 

 not uniform, the variations being greater than in the case 

 of the sun. Hipparchus appears to have been the first to 

 discover that the part of the moon's path in which the 

 motion is most rapid is not always in the same position on 

 the celestial sphere, but moves continuously ; or, in other 

 words, that the line of apses ( 39) of the moon's path 

 moves. The motion is an advance, and a complete circuit 

 is described in about nine years. Hence arises a fourth 

 kind of month, the anomalistic month, which is the period 

 in which the moon returns to apogee or perigee. 



To Hipparchus is due the credit of fixing with greater 



* The name is interesting as a remnant of a very early supersti- 

 tion. Eclipses, which always occur near the nodes, were at one 

 time supposed to be caused by a dragon which devoured the sun 

 or moon. The symbols 8 <Q still used to denote the two nodes 

 are supposed to represent the head and tail of the dragon. 



