A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 5 



and that they are disposed in a general way across the northern hemisphere.' 

 (See plates i. to vii. inclusive.) 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, though faintly, some of the bright bands which, 

 radiating from certain crater-like pits, extend for great distances over the surface. 

 So, too, they may see at the first 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 best modern telescopes under the most suitable conditions of 

 observation, the moon is seen as it would be by the unaided eye if it were not more 

 than about forty miles from the observer. The conditions of this seeing are much 

 more favorable than those under which we behold a range of terrestrial moun- 

 tains at that distance, for the reason that the air, and especially the moisture, in 

 our atmosphere hinders and confuses the light, and there is several times as much 

 of this obstruction encountered in a distance of forty miles along the earth's 

 surface as there is in looking vertically upwards. 



Seen with the greater telescopes, the surface of the moon may reveal to able 

 observers, in the rare moments of the best seeing, circular objects, such as pits, 

 which are perhaps not more than five hundred feet in diameter. Elevations of 

 much less height may be detected by their shadows, which, because there is no 

 trace of an atmosphere on the moon, are extraordinarily sharp, the line between 

 the dark and light being as distinct as though drawn by a ruler. Elongate 

 objects, such as rifts or crevices in the surface, because of their length, may be 

 visible even when they are only a few score feet in width, for the same reason that 

 while a black dot on a wall may not make any impression on the eye, a line no 

 wider than the dot can be readily perceived. Owing to these conditions, the 

 surface of the moon has revealed many of its features to us, perhaps about as well 

 as we could discern them by the naked eye if the sphere were no more than 

 twenty miles away. 



Separated from all theories and prepossessions, the most important points 

 which have been ascertained as to the condition of the moon's surface are as 

 follows : 



The surface differs from that of the earth in the fact that it lacks the envel- 

 opes of air and water. That there is no air is indicated by the feature above 

 noted, that there is no diffusion of the sunlight, the shadows being absolutely black 

 and with perfectly clean-cut edges. It is also shown by the fact that when a star 

 is occulted or shut out by the disc of the moon it disappears suddenly without its 

 light being displaced, as it would be by refraction if there were any sensible 



' It is 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 customary to place the west on terrestrial 

 maps. 



