ASTRONOMY. 105 



104. The difference between the solar and si- 

 derial day, has already been shewn to consist 

 of 3 m 56 s 33* of siderial, or 3 m 55 s 49* of solar 

 time ; but the solar day thus defined, is only 

 the average, or the mean 5 for, the time actual- 

 ly employed by the sun to return from the me- 

 ridian to the meridian again, is different at dif- 

 ferent seasons of the year, 92. 



105. If the sun moved regularly forward in 

 the equator, at the rate of 59' 8".3, every day, the 

 solar days would be all equal, and their lengths 

 as above determined ; but as ^the sun neither 

 moves in the equator, nor moves in the eclip- 

 tic, at a uniform rate, there are two causes that 

 affect the length of the solar day. 



The time, therefore, which is reckoned by the arrival 

 at the meridian of an imaginary sun, or one which 

 is supposed to move in the equator, is calledmean 

 solar time. When it is reckoned by the arrival 

 the real sun on the meridian, it is called apparent 

 time. 



106. The difference between the right ascen- 

 sion of the sun, and his mean longitude, con- 

 verted into mean solar time, is the difference 



between 



