ASTRONOMY. 443 



disc is eclipsed, the quantity of the eclipse is said 

 to be six digits. When the diameter of the shadow 

 through which the moon must pass, is greater 

 than the diameter of the moon, the quantity of 

 the eclipse is said to be more than 1 2 digits ; thus 

 if the diameter of the moon is to that of the shadow 

 as four to five, then the eclipse is said to be 1.5 

 digits. The duration of a lunar eclipse is various, 

 but never exceeds two hours. 



The eclipses of the sun are owing to a different 

 cause than those of the moon. They are occasioned 

 by the moon's coming directly between us and the 

 sun, and, therefore, obstructing our view of it. 

 When the moon happens to be in conjunction with 

 the sun, or between the sun and the earth, viz. at 

 the time of the new moons, the shadow of the 

 moon falls upon the surface of the earth ; hence, 

 properly speaking, such eclipses should be called 

 eclipses of the earth. But the whole disc of the 

 earth cannot be involved in the shadow of the 

 moon, because the moon is much smaller than the 

 earth, and the shadow of the moon is conical. 

 Thus, in Plate 24. Fig. 3., the rays of the sun 

 S, being intercepted by the moon L, form the 

 conical shadow C D G, which falling upon the 

 surface of the earth, entirely deprives the portion 

 of it upon which it falls, of the sun's light, and, 

 of course, the inhabitants of that part of the earth 

 will have a total eclipse of the sun. Beyond the 

 dense conical shadow C D G, there is a diverging 

 half shadow, or penumbra, CDEF, which is oc- 

 casioned by the moon's intercepting only a part of 

 the sun's rays from those places which fall within 

 this penumbral cone, and are out of the dense 

 shadow. Thus from the part of the earth Z, the 

 portion Y Y B, of the sun only can be seen ; con- 



