64 



they are mere optical illusions, and do not affect its real motion 

 of rotation." 



It would scarcely be credible that this should have proceeded 

 from such a man as La Place, were it not in perfect consistency 

 with the prevalent opinions of astronomers on the subject of the 

 moon's rotation. " An observer," he says, " placed at the centre 

 of the moon, supposed transparent, zvillsee the earth and its visual 

 ray revolve about himP The meaning of this passage is not very 

 obvious ; but probably it is this : The observer must be perfectly 

 separate from, and independent of, the moon's mass, and capable 

 of revolving about the earth unaffected by the moon's motion. In 

 this case, says La Place, he might revolve without rotating, while 

 the moon does rotate in the direction alleged, from west to east : 

 and the result would be, that the same point of the moon's cir- 

 cumference, with the radius-vector and the earth's visual ray, or 

 the earth herself, would appear successively on every side of the 

 observer, or, as La Place says, would revolve about him. But 

 the observer's apparent standing still in these circumstances is 

 nothing else than that retro-rotation from east to west which has 

 been mentioned so often ; and the fact of his retro-rotation might 

 be proved by attaching the observer by a cord to that point of the 

 moon's circumference that is always in one with the radius. At 

 the completion of his revolution the cord would be drawn round 

 about him ; and this could not have been occasioned by the radius 

 revolving about him, for that is physically impossible. He will 

 be placed in circumstances precisely similar to those of the walk- 

 ing gentleman, whose case we have so recently considered ; and 

 either gentleman might be very well represented by a thimble or 

 a cork placed on the axis of the magnetic card of a marine com- 

 pass carried round a circle. As the box revolves, with the same 

 point of its circumference continually directed to the centre of 

 the orbit, that point, with the centre itself and the radius-vector, 

 will undoubtedly appear successively on every side of the gen- 

 tleman's representative ; not, however, because they revolve about 

 him, which is impossible, but because he is continually turning 

 himself round in a direction contrary to that which is taken by 

 his own axis and the box that contains it, in performing their or- 

 bital revolution. 



La Place adds, '^ since the earth's visual ray always intersects 

 nearly the same point of the lunar surface, it is evident that this 

 point must revolve round the spectator, in the same time, and 

 in the same direction as the earth." So here we have a broad 

 assertion of the proposition, that not only that point of the 

 moon's circumference which remains in one with the radius- 

 vector, but also the radius itself, and the earth which it repre- 

 sents, actually revolve round about the moon!s centre. Such a re- 



