170 OUTLINES OF NATURAL PHILOSOPHY. 



into account, as the times of their stations, &c. 

 may not yet have been sufficiently determined by 

 observation. 



f. On the strength of this evidence, we shall assume 

 the motion of the Earth as a fact, and try whether 

 it is consistent with the other phenomena of the 

 planetary motions. 



Orbits of the Planets. 



168. If a planet be observed twice in the same 

 node, the node in the interval being supposed 

 to remain fixed, the position of the line of the 

 nodes may be determined, and also the distance 

 of the planet from the sun at the times of ob- 

 servation, 



a. Let a superior planet be observed in its node 

 N, (fig. 19.), from the Earth at E, and after the 

 planet has made an entire revolution, and return- 

 ed to the point N, let the Earth be at E'. Then, 

 from the time, and the construction of the Earth's 

 orbit, EE' is given, and the angles SEE', SE'E. 

 But the angles SEN, SE'N, are known by obser- 

 vation 5 therefore the angles EE'N, E'EN, as also 



the 



