ECLIPSES. 67 



sions are dealt with precisely as those of the earth's shadow, 

 with simple change of data ; and its average length is readily 

 found to be 232,000 miles. As the moon's distance from the 

 earth varies, the shadow is sometimes just long enough to reach 

 the earth, and sometimes more than this, sometimes less. 



Supposing as before an observer at the apex of this conical 

 shadow ; he would see the sun not at all — for the moon 

 would blot out that luminary exactly, no more and no less. 



If he went nearer to the moon, it is clear that he would go 

 farther into the cone of the shadow, and that the moon would 

 more than cover the sun. If, on the contrary, he went farther 

 away, he would be clear of the shadow apex. The moon 

 would not suffice to blot out the sun. A portion of the sun's 

 disk would be visible around the black circle of the moon. 

 All this is realized in fact ; the only difference in the working 

 being, that in consequence of the moon's varying distance 

 from the sun, the point of the shadow cone falls short of the 

 observer on the earth's surface, reaches him exactly, or passes 

 beyond him several thousand miles into the body of the earth 

 (as it were) and even beyond it on the other side. It must be 

 clear that in the first case the observer sees an annular eclipse ; 

 in the second a total eclipse, lasting but for an instant, and in 

 the last case a total eclipse lasting some little time. 



We have now but to remember that the moon is sometimes 

 in the ecliptic, sometimes above it, and sometimes below it. 

 Hence the irregularly recurring solar eclipse. The moon 

 comes to her nodes at periods irregular, but entirely calculable. 

 Her shadow points at, reaches or more than reaches the earth 

 at periods irregular, but entirely calculable. Whoever wishes 

 to have the "figures" of an eclipse may find them in the 

 almanac. The physical explanation of a solar eclipse is so 

 readily set out by a little orderly thought that it is not easily 

 lost when once acquired. 



It is not usually realized that we see, to all intents and 

 purposes, a total eclipse of the sun ever}^ time a cloud passes 

 fairly across his face. The sun is obscured, and we see the 



