4 A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 



such as characterizes the earth, unless, as is possible, there are irregularities of 

 figure on the unseen part of the sphere. It is essentially globular in form. The 

 fact that the moon is not flattened at its poles probably 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. c, 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 -^j 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 towards 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 matter cannot be explained without giving in mathematical form 

 a full account of the motion of our satellite, which is one of the most compli- 

 cated of astronomical problems. An excellent non-mathematical presentation of 

 the question, which affords a sufficient idea of it, may be found in The Moon, by 

 Richard A. Proctor, pp. 117 ei seq., D. Appleton & Co., New York, 1878. 



As noted below, there is some accessible information going to show that even 

 beyond the extreme field revealed by the librations the surface of the moon has 

 the same character as that which is visible. Thus we find that up to the limits 

 of the visible part there is no sign of change in the nature of the surface. It 

 is therefore reasonable to conclude that the same characteristics extend for some 

 distance beyond the limits of vision. We also note on the verge of the unseen 

 field the hither margins of certain ring-shaped structures, evidently of large size, 

 the so-called volcanoes, so that it is fair to conclude that these features are con- 

 tinued on the unseen part. Moreover, there are some light-colored bands, such 

 as on this side of the moon always radiate from crater-like pits, which apparently 

 come over from such centers on the unseen part. These several facts, taken to- 

 gether, make it eminently probable that the unseen four-tenths of the lunar sur- 

 face in no essential way differs from that we observe. It is, indeed, altogether 

 likely that we see every type of structure that exists on the moon, and that a 

 view of its whole area would add nothing essentially new to our knowledge of 

 the sphere. 



Seen by persons of ordinarily good vision, even at a distance of about 240,000 

 miles, the moon reveals much of its surface shape, structure, and color ; it is evi- 

 dent that the color varies greatly from very bright areas to those which are rela- 

 tively dark, that the latter are somewhat less in total extent than the former 



