524 ADDITIONS. 



might be a preparation for the reception of the Copernican system; 

 but it is very different from the doctrine that the Sun is the centre of 

 the Planetary Motions. 



CHAPTER II. 

 THE COPERNICAN THEORY. 



The Moon's dotation. 



THAVE said, in page 264, that a confusion of -mind produced by 

 the double reference of motion to absolute space, and to a centre of 

 revolution, often leads persons to dispute whether the Moon, while she 

 revolves about the Earth, always turning to it the same face, revolves 

 about her axis or not. 



This dispute has been revived very lately, and has been conducted 

 in a manner which shows that popular readers and writers have made 

 little progress in the clearness of their notions during the last two or 

 three centuries ; and that they have accepted the Newtonian doctrines 

 in words with a very dim apprehension of their real import. 



If the Moon were carried round the Earth by a rigid arm revolving 

 about the Earth as a centre, being rigidly fastened to this arm, as a 

 mimic Moon might be, in a machine constructed to represent her mo- 

 tions, this contrivance, while it made her revolve round the Earth, 

 would make her also turn the same face to the Earth : and if we were 

 to make such a machine the standard example of rotation, the Moon 

 might be said not to rotate on her axis. 



But we are speedily led to endless confusion by taking this case as 

 the standard of rotation. For the selection of the centre of rotation in 

 i system which includes several bodies is arbitrary. The Moon turns 

 all her faces successively to the Sun, and therefore with regard to the 

 Sun, she does rotate on her axis ; and yet she revolves round the Sun 

 as truly as she revolves round the Earth. And the only really simple 

 and consistent mode of speaking of rotation, is to refer the motion not 

 to any relative centre, but to absolute space. 



This is the argument merely on the ground of simplicity and con- 

 sistency. But we find physical reasons, as well as mathematical, for 

 referring the motion to absolute space. If a cup of water be carried 

 x>::nd a centre so as to describe a circle, a straw floating on the surface 



