LUNAR ECLIPSE 519 



of which, if extended, will fall before the opaque body. If we receive the 

 shadows so projected at vi n, for example, on a white sheet, we have in the 

 centre a dark circle, which is the umbra, surrounded by the penumbra, 

 which gradually decreases in intensity towards the exterior (see fig. 573). 

 The farther we hold the sheet from the body producing the shadow, the 

 umbra decreases, and the penumbra is enlarged. For where (in solar eclipses) 

 the umbra falls there is totality; within the penumbra partial eclipse only. 



Lunar Eclipse. Let A (fig. 573) be the sun, and B the earth, the length 

 of the umbra of the latter will 

 exceed 108 diameters of the 

 earth. Since the moon is only 

 about thirty terrestrial diameters 

 distant from the earth, and as 

 the diameter of the earth's 

 shadow, at this distance, is nearly 



three times as large as the ap- ** 



parent diameter of the moon, it Fig. 573 .- Lunar eclipse, 



follows that when the latter enters this shadow, she must be totally eclipsed, 

 for at those places where the moon's shadow falls there is total eclipse. If 

 the moon's orbit were coincident with the ecliptic, or if both moon and earth 

 moved round the sun in the same plane, there would be an eclipse at every 

 conjunction, and at every opposition, i.e., a solar eclipse would happen at 

 every new moon, and a lunar eclipse at every full moon. But we have seen 

 that the lunar orbit cuts the ecliptic only in two points ; consequently an 

 eclipse of the moon is possible only when, at the 

 time of opposition, the moon is in one of her nodes, 

 or in close proximity to it, which can only occur 

 twenty-nine times in the space of eighteen years. 



A lunar eclipse begins on the eastern margin 

 of the moon, and is either total, when her whole disc 

 enters the umbra, or partial, when only part of her 

 disc is in the shadow. A total eclipse may last for 

 two hours. 



We shall understand this better, perhaps, with 

 the diagrams. 



Solar Eclipses. When the moon and the sun are 

 in conjunction, the moon's place may be represented by 

 M (fig. 574) between the earth, T, and the sun, s. If 

 this conjunction occur when the moon is in one of her 

 nodes, or within 1 6 of it, the shadow of the moon will 

 fall upon the earth, and the sun will be eclipsed. At 

 other places the sun will not be entirely covered ; and 

 if the moon be moved farther off, so that its shadow 

 will not reach the earth, and so not cover the sun up completely, we shall 

 have an annular eclipse, because a rim of the sun will be visible. 



ig. 574. Solar eclipse. 



