276 Astronomy. [Lecture 18. 



the present. It is very well done, though not 

 in an elementary manner, in Laplace's elegant 

 Systeme du Monde. 



The fixed stars appear every day to make an 

 entire revolution round the earth. The sun, I 

 have said, makes the same apparent diurnal re- 

 volution. But the diurnal motion of the sun 

 is apparently slower than that of the fixed stars. 

 It is almost needless to repeat to you that these 

 appearances are caused by the daily rotation of 

 the earth upon its axis, which is accomplished 

 in 23 hours 56 minutes and 4 seconds. If, 

 however, the earth only turned upon its axis; 

 and if while it turned in this manner it did not 

 advance in its orbit, the apparent diurnal move- 

 ments of the sun and fixed stars would always be 

 the same. The stars which had passed once 

 over the same meridian , with the sun would 

 constantly repeat the same movement in the 

 same time; the winter and the summer nights 

 would at the same place present the same con- 

 stellations. But because of the annual motion 

 of the earth from west to east round the sun, in 

 which it advances about 59 minutes and 8 

 seconds o^a degree in a day, the sun appears to 

 advance in the same proportion in the ecliptic. 

 This constitutes the difference between solar and 

 sidereal time, in explaining which I shall make 

 use both of the figure and the words of Mr. 

 Ferguson. 



" Let ABCDEFGHIKLM be the earth's 



