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Prof. Olmsted on the Zodiacal Light 317 



3. Motions. — The zodiacal light sometimes moves forward in 

 the order of the signs, it is sometimes stationary among the stars, 

 and sometimes retrograde- Beginning with morning observa- 

 tions in August, and noting its positions from day to day, we see 

 it first stretching across the middle of the constellation of the 

 Twins.* The vertex moves slowly along through the constella- 

 tions Gemini, Cancer, and Leo, being, on the 13th of November, a 

 little east of Gamma Leonis,t having, in three months shifted its 

 place eastward nearly three signs, and consequently nearly kept 

 pace with the sun in its annual revolution, maintaining an average 

 V elongation from that body of 90 degrees. After the middle of 



November its light fades away in the east, its vertex becomes 

 nearly stationary and of course its elongation westward of the 

 sun diminishes, until the early part of January, when it is hardly 

 visible at all in the morning sky. In the mean time, this light 

 has been rapidly rising in the evening sky. and to this we will 

 next direct our attention. 



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

 portions reach beyond Capricornus, its vertex extending to the 

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

 times 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 slov/er than before, hardly 

 gaining one sign for the next three months, scarcely ever being 

 distinctly visibTe beyond Castor^ although neitlier the want of 

 elevation above the southern horizon, nor the length of the 

 twilight, would prevent 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 disappears altogether. 



These facts respecting the zodiacal light are derived chiefly 

 from my own observations, made and recorded at different times 

 during the six vears following 1833; but on comparing them 

 with the observations of Cassini made towards 170 years ago, a 

 near correspondence will be found between them; and the same 

 will be the case if the comparison be made with the tabular view 

 of observations collected from various authorities, as given by 

 Houzeau in 1843. 



Iti some cases, the apparent progress of this body through the 

 signs corresponds so nearly to that of the sun, as to suggest the 

 i<lea that it is something attached to the sun, and has an apparent 

 niotion due to the same cause, namely, the motion of the earth 

 in its orbit. In other cases, however, its movements are too sud- 

 den and too unlike those of the sun to permit such a conclusion. 



« 



f Cassini placed it in 168^-at ;t Leonid. 



milkv 



Second Series, Vol XII, No. So!— Nov.. 1851. 41 



