A Short History of Astronomv 



[Cn. II. 



eclipse of the sun will take place ; if not, there will be no 

 eclipse. It is an easy calculation to determine (in fig. 26) 

 the length of the side N s or N M of the triangle N M s, 

 when s M has this value, and hence to 

 determine the greatest distance from the 

 node at which conjunction can take place 

 if an eclipse is to occur. An eclipse of 

 the moon can be treated in the same way, 

 except that we there have to deal with the 

 moon and the shadow of the earth at the 

 distance of the moon. The apparent size 

 of the shadow is, however, considerably 

 greater than the apparent size of the moon, 

 and an eclipse of the moon takes place if 

 the distance between the centre of the moon and the centre 

 of the shadow is less than about i. As before, it is easy 

 to compute the distance of the moon or of the centre of the 

 shadow from the node when opposition occurs, if an eclipse 

 just takes place. As, however, the apparent sizes of both 

 sun and moon, and consequently also that of the earth's 

 shadow, vary according to the distances of the sun and 



FIG. 27. The sun 

 and moon. 



FIG. 28. Partial eclipse of 

 the moon. 



FIG. 29. Total eclipse of 

 the moon. 



moon, a variation of which Hipparchus had no accurate 

 knowledge, the calculation becomes really a good deal more 

 complicated than at first sight appears, and was only dealt 

 with imperfectly by him. 



Eclipses of the moon are divided into partial or total, 

 the former occurring when the moon and the earth's 

 shadow only overlap partially (as in fig. 28), the latter 



