Attempt to apply instrumental measurement to the Zodiacal Light

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                                his observations were confined to his own high Northern latitude; 
and were therefore affected to a great and unknown extent by 
circumstances of climate and geographical position. He had much 
wished to eliminate these effects by means of observations made 
in the Southern hemisphere, but unfortunately was not able 
to obtain any: and indeed those which have been made by the 
author, & recorded in this paper are perhaps the first which have been 
published, and brought to bear on the theory of the subject. 
Cassini's conclusions were, that the Zodiacal Light 
is a flat luminous ring encircling the Sun, nearly in the plane 
of his equator, and is therefore seen always more or less in profile, 
and perfectly so at two periods of the year, April & August, 
when little Saturn's ring, and for similar reasons, he supposed it 
to vanish from our sight: explaining the non-visibility at <u>any<\u> 
period between those two months, as produced mainly by the 
overpowering effect of the lengthened summer twilight. But 
these ideas, on being tested by the Cape observations, completely fall to the ground; for during the <u>whole<\u> period of invisibility to 
Cassini, (caused in reality wholly by the lengthened twilight of 
summer in his Northern hemisphere), this phenomenon was 
most visible at the Cape, as winter then prevailed <s>during<\s> in the 
Southern Hemisphere. And indeed the very reverse effect from 
that expected by Cassini should follow, when a transparent & oblate 
luminous ring is viewed in profile, for it will then be seen at 
its brightest, on account of all the infinitely small light-gi=
ving particles being brought closer together; so small are they, 
that they can by no means be distinguished separately, or 
when thinly scattered over the sky; but only make themselves 
sensible to the eye when they are crowded together in a smaller 
space. The idea, moreover, of the Zodiacal Light being in the 
form of a <u>ring<\u> at all, is discountenanced by the observed ap=
=pearances, they being all conformable to that phenomena which 
would be afforded by a thin <u>lenticular<\u> body, excentrically situated, 
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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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