I 

 ASTRONOMY. 81 



The mean interval of time between the sun's passing 

 the meridian one day, and his passing it the next, 

 is called a mean solar day. 



The solar exceeds the siderial day by 3 m 56 s 33* of 

 siderial time ; or by 3m 55 55* of solar time; and 

 the lengths are in the ratio of 1.0027399 to 1. 



81. The sun does not move eastward in the 

 plane of the equator, but in the plane of a cir- 

 cle cutting it obliquely in the two opposite 

 points, already referred to, of the Vernal and 

 Autumnal Equinoxes, and making, with it, an 

 angle of 23 27' 30" nearly. 



a. That the sun's motion is all in one plane, may be 

 shewn by observing his right ascension and decli- 

 nation, every day at noon, and marking it offupon 

 a globe, on which a great circle, representing the 

 equator, has already been described ; or, which is 

 better, by remarking, that the sine of the right as- 

 cension has always the same ratio to the tangent 

 of the declination. This could not be, unless the 

 plane passing through the sun and the vernal equi- 

 nox, made a constant angle with the equator, such 

 that the radius had to its tangent a ratio the same 

 with the preceding. 



The circle which the sun thus appears to describe in 

 the heavens, is called the Ecliptic, and the angle 

 which it makes with the equator, is called the Obli- 



VOL. II. F quity 



