Zodiacal Light. 159 



We have seen that about the 25th of November its upper 

 portions reach beyond Capricornus, its vertex extending off 

 the right arm of Aquarius. From this time it moves onward, 

 sometimes more rapidly than the sun, but with an average 

 elongation of 90°, until about the 24th of February, when it 

 reaches a point a little south of the Pleiades. From the 

 latter part of February, its progress eastward has seemed to 

 me slower than before, hardly gaining one sign for the next 

 three months, scarcely ever being distinctly visible beyond 

 Castor, although neither the want of elevation above the 

 southern horizon, nor the length of the twilight, would pre- 

 vent its being seen beyond this if in reality it existed there. 

 Finally, early in April it rapidly fades away, and soon after 

 the first of May it disappears altogether. 



These facts respecting the zodiacal light are derived 

 chiefly from my own observations, made and recorded at 

 different times during the six years following 1833 ; but on 

 comparing them with the observations of Cassini, made to- 

 wards 170 years ago, a near correspondence will be found 

 between them ; and the same will be the case if the compari- 

 son be made with the tabular view of observations collected 

 from various authorities, as given by Houzeau in 1843. 



In some cases the apparent progress of this body through 

 the signs, corresponds so nearly to that of the sun, as to sug- 

 gest the idea that it is something attached to the sun, and 

 has an apparent motion due to the same cause, namely, the 

 motion of the earth in its orbit. In other cases, however, 

 its movements are too sudden, and too unlike those of the 

 sun to permit such a conclusion. 



At one time, as we have seen, its elongation from the sun 

 increases rapidly from 90° to 120°; at other times it becomes 

 for considerable periods stationary among the stars, and even 

 retrograde ; facts which seem to imply motions of its own, 

 independent of the sun and the earth ; and such motions in 

 any body thus situated, though they might be greatly modi- 

 fied by perspective, can hardly be any other than motions of 

 revolution. On this subject La Place has the following re- 

 marks, at the end of his chapter "• on the figure of the atmo- 

 sphere of the sun.'* 



