168 OUTLINES OF NATURAL PHILOSOPHY. 



This is evident, because CF' is equal to D'B, and 

 makes the same angle with CD. 



Hence it is evident, that an object, without being 

 really at rest, may be apparently so, if the observer 

 is in motlori, and may even acquire an apparent 

 motion, in a direction contrary to its real. Thus 

 also the observer and the object both moving with 

 perfect regularity, both describing concentric 

 circles, for example, with uniform velocities, and 

 directed the same way, the pne may become sta- 

 tionary in respect of the other, and even acquire 

 a motion in an opposite direction. 



167* Suppose f? to be the distance of a planet 

 (imagining it to describe a circle round the sun) 

 from the sun, or the radius of its orbit, and t 

 the elongation from the sun, at which it appears 

 stationary, the radius of the circle in which the 

 observer must move, in order to see the planet 

 stationary at that elongation, being called *r, is 

 found from the equation, a? + d x = d* cot* e ; 





. This follows from what KEIL has demonstrated in 

 his Astronomy, Sect. 27. ; LEMONIER, Institutions 



Astronomiques, 



