217 



A temporary star may be regarded as a sphere having its axis of 

 rotation oblique to the direction of the star's place, as seen from the 

 earth. This sphere, moreover, may be presumed to be in a great 

 measure opaque; insomuch, that but a small spot on that portion of 

 it, turned toward the earth, would be luminous, and the situation of 

 this spot be similar to that of a star on the celestial sphere, near to 

 the circle of perpHual occultation. Such spot would come some- 

 what rapidly into view, acquiring very speedily an almost maximum 

 brightness: the subsequent diminution of its brightness would, more- 

 over, be as rapid as its increase had been, and during by far the 

 larger part of the star's rotation, the luminous portion, and of course 

 the star itself, would be invisible. This supposes the star to be very 

 large, or otherwise excessively bright, or else comparatively near; 

 or some or all of these combined. 



IV. On the Dragging of the Shadows of the Earth and other Pla- 

 nets, as well as those of their Satellites. 



Prof. A. remarked, that while the tangent ray, which at any in- 

 stant was situated in the limit of the earth's shadow, was subject to 

 the progressive motion of light, the earth itself moved onward with a 

 velocity due to its annual motion in its orbit. The limit of the sha- 

 dow would therefore pass through the points which the successive 

 tangent rays, after they had left the earth, had at any instant reached. 

 The whole shadow would, therefore, drag or fall behind the direction 

 of the line joining the centres of the sun and earth. The angular 

 amount of this dragging of the shadow would, however, be equal and 

 opposite to the sun's annual aberration; insomuch, that the direc- 

 tion oi" the shadow would be the opposite to that of the apparent place 

 of the sun, as affected by the annual aberration. A closer investiga- 

 tion would indicate an essentially similar result with respect to the moon 

 and planets, when their shadows or penumbrse fell upon the earth. 

 Hence the phenomenon of the dragging would be wholly masked, in 

 the case of an eclipse of either sun or moon, or that of the transit of 

 an inferior planet. When, however, the earth was not the body in 

 question, or the shade did not fall upon the spectator, as in the case 

 of an eclipse of one of Jupiter's satellites, an equation must be due to 

 the dragging of the shadow, though the circumstances might render 

 its effect insensible to observation. These and the preceding obser- 

 vations were illustrated by diagrams, and, — 



