43l 



Hipparchus 



57 



moon (M') would be in the opposite direction to the sun 

 as seen from the earth, and would consequently be obscured 

 by the shadow of the earth. 



As, however, the moon's path is inclined to the ecliptic 

 ( 40), the latitudes of the sun and moon may differ by 

 as much as 5, either when they are in conjunction, i.e. 

 when they have the same longitudes, or when they are 



FIG. 25. The earth's shadow. 



in opposition, i.e. when their longitudes differ by 180, 

 and they will then in either case be too far apart for an 

 eclipse to occur. Whether then at any full or new moon 

 an eclipse will occur or not, will depend primarily on the 

 latitude of the moon at the tim^, and hence upon her 

 position with respect to the nodes of her orbit ( 40). If 

 conjunction takes place when the sun and moon happen 



M 



S ECLIPTIC 5' 



FIG. 26. The ecliptic and the moon's path. 



to be near one of the nodes (N), as at s M in fig. 26, the 

 sun and moon will be so close together that an eclipje 

 will occur ; but if it occurs at a considerable distance from 

 a node, as at s' M', their centres are so far apart that no 

 eclipse takes place. 



Now the apparent diameter of either sun or moon is, 

 as we have seen ( 32), about | ; consequently when their 

 discs just touch, as in fig. 27, the distance between their 

 centres is also about J. If then at conjunction the dis- 

 tance between their centres is less than this amount, an 



