SURFACE FEATURES OF THE MOON WRIGHT 173 



fact tliat mosseiigers are continually reaching us from the moon in 

 the form of reflected sun's rays; they will teach us much if we can 

 decipher and interpret their messages correctly. 



THE MOON'S SURFACE 



The general features of the moon's surface are shown in plates 2 

 to 4, The dark smooth areas of plates 2 and 4 are called seas or 

 maria ; the lighter areas bordering the maria are the mountains ; the 

 features of circular outline are called craters because of their resem- 

 blance to terrestrial craters. The craters dominate many parts of 

 the moon's surface and are remarkable for their range in size and 

 for their frequency. Of small crater^ there are literally thousands 

 spread over the surface of the moon. The larger craters greatly ex- 

 ceed in dimensions terrestrial craters. Many of the craters, located 

 in the maria, have smooth floors, level with the ground of the sur- 

 rounding country; other craters are much deeper and less smooth; 

 in many of these craters there is a central hill or series of peaks 

 which rise from the crater floor; on several of these peaks there is 

 perched, in turn, a small crater (pi. 3). The area covered by a 

 mare is greater than that of any one of the great plains regions of 

 the earth. Mare Imbrium, which occupies the central portion of 

 plate 2, is 800 miles across. The maria are relatively late formations 

 and spread, as floods over preexisting craters and other features, sub- 

 merging them either completely or nearly so. 



One of the most impressive craters on the moon is Copernicus 

 (pi. 3) ; it is 56 miles across and 13,500 feet deep, about as deep as 

 Mount Blanc is high, and with central hills rising 2,400 feet above 

 its floor. The simplest method for measuring the elevation of a lunar 

 feature above the adjacent country is to ascertain the length of its 

 shadow when it is near the terminator or the limit of illumination 

 across the moon's disk. We know at any given time and for any 

 point on the moon's surface the angle which the sun's rays make 

 with the vertical to the moon's surface at that point, so that it is 

 a simple task to compute from the given angle and the length of the 

 shadow the height of the feature casting the shadow. Another 

 method is based on the shift in longitude relations between adjacent 

 features of different elevations with changes in libration. Still an- 

 other method is the stereoscopic method, which also is based on phe- 

 nomena due to libration. The terraced inner walls of Copernicus are 

 conspicuous ; also the rays or streaks which emanate from it and ex- 

 tend for great distances across Mare Imbrium. The most pro- 

 nounced rays, however, radiate from Tycho (pi. 4) ; this crater is 

 54 miles across and 17,000 feet deep; a central hill rises 5,200 feet 

 from its floor. It is located in a part of the moon which is domi- 

 nated by craters large and small and of different ages. The more 



