152 



SE VENTEENTH CENTUR Y. 



PT. III. 



and the planets pull the sun ; but our sun has 700 times 

 more atoms in it than all the planets put together, and so it 

 keeps them moving round it. In the same way our earth 

 has eighty times more atoms in it than our moon, and so it 

 keeps the moon moving round it. 



In this way the force of gravity keeps all the different 

 planets in their paths or orbits. It does not set them moving ; 

 some other force unknown to us first started them across the 

 sky gravitation is only the force which determines the direc- 

 tion in which they move. 



It was a grand thing to have discovered this force, but it 

 would have been of little value to Astronomy to know that 

 the heavenly bodies attracted each other unless it could also 

 be known how much influence they have upon each other. 

 This also Newton worked out accurately. You will remem- 

 ber that Kepler had shown that planets move in ellipses, 

 having the sun in one of the two foci (see fig. 10, p. 99). 

 Knowing this, Newton was able to calculate how much the 

 sun attracts a planet when it is near, and how much when it 

 is far off, so as to make it move in an ellipse ; and he found 



that exactly as much as the 

 square of the distance increases, 

 so much the attraction de- 

 creases ; that is, the attraction 

 grows less and less at a regular 

 rate as you go farther away 

 from the body that is pulling. 

 For instance, suppose that 

 at the point i, fig. 24, a planet 

 was one million of miles away 

 from the sun, and was being 

 attracted with immense force. When it arrived at the point 



',3 



6 



