LUNAR ECLIPSES. 



in the exterior and non-luminous atmosphere forming the corona 

 necessary to explain their appearance, we must suppose their 

 height to amount to nearly a twentieth part of the sun's diameter, 

 that is to 44000 miles. 



The fact that they are gaseous and not solid matter appears, 

 therefore, to be conclusively established by their enormous mag- 

 nitude, the great height above the surface of the sun at which 

 they are placed, their faint degree of illumination, 

 and the circumstances of their being sometimes 

 detached at their base from the visible limb of 

 the sun. These circumstances render it probable 

 that these remarkable appearances are produced 

 by cloudy masses of extreme tenuity, supported, 

 and probably produced in an extensive spherical 

 shell of non-luminous gaseous matter, surrounding 

 and rising above the luminous surface of the sun 

 to a great altitude. 



Fig. 19. 



LUNAR ECLIPSES. 



39. As the earth moves in its orbit round 

 the sun it projects behind it a conical shadow, 

 the axis of which is always directed to that 

 point of the heavens which is in immediate 

 opposition to the centre of the sun. A section 

 of the sun s, and the earth e, and of the conical 

 shadow made by a plane passing through the 

 centres of the sun and earth, is shown in fig. 19. 



It will be evident whenever the globe of the 

 moon or any part of it enters within the limits of 

 the conical shadow a f a', it will be deprived of 

 the sun's light, and will consequently be eclipsed ; 

 the circumstances, therefore, which determine 

 lunar eclipses will depend upon the transverse 

 section of the shadow at the moon's distance from 

 the earth, this distance being about one-third of 

 the total length of the conical shadow. 



It is found by calculation that its greatest appa- 

 rent diameter, as seen from the earth, is 45' 42", 

 and its least 37' 49". Its mean magnitude is, 

 therefore, 41' 45". 



40. The section of the shadow may, therefore, be regarded as a 

 dark disc, whose apparent semi-diameter varies between 37' 49" 

 and 45' 42", and the true place of whose centre is a point on the 

 ecliptic 180 behind the centre of the sun. A lunar eclipse is 



181 



