88 IN STARRY REALMS. 



considerable interest. With the aid of a good telescope 

 the moon is observed to approach close to the star, and 

 then to pass in front of it, whereupon instantly the star is 

 extinguished. The sudden character of this phenomenon 

 is that which generally strikes the observer, for he can, 

 in fact, observe with accuracy the second of time when 

 the extinction of the star takes place. This particular 

 class of observation can be made with such definiteness 

 and with such precision that it becomes of much value for 

 the determination of the position of the moon itself. If, 

 however, the moon were surrounded by an atmosphere, it is 

 quite clear that the phenomenon attending the occultation 

 of a star would be something wholly different from that 

 which we actually find it to be. In proof of this we need 

 only refer to the circumstances which can be observed 

 here when a star is in process of setting. As the star 

 gets lower and lower it gradually becomes fainter and 

 then passes to extinction. Indeed, though we talk of the 

 star " setting," no one has ever yet seen a star " set," 

 the fact being that the star has become quite invisible long 

 before it apparently reaches the horizon. If there were 

 an atmosphere surrounding the moon at all comparable 

 with that surrounding our earth, the occulted star would 

 gradually decline in lustre and be extinguished long 

 before it reached the edge of the moon. Even with a 

 lesser degree of atmosphere the place of the star as well 

 as its appearance would be largely affected by refraction, 

 and this could not fail to be noticed by the discrepancies 

 it would produce in the position of the star. 



From these various considerations it becomes certain 

 that there is no atmosphere surrounding the moon, which 



