C 24> Astronomy. [Lecture 24-. 



(fig. 10), and that of the sun from A to B, it 

 will easily be seen that an eclipse of the sun 

 must always begin in its western edge A. 



As the moon is considerably less than the 

 earth, her. shadow forms a cone NOC (fig. 10) 

 whose section is much less than the earth, so 

 that only a small portion, D E, of the earth is 

 involved in the shadow at one time. Hence it 

 is that an eclipse of the sun is not perceived at 

 the same instant in every part of the hemisphere 

 which is thus turned towards the sun, and that 

 in some parts it will not be seen at all. More- 

 over, in different situations, different parts of the 

 sun's disc will appear eclipsed ; for those who 

 are in F see him eclipsed in the part I B, and 

 those who are in G see him eclipsed in the part 

 KA. On the contrary, an eclipse of the moon 

 is perceived at the same moment in every part 

 of the earth where this planet is visible, and ap- 

 pears every where to occupy the same portion of 

 her disc. It is for this reason that eclipses of 

 the sun are much less frequent in any particular 

 place than eclipses of the moon. 



If the moon's nodes constantly corresponded, 

 with the same points in the heavens, the eclipses, 

 whether of the sun or the moon, would take 

 place in the same months, and even on the same 

 days ; but as the nodes shift backwards, or con- 

 trary to the earth's annual motion about 19 {- 

 degrees in the year, the same node will come 

 round to the sun about 19 days sooner every 



