ASTRONOMY. 



mean. See KEIL'S Astronomy ', Sect. 25. p. 322., 

 &c. 



f. When the equation of time is a maximum, the 

 length of the mean and apparent day is the same. 

 When the equation of time is nothing, the lengths 

 of the apparent and the mean day differ most from 

 one another. On the 16th of June, for example, 

 when the equation of time vanishes, the day is 

 12".5 shorter than the mean solar day. The dif- 

 ference becomes less and less till the 26th or 27th 

 of July, when the equation of time is stationary or 

 a maximum^ being then 6 ra 5 s nearly; and the 

 apparent day is then in length equal to the mean. 

 The difference of the days increases from thence ; 

 and on the beginning of September, the apparent 

 day is 1 8 s longer than the mean, Sac. 



d. Clocks ought to be regulated by the mean solar 

 time; and when they are adjusted by the sun's 

 passing the meridian, the equation of time must be 

 applied. 



e. The mean length of the day is thus accurately de- 

 termined. But in the reckoning of time for the 

 purposes of astronomy, and of civil life, we must 

 not count by days only, but by that assemblage of 

 days which constitutes a year, and which is natu- 

 rally pointed out as a division of time by the re- 

 turn of the seasons. The sort of incommensura- 

 bility that exists between the lengths of the day 

 and of the sun's revolution, renders it somewhat 



difficult 



