SOLAR AND LUNAR ECLIPSES. 419 



earth and the sun, the shadow of the moon reaches the 

 earth, and we have an eclipse of the sun. The distance 

 of the moon from the earth is, however, variable. If 

 the moon, when an eclipse takes place, is at its least 

 distance from the earth, then the extreme part of the 

 umbra reaches the earth and obscures a small portion 

 of the latter ; at those parts of the earth which are 

 situated within the umbra the eclipse is total, while at 

 those situated within the penumbra the eclipse is 

 partial. 



When the moon is at its greatest distance the umbra 

 does not reach the earth at all ; the moon then appears 

 to be smaller than the sun, and those who are exactly 

 behind the point of the umbra observe an annular 

 eclipse. In most eclipses the moon does not pass 

 exactly through the straight line between sun and 

 earth ; in that case only a portion of the penumbra 

 reaches the earth, and hence most eclipses of the sun 

 are only partial. 



When the moon passes through the umbra of the 

 earth an eclipse of the moon take place, which is either 

 total or partial, according as the moon enters completely 

 or only partly into the umbra. The portions of the 

 umbra of the moon which can reach the earth can only 

 be of small extent, and only a small portion of the sur- 

 face of the earth can be at any time eclipsed by it. 

 But it is different in the case of the earth's umbra, 

 which at the distance of the moon from the earth has 

 still a diameter which is three times greater than the 

 diameter of the moon, and can thus obscure the latter 

 very easily. When the moon enters the penumbra of 



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