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CHAPTER II. 

 The Circular Orbits of the Planets round the Sun. 



THE orbit which the earth describes round the 

 sun is very nearly a circle : the sun is about one 

 thirtieth nearer to us in winter than in summer. 

 This nearly circular form of the orbit, on a little 

 consideration, will appear to be a remarkable 

 circumstance. 



Supposing the attraction of a planet towards 

 the sun to exist, if the planet were put in motion 

 in any part of the solar system, it would describe 

 about the sun an orbit of some kind ; it might be 

 a long oval, or a shorter oval, or an exact circle. 

 But if we suppose the result left to chance, the 

 chances are infinitely against the last mentioned 

 case. There is but one circle ; there are an 

 infinite number of ovals. Any original impulse 

 would give some oval, but only one particular 

 impulse, determinate in velocity and direction, 

 will give a circle. If we suppose the planet to 

 be originally projected, it must be projected per- 

 pendicularly to its distance from the sun, and 

 with a certain precise velocity, in order that the 

 motion may be circular. 



In the basin to which we have compared the 

 solar system, the adjustment requisite to produce 

 circular motion would require us to project our 



