232 A Short History of Astronomy [CH. ix. 



the case of the large planets Jupiter and Saturn (cf. chap- 

 ter vn., 144)- It was, however, proved by Newton that 

 in any system of bodies, such as the solar system, moving 

 about in any way under the influence of their mutual 

 attractions, there is a particular point, called the centre of 

 gravity, which can always be treated as at rest ; the sun 

 moves relatively to this point, but so little that the distance 

 between the centre of the sun and the centre of gravity can 

 never be much more than the diameter of the sun. 



It is perhaps rather curious that this result was not seized 

 upon by some of the supporters of the Church in the con- 

 demnation of Galilei, now rather more than half a century 

 old; for if it was far from supporting the view that the 

 earth is at the centre of the world, it at any rate negatived 

 that part of the doctrine of Coppernicus and Galilei which 

 asserted the sun to be at rest in the centre of the world. 

 Probably no one who was capable of understanding 

 Newton's book was a serious supporter of any anti- 

 Coppernican system, though some still professed them- 

 selves obedient to the papal decrees on the subject.* 



* Throughout the Coppernican controversy up to Newton's time 

 it had been generally assumed, both by Coppernicans and by their 

 opponents, that there was some meaning in speaking of a body simply 

 as being " at rest " or " in motion," without any reference to any 

 other body. But all that we can really observe is the motion of one 

 body relative to one or more others. Astronomical observation tells 

 us, for example, of a certain motion relative to one another of the 

 earth and sun ; and this motion was expressed in two quite different 

 ways by Ptolemy and by Coppernicus. From a modern standpoint 

 the question ultimately involved was whether the motions of the 

 various bodies of the solar system relatively to the earth or relatively 

 to the sun were the simpler to express. If it is found convenient to 

 express them as Coppernicus and Galilei did in relation to the 

 sun, some simplicity of statement is gained by speaking of the sun 

 as "fixed" and omitting the qualification "relative to the sun" in 

 speaking of any other body. The same motions might have been 

 expressed relatively to any other body chosen at will : e.g. to one of 

 the hands of a watch carried by a man walking up and down on the 

 deck of a ship on a rough sea; in this case it is clear that the motions 

 of the other bodies of the solar system relative to this body would be 

 excessively complicated ; and it would therefore be highly inconvenient 

 though still possible to treat this particular body as " fixed." 



A new aspect of the problem presents itself, however, when an 

 attempt like Newton's is made to explain the motions of bodies of 

 the solar system as the result of forces exerted on one another by 



