68 BROWN : 



moving shadow. And, indeed, the moon's shadow may be 

 seen by those suitably raised above a level, far extended hori- 

 zon. Its edge is plainly enough defined and it advances east- 

 ward with a very impressive velocit}^ 



The moon's shadow, then, if it reach so far, is cut by the 

 earth's surface at varying distances from the apex. It may 

 be circular, if the shadow strikes the earth vertically, and 

 may be from i6o to 170 miles in diameter at the intersection. 

 Observers within the shadow see a total eclipse, with optical 

 effects well calculated to terrify those who in old times beheld, 

 but did not understand. The path of the shadow and the 

 day, hour and minute of its passing are calculated beforehand, 

 and though the number of conditions to be taken into account 

 makes the computation intricate, it is perfectly within the 

 scope of ordinary mathematics. 



If what was said just above regarding penumbra be 

 remembered, it will suffice to say here, that the moon's 

 shadow is precisely like the earth's in this respect, and that 

 those within the penumbra see a partial eclipse instead of a 

 total eclipse. 



There are few that have not seen an eclipse of the sun, 

 but very few that have seen a total eclipse. And yet there 

 are those who will hear a new fact when they are told that 

 about four solar eclipses happen for every three lunar eclipses. 

 More than half the earth sees every lunar eclipse. But the 

 track of the moon's shadow has a small width and length, 

 which under favorable circumstances may make up about one 

 two-hundredth part of the earth's surface. And even this 

 does not mean that one man in two hundred sees the phenom- 

 enon of a total eclipse when it occurs ; for the track may pass 

 in great part over desert land or over the open ocean. 



We have seen that the moon's diameter is 2163 miles 

 (nearly) and that she moves eastward about her own diameter 

 in an hour. This, then, would be the rate at which the 

 moon's shadow would pass the observer eastwardly. But the 

 earth also is rolling eastward, and it is the difference in the 



