106 GENERAL DESCRIPTION OF THE MOON. 



account of the motion of our satellite, which is one of the most com- 

 plicated of astronomical problems." 



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

 that even beyond the extreme field revealed by the librations the sur- 

 face 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, the so-called volca- 

 noes, evidently of large size, so that it is fair to conclude that these 

 features are continued 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 apparentl}' come over from such centers 

 on the unseen part. These several facts, taken together, make it 

 eminently probable that the unseen four-tenths of the lunar surface in 

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

 likely that we see ever}' type of structure that (^xistson 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 ordinaril}' good vision, even at a distance of 

 about 240, (XH) miles, the moon reveals much of its surface shape, struc- 

 ture, and color; it is evident that the color varies greatly from very 

 bright areas to those which are relatively dark, that the latter are 

 somewhat less in total extent than the former, and that they are dis- 

 posed in a general way across the northern hemisphere.'' Persons of 

 more than usually good vision may, under favorable conditions, see on 

 the edge of the illuminated area the ragged line of the sunlight, which 

 indicates that the surface is very irregular, the high points coming 

 into the day before the lower are illuminated. Such persons at time 

 of full moon can also note, thougli faintly, some of the bright ])ands 

 which, radiating from certain crater-like pits, extend for great distances 

 over the surface. So, too, they may see at the tirst stage of the new 

 and the last of the old moon, the light from the sunlit earth slightly 

 illuminating the dark part of the lunar sphere, or, as it is often termed, 

 the old moon in the arms of the new. 



With the ])est modern telescopes under the most suitable conditions 

 of observation the moon is seen as it would be b}' the unaided eye if 



«An excellent noninathematical presentation of the question, which affords a 

 sufficient idea of it, may be found in The IMoou, by Richard A. Proctor, p]i. 117 

 et seq., D. Appleton & Co., New York, 1878. 



f'ltis well to note the fact that in a celestial telescope objects are seen in reverse 

 position, or "upside down." For convenience they are usually so depicted on maps 

 and pictures of the moon; the north pole at the bottom, and the east where it is cus- 

 tomary to place the west on terfestrial maps. 



