520 SCIENTIFIC RECREATIONS. 



The lunar umbra extends from the moon by a space about equal to her 

 distance from the earth, and hence only a small portion, d, of the earth's 

 surface enters the lunar umbra. To the inhabitants of this part of the earth 

 the sun will be totally eclipsed, and the eclipse will be annular if only the 

 margin of the sun's disc remain uneclipsed by the lunar shadow. This is only 

 possible when the moon is in her apogee, or greatest distance from the earth, 

 where her apparent diameter is less than that of the sun, which it cannot in 

 general exceed more than i' 38". Hence the duration of a total eclipse 

 of the sun cannot be more than 3 \ minutes. 



On the contrary, the penumbra of the moon is diffused over a much 

 larger portion, n m, of the surface of the earth, since its section is five-ninths 

 of the earth's diameter. The inhabitants of this portion of the earth do not 

 receive light from all parts of the sun, consequently a part of this luminary 

 is invisible to them, and the eclipse is said to \*z. partial. 



Solar eclipses commence on the western margin of the sun, and advance 

 to the eastern. On account of the proximity of the moon to us, an eclipse 

 of the sun is, in all places above the horizon of which the sun appears, 

 visible neither at the same time, nor is it of equal duration, nor of equal 

 extent : in some parts it may not be visible at all. In favourable situations, 

 the diameter of the umbra, where it reaches the earth, amounts to about 167 

 miles, and on this small strip of the earth's surface only can the sun appear 

 totally eclipsed. 





Fig 575. Lord Rosse s monster telescope. 



