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316 Prof. Ohnsled on the Zodiacal Light, 



the duration of twilight at its minimnm, the apparent elongation 

 ought to be greatest of all; whereas it is then only 60^, while 

 from the 21st of November to the 18th of Decemberj 1837, we 

 found it increase from 90^ to 120^, and this at a season of the 

 year when the elevation above the southern horizon is near its 

 minimum, and the duration of twilight is longer than 'before. t 



Nor is this an anomalous fact; the elongation has uniformly ap- ] 



peared greater in the west, during the months of December and 



January, than daring March and April. Again, at the winter 



solstice the elevation is much greater in the morning than in the 



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evening ; but the light is far more conspicuous in the west than 

 in the east. 



2. Direction. — The general direction of the zodiacal light is, 

 as its name imports, from the sun along the zodiac. Cassini and 

 Mairan thought that its axis lay nearly or quite in the plane of 

 the solar equator, making an angle with the ecliptic of seven and 

 a C[uarter degrees; and, accordingly, that its nodes must be in 

 the part of the ecliptic which the earth traverses in June and 

 November- But Cassini himself remarked, that the direction of 

 the axis is not always the same. On several occasions the vertex 

 appeared to him to veer to the northward of its previous direc- 

 tion, so that, while it would at one time just graze Alpha Arielis 

 on its northern border, shortly afterwards that star would be 

 wholly within it. Before I had met with these statements in 

 Cassini, I had several times remarked the same changes in the 

 direction of the axis, the vertex sometimes lying in the ecliptic 

 itself Nor, as I think, will the observations warrant the conclu- 

 sion that the axis of this body cuts the sun and consequently lies 

 across the ecliptic in the plane of a great circle. On the 19th of 





January, 1835, the northern border was 8^ south of Castor, and 

 the vertex directed to a point south of the Pleiades. Conse- 

 quently, its axis could not have been far from the ecliptic. But, 

 on the 20th of March, the vertex reached above tlie 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 equa- 

 tor is not constant. In the year 1843. M. Houzeau published 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 occa- 

 sional 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. 



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