448 Eclipse of the Moon. 



well known that the planets, primary and secondary, in their 

 circuit round the sun, must project a cone of darkness point- 

 ing into the void behind them, which becomes visible, as a 

 shadow, when it sweeps across any thing to be obscured by it. 

 The shadow must consist of two parts; one of which includes 

 the space in which the sun is totally hidden by the planet : 

 this ends in a point, and is surrounded by the other part of 

 the shadow, which extends widening, as far as the sun's rays 

 serve to give a boundary to it, including all the space in which 

 the planet can appear to touch upon, or obscure any por- 

 tion of the sun's disc : this is the penumbra. Hence arise 

 the appearances of the moon when eclipsed in the earth's 

 shadow. A section of this shadow would present a darker 

 disc, included by a rim of ill-defined obscurity, across which 

 the full orbed satellite passes in entering or leaving the 

 deeper shade which circumscribes it. The moon's appearance 

 in both cases is much modified by the earth's atmosphere. 

 The breadth of .the exterior lighter shadow ought to be gene- 

 rally, at the distance of the moon, greater than the breadth of 

 the moon's disc, but in the late eclipse it was only about one- 

 third of this breadth ; this arises from the bending of the sun's 

 rays in passing through the earth's atmosphere, in such a 

 manner that the greatest proportion of the sun's light reaches 

 any part of the moon's disc, even after a large part of the 

 earth is interposed between them. From the same cause it 

 is that the moon scarcely ever disappears, even in the darkest 

 part of the earth's shadow. In fact, if the atmosphere ex- 

 erted no influence on the sun's light, except that of bending 

 it, the darker cone of the earth's shadow would be so much 

 shortened that it would fall within the moon's orbit ; and that 

 luminary, in passing behind the earth, would be subjected to 

 only slight variations of brilliancy; when opposite the earth, 

 it would then be intersecting a cone of condensed light, and 

 shining with greater radiance than usual: the earth's atmo- 

 sphere being to the moon like the rim of a gigantic lens, appear- 

 ing to augment the objects situate at a due distance beyond it. 

 Such, however, is the absorbing power of the denser portions 

 of the earth's atmosphere, that only a small portion of tin 

 light which it would concentrate, if perfectly transparent, 

 can pierce through it to the moon. 



La Place, who, in the Systeme clu Blonde, mentions these ' 

 appearances, has calculated that so little of this refracted 

 light reaches the moon, that its centre cannot then be brighter 

 than one-fiftieth part of its brilliancy at full moon, and must 

 generally be much less illuminated than this. The appear- 

 ance of the moon will depend considerably on the nature of 

 the outline of that hemisphere of the earth turned towards it ; 

 and if the atmosphere over the bounding meridian be much 



