63 



result will be admitted to be the effect of his retro-rotation. Bat, 

 when precisely the same effect is produced gradually during an 

 orbital revolution, what is so likely to produce it as the same cause^ 

 the revolving body's retro-rotation on its axis, in a direction con- 

 trary to that of the revolution performed by the axis itself? 



The Marquis de la Place, in his " System of the World," 

 translated by J. Pond, F. R. S, (the late Astronomer Royal,) 

 Book i. ch. iv. p. 59 — 62, says, "The lunar disk contains a great 

 number of invariable spots which have been observed and ac- 

 curately described. They shew us that this body always presents 

 to us very nearly the same hemisphere ; it turns then upon itself 

 in a period equal to its rotation round the earth, for if we ima- 

 gine an observer placed at the centre of the moon, supposed trans- 

 parent, he will see the earth and its visual ray revolve about him, 

 and since this 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. Nevertheless, continued observation of the lunar disk 

 has discovered some small diversity in these appearances ; the 

 spots are perceived to approach to and recede alternately from 

 the limb ; those that are very near the extremity appear and 

 disappear successively by periodical oscillations, which have been 

 distinguished by the name of the libration of the moon. To form 

 a just idea of the principal causes of this phenomenon, we should 

 consider that the disk of the moon, seen from the centre of the 

 earth, is terminated by a great circle of the lunar globe, perpen- 

 dicular to the radius drawn from this centre to that of the globe. 

 It is upon the plane of this great circle that the hemisphere of 

 the moon is projected, directed towards the earth, and its appear- 

 ances arise from its motion of rotation relatively to its radius- vec- 

 tor: if it was without a motion of rotation this radius-vector 

 would trace at every lunar revolution, the circumference of a 

 great circle upon its surface, every point of which would be suc- 

 cessively turned to us ; but at the same time that the radius- vec- 

 tor traces this circumference, the lunar globe by its revolution 

 brings always very nearly the same point of its surface to this 

 radius, and consequently the same hemisphere towards the earth. 

 The inequalities of the motion of the moon produce some small 

 variations in the appearances, for the motion of rotation not par- 

 taking in a sensible manner of these inequalities, it is variable 

 with respect to its radius-vector, which thus intersects its surface 

 at different points. The lunar globe, therefore, makes, with re- 

 spect to this radius, oscillations corresponding to the inequalities 

 of its motion, which causes some part of its surface to bie alter- 

 nately concealed, and exposed to our observation. All these 

 causes produce only an apparent libration of the lunar globe ; 



