158 Professor Denison Olmsted on the 



Consequently, its axis could not have been far from the 

 ecliptic. But on the 20th of March, the vertex reached 

 above the Pleiades, and the axis had perceptibly veered 

 northward from the ecliptic. These observations, taken in 

 connection with those of Cassini, indicate that the supposed 

 relation of this body to the solar equator is not constant. In 

 the year 1843, M. Houzeau publishe'd an article in the 

 Astronomische Nachrichten, in which he investigated the 

 plane of symmetry of the zodiacal light, from data derived 

 from a comparison and digest of all the observations he 

 could collect. He makes the inclination less than half that 

 of the solar equator, and the place of the nodes of course 

 quite different from that assigned to them by Cassini. 



If, then, as is demonstrated by Houzeau, the normal 

 place of the axis gives it an inclination of only about 3^°, 

 the great occasional deviations from this direction confirm 

 our remark, that the course of the zodiacal light along the 

 zodiac is not always the same, but is subject to vary with the 

 seasons of the year. 



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 

 observations in August, and noting its positions from day to 

 day, we see it first stretching across the middle of the constel- 

 lation of the Twins.* The vertex moves slowly along through 

 the constellations Gemini, Cancer, andLeo; being, on the 13th 

 of November, a little east of Gamma Leonis,f having, in three 

 months, shifted its place eastward nearly three signs, and, 

 consequently, nearly kept pace with the sun in its annual re- 

 volution, maintaining an average elongation from that body 

 of 90°. 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 meantime, this light has been rapidly rising in the 

 evening sky, and to this we will next direct our attention. 



• Above this point the light is blended with that of the milky way. 

 t Cassini placed it in 1686 at ;^ Leonis. 



