GALILEO AND HIS JUDGES. 153 



must become of the old systems of astronomy ? What 

 must befall Ptolemy and even Tycho Brahe ? 



It is obvious that they could do nothing but col- 

 lapse. If the law of gravitation were once admitted 

 to be true, the idea of the Sun revolving round the 

 Earth must be dismissed as impossible. Here it is 

 right to remark that (assuming the law of universal 

 gravitation) it is not, strictly and scientifically speak- 

 ing, correct to say that any one heavenly body re- 

 volves round another, but that they both revolve 

 round their common centre of gravity. In the case 

 of the Earth and the Sun, so vastly superior is the mass 

 of the latter that the centre of gravity is far away 

 within his volume, and the disturbance exercised on 

 him by the Earth is scarcely appreciable ; so also, in 

 the case of the Moon and the Earth, the centre of 

 gravity is within the latter, but at a considerable 

 distance from its own centre ; and here there is a 

 distinctly appreciable oscillation of the Earth, arising 

 from this very cause, during each revolution of the 

 Moon in her orbit. When two bodies are more nearly 

 equal in mass, as is probably the case with the double 

 stars that have been observed in recent times, then 

 the two revolve round a centre of gravity lying 

 between them, exterior to both of them. It is be- 

 lieved that this is actually the fact in the instance I 

 am here alluding to of the double stars, and there is 

 some reason for supposing that the curve in which 

 they revolve is an ellipse. This, if true, would clearly 

 indicate that the law of gravitation, as stated by 



