Attempt to apply instrumental measurement to the Zodiacal Light

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                                "of the sun's equator, which is 7[degree] inclined to the zodiac, and 
which plane seen from the sun, intersects the ecliptic in longi=
=tude 78[degree] and 258[degree], or so much in advance of the equinoctial 
points. In consequence it is seen to the best advantage at, or 
a little after, the equinoxes, after sunset at the spring, and 
before sunrise at the autumnal equinox, not only because 
the direction of its apparent axis lies at those times more 
perpendicular to the horizon, but also because at those epochs 
we are approaching the situation in which it is seen most 
completely in section. 
"At the vernal equinox the appearance of the Zodiacal 
light is that of a pretty broad pyramidal, or rather lenticular, 
body of light, which begins to be visible as soon as the twilight 
decays. It is very bright at its lower or broader part near the 
horizon, and (if there be broken clouds about) often appears like 
the glow of a distant conflagration, or of the rising moon, only 
less red & giving rise, in short, to amorphous masses of light, such 
as have been noticed by some as possibly appertaining to the comet. 
At higher altitudes its light fades gradually, and is seldom traceable 
much beyond the Pleiades, which it usually however attains and 
involves; and (what is most to my present purpose) its axis 
at the vernal equinox is always inclined (to the Northward of 
the equator) at an angle of between 60[degree] & 70[degree] to the horizon; and 
it is most luminous at its base, resting on the horizon, where 
also it is broadest, occupying, in fact, an angular breadth of some=
=where about 10[degree] or 12[degree] in ordinary clear weather." 
The ring hypothesis of Cassini has however been followed 
in a greater or less degree by La Place, Schubert and Poisson, 
as well as by Humboldt, who is an observed, and publishing 
in 1844, is the latest of all the authorities. 
His description of the general appearance of the light 
is most vivid and truthful, and can perhaps only be fully 
appreciated by those who have seen it under similar 
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Charles Piazzi Smyth
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Cite as

Attempt to apply instrumental measurement to the Zodiacal Light , 1840. From The Royal Society, AP/30/18



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