The Primary Planets. 



295 



TABLE of the inclination of the orbits of the pri- 

 mary planets from the plane of the ecliptic. 



These orbits differ greatly in extent in pro- 

 portion as the planets are respectively more or 

 less distant from the central star of the system, 

 the sun. The means by which these distances 

 are ascertained have been mentioned before, 

 when we spoke of the second law of Kepler. 

 But it must be evident that we must know the 

 distance of some one planet from the sun, before 

 we can compute the distance of any other by com- 

 paring the time of its orbit with that, the distance 

 of which from the sun is known. 



As we exist upon the earth, our calculations 

 must originate from the planet we inhabit. 

 Here only we have certain grounds, and, what- 

 ever we measure of the arch of the heavens 



