GENERAL DESCRIPTIOK OF THE MOON. 105 



It is not certain at just what stagfe in the concentration of a nebula, a 

 planet or a satellite ma}" be set off from the parent bod}"; nor can the 

 present distance of the satellite from the main sphere be assumed as 

 that at which the parting took place. It is possible that the concen- 

 tration of the parent body had gone so far that the diffused or nebu- 

 lous stage of its materials had been passed by and the more advanced 

 stage of igneous fluidity entered on. It is, however, more likely that 

 in all cases the separation occurred while the particles of matter were 

 divided as they are in a gas or vapor. As soon as the two spheres are 

 separated from one another, and so long as they remain in any meas- 

 ure fluid, the difference in their gravitative attraction on the nearer 

 and more remote part of their masses induces tides, and the effect of 

 these tidal movements, as has been shown by Prof. George Darwin, is 

 necessarily to impel the two bodies farther apart. It seems certain 

 that before the earth and the moon became essentially rigid, as they 

 now are, the effect of these tides in driving them apart must have been 

 great enough to account for a considerable part of the interval which 

 now separates them. 



In the present condition of the moon it is a sphere having a com- 

 puted diameter of 2,159.0 miles and its mean distance from the earth 

 238,818 miles. So far as has been determined the moon exhibits no 

 trace of flattening at the poles, such as characterizes the earth, unless, 

 as is possible, there are irregularities of flgure on the unseen part of 

 the sphere. It is essentially globular in form. The fact that the moon 

 is not flattened at its poles proba))ly indicates that if it once rotated 

 in the manner of the planet it ceased to do so before it became solid. 



The measure of density of the moon — i. e., the proportion of its 

 weight to its bulk — is only about six-tenths that of the earth. While 

 the earth's mean density is nearly .5.7 times that of water, that of the 

 moon is about 3.5 times as great. Thus the total gravitative force of 

 the lunar mass is to be reckoned as only about one eighty-first of that 

 of our planet. 



As the moon revolves on its polar axis but once in about a month, 

 and at a rate that tends to keep the same part of its surface turned 

 toward the earth, we should, but for the phenomenon of librations, 

 see no more than one-half of its superficial area. Owing, however, to 

 this feature, which is due to certain complications of the moon's 

 exceedingly varied movements, the satellite in effect sways in relation 

 to the earth so that at certain times we see farther to the east and at 

 others farther to the west of its center, and in the succession of these 

 movements we are able to behold somewhat more than one-half the 

 total area — in fact, about six-tenths of it. It is impossible to set forth 

 in this writing the reasons for the librations of the moon, as the mat- 

 ter can not be explained without giving in mathematical form a full 



