ASTRONOMY. 115 



The rate of diminution in the maximum of the equa- 

 tion, is about 18".79 in a century. 



If the rate of this diminution were to continue uni- 

 form, (which, however, we have not a right to sup- 

 pose), the earth's orbit wouid become a circle in 

 about 36300 years. 



113. From the changes in the place of the 

 apsides, in the equation of the centre of the 

 sun's orbit, and in the obliquity of the ecliptic, 

 the equation of time undergoes a change from 

 one age to another. 



a. The equation of time is equal to the mean longi- 

 tude) minus the true right ascension of the sun. 

 The former of these is not affected by the place of 

 the apogee or perigee, and depends only on the 

 distance of time from the equinox. The latter, 

 the true right ascension, is dependent on ail the 

 three elements just mentioned. The true longi- 

 tude of the sun, from which the right ascension is 

 deduced, depends on the place of the perigee, and 

 on the equation to the centre, both of which vary ; 

 and it involves also the reduction from the eclip- 

 tic to the equator, which depends on the angle 

 these circles make with one another. The equa- 

 tion of time, therefore, answering to the same day 

 of the year, varies in different ages ; but for any 

 age, past or future, maybe computed on the prin- 

 ciples already explained. 



*. The 



