The Primary Planets. 287 



tractions of the other planets. Let A E P G 

 (PL XXVI. fig. 112.) be an ellipsis, or the course 

 of a planet. The central star or sun is at S, 

 which is one of the foci. 



The second law of Kepler is, that the squares 

 of the times of the revolutions of the planets are 

 as the cubes of their mean distances from the sun. 

 That is, if we compare the square of the time 

 which any two of the primary planets occupy in 

 completing their orbits, we shall find between 

 these two squares the same proportion as between 

 the cubes of the mean distances S E of these two 

 planets from the sun. Thus, if we know the 

 times of the revolution of two planets, we can 

 thence compute what are their respective dis- 

 tances from the sun ; and if we are made ac- 

 quainted with the true distance of the one, we 

 shall easily find the true distance of the other, as 

 indeed the distances of all of which we know the 

 time of their periodical revolutions. 



Thus, if we suppose the planet Venus to re- 

 volve round the Sun in 224 days, and the Earth 

 in 365 ; and if we admit the mean distance 

 of the earth from the sun to be 95 millions of 

 miles then, as the square of 365 is to the square 

 of 224, so will be the cube of 95,000,000 to a 

 fourth number, which will show the cube of 

 Venus's mean distance from the sun ; and if the 

 cube-root of this number is found, it will give 

 about 68 millions of miles for the mean distance 

 of Venus from the Sun. 



