2 OUTLINES OF NATURAL PHILOSOPHY. 



having all made a progress toward the west, 

 where some have set, while others have risen in 

 the east. More attentive and continued obser- 

 vation shews, that they all describe circles ha- 

 ving the same point for their Pole, or that from 

 which the parts of the same circumference are 

 all at an equal distance. 



The stars, therefore, move exactly as if they were 

 fixed in the surface of a sphere, which revolved on 

 its axis in a space of time nearly equal to 24? hours. 

 This motion, which is common to all the heavenly 

 bodies, is called the Diurnal Motion. 



3. If we observe the place of the Moon on 

 two successive nights, we shall find that she 

 changes her position among the stars, and ad- 

 vances to the eastward at the rate nearly of 13 

 degrees in 24< hours ; so that she completes the 

 circle of the heavens in 27 days nearly. 



a. Not only the Moon, but the Sun, and ten other 

 stars, (five of them visible only with the telescope), 

 besides the diurnal motion, have motions east- 

 ward, relatively to the other stars. 



b. The stars which have no sensible motion relatively 

 to one another, are called Fixed Stars ; the others 

 are called Planets, and will afterwards be more 

 fully considered. Besides the planets, there are 

 also stars which do not remain permanently visible, 

 and which have motions of their own. These are 



the 



