86 The Earth considered as a Planet. 



plane of its orbit ; but the earth's annual 

 motion in an elliptic orbit, causes the sun's 

 apparent motion in the heavens to be un- 

 equal. When the sun's annual motion 

 in the heavens appears slowest, it is noon, 

 or 12 on the sun-dial, before it is the 

 same hour on a true-going clock ; and 

 when quickest, it is 12 by the clock be- 

 fore the sun be over our meridian. 



Although the earth be said to complete 

 an orbit round the sun in the course of a 

 year, it does not return exactly to the 

 place it set out from, neither is its cir- 

 cuit completed exactly in a year. Adopt- 

 ing, therefore, the apparent motion in- 

 stead of the real ; if the sun set out as 

 from any star, or other fixed point in the 

 heavens, at the moment it is departing 

 from the equinoctial, or from either tro- 

 pic, it will come to the same equinox, or 

 tropic again, 20 min. 17- sec. of time, 

 or 50 sec. of a degree, before it completes 

 its circuit round the heavens, so as to ar- 

 rive at the same fixed star or point from 

 whence it sets out ; for the equinoctial 

 points recede 50 seconds of a degree west- 

 ward every year, contrary to the sun's 

 annual progressive motion. This is call- 

 ed the Precession of the Equinoxes, 



