46 LUNAR AND SOI^AR ECLIPSES. [SECT. v. 



diameter of the sun, as seen from the centre of the moon. 

 The mean duration of a revolution of the sun, with regard 

 to the node of the lunar orbit, is to the duration of a synodic 

 revolution (K 112) of the moon as 223 to 19. So that, after 

 a period of 223 lunar months, the sun and moon would return 

 to the same relative position with regard to the node of the 

 moon's orbit, and therefore the eclipses would recur in the 

 same order were not the periods altered by irregularities in 

 the motions of the sun and moon. In lunar eclipses, our 

 atmosphere bends the sun's rays which pass through it all 

 round into the cone of the earth's shadow. And as the hori- 

 zontal refraction (K 113) or bending of the rays surpasses 

 half the sum of the semidiameters of the sun and moon, di- 

 vided by their mutual distance, the centre of the lunar disc, 

 supposed to be in the axis of the shadow, would receive the 

 rays from the same point of the sun, round all sides of the 

 earth, so that it would be more illuminated than in full 

 moon, if the greater portion of the light were not stopped or 

 absorbed by the atmosphere. Instances are recorded where 

 this feeble light has been entirely absorbed, so that the moon 

 has altogether disappeared in her eclipses. 



The sun is eclipsed when the moon intercepts his rays 

 (N. 114). The moon, though incomparably smaller than the 

 sun, is so much nearer the earth, that her apparent diameter 

 differs but little from his, but both are liable to such varia- 

 tions, that they alternately surpass one another. Were the 

 eye of a spectator in the same straight line with the centres 

 of the sun and moon, he woulcl see the sun eclipsed. If 

 the apparent diameter of the moon surpassed that of the sun, 

 the eclipse would be total. If it were less, the observer would 

 see a ring of light round the disc of the moon, and the eclipse 

 would be annular, as it was on the 17th of May, 1836. If 

 the centre of the moon should not be in the straight line 

 joining the centres of the sun and the eye of the observer, the 

 moon might only eclipse a part of the sun. The variation, 

 therefore, in the distances of the sun and moon from the 



