80 



ECLIPSES. 



Fisr. 1. 



forms what is called the earth's penumbra. The ray m a, fig. 4, from the top of the 

 sun's disk. passes to the point/, while the ray n a from the lowest point of the 

 sun's disk passes to the point c. The space between a /and a c will be par- 

 tially illuminated by the sun. If a spectator were placed anywhere in that 

 space, he would see a portion of the upper limb of the sun, and would see more 

 of it the nearer he might be to c, and less of it the nearer he might be to/. 



As he would see the sun, he would of course receive a portion of its light. 

 Thus that part of the space included between a / and a c, which is near a/, 

 receives light from a small portion of the upper limb of the sun, while that part 

 which is near c c receives light from nearly the whole of the sun ; and in short, 

 proceeding from a f to a c, the light received from the sun will be gradually 

 increased. 



Fig. 4. 





In like manner, the ray m b proceeding from the upper limb of the sun and 

 continued to d, will include between it and the ray b f a. space which is only 

 partially illuminated, and will be subject to the same observations as we have 

 made respecting the space between a / and a c. 



When any object which receives its light from the sun passes between fhs 

 lines a c and b d, it will be either wholly or partially deprived of the sun's light. 

 If it be outside the limits 6/and a/ it will be only partially obscured ; but if 

 it be within these limits, it will be altogether darkened. 



The length of the line of being incomparably less than the distance of any 

 body in the universe from the earth except the moon, but being on the contrary 

 considerably greater than the distance of the moon, it follows that the only 

 body in the system which can be deprived of light by the earth's shadow is the 

 moon, and that whenever that object is in opposition to the sun, and at the same 

 time so near the ecliptic as to be included between the lines ac and b d, it will 



