THE MOON. 



305 



HAS THE MOON AN ATMOSPHERE ? 



In order to determine whether or not the globe of the moon is surrounded \ 

 with any gaseous envelope like the atmosphere of the earth, it is necessary ; 

 first to consider what appearances such an appendage would present, seen at 

 the moon's distance, and whether any such appearances are discoverable upon 

 the moon. 



According to ordinary and popular notions, it is difficult to separate the idea 

 of an atmosphere from the existence of clouds ; yet to produce clouds some- 

 thing more is necessary than air. The presence of water on the surface is 

 indispensable, and if it be assumed that no water exist, then certainly the ab- 

 sence of clouds is no proof of the absence of an atmosphere. Be this as it 

 may, however, it is certain that there are no clouds upon the moon, for if there 

 were, we should immediately discover them, by the variable lights and shadows 

 they would produce. If there is, then, an atmosphere upon the moon, it is one 

 entirely unaccompanied by clouds. 



One of the effects produced by a distant view of an atmosphere surrounding 

 a globe, one hemisphere of which is illuminated by the sun, is, that the bounda- 

 ry, or line of separation between the hemisphere enlightened by the sun and 

 the dark hemisphere, is not sudden and sharply defined, but is gradual the 

 light fading away by slow degrees into the darkness. This is an effect pro- 

 duced by a portion of the atmosphere which extends over the dark hemisphere 

 being illuminated by the sun. Let A B (fig. 2) be a diameter of the moon 

 separating the enlightened hemisphere A M B from the dark hemisphere A N 

 B. Let C E D F be the upper surface of the atmosphere. Let S T be rays 

 from the sun touching the moon at A B. It is evident that the portion of the 

 atmosphere included between A T and C T, and that between B T and D T, 



Fig. 2. 



will be illuminated by the sun ; and if the moon be viewed from a distant point 

 G, then these latter portions of the atmosphere will be seen throwing a faint 

 light on a portion of the dark hemisphere, which light will become gradually 

 fainter till it dies away. This is the effect which on the earth is the cause of 

 the morning and evening twilight. 



Now, if such an effect as this were produced upon the moon, it would be 

 discoverable by us with the naked eye, and still more certainly with the tele- 

 scope. When the moon is a crescent, its concave edge is the boundary which 

 separates the enlightened from the dark hemisphere. When it is in the quar- 



ao 



