Attempt to apply instrumental measurement to the Zodiacal Light

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                                A few days after the last observation in October 1845 I left the Cape, & 
the passage thence by S<sup>t<\sup> Helena through the Tropics was so uniformly 
cloudy, that I was unable to obtain another satisfactory look at the 
Zodiacal Light. And ever since my residence in Edinburgh, in 
the middle of a city glowing at night with gas, and seething 
with smoke, and under a sky but rarely clear, nd when it 
is so, not unfrequently illuminated by the Aurora Borealis, - 
I have been equally unfortunate. 
To be able to make a good observation of the Zodiacal 
light the sky should be quite free from clouds, the air pure & 
transparent; not the slightest vestige of the twilight remaining, 
the milky way far from the neighbourhood, nor the Moon 
visible, or the brighter planets, (such as Venus and Jupiter) near. 
If these circumstances be secured, and a person looks out at 
that period of the year as herein after detailed, when the ecliptic 
makes a large angle with the horizon of the place of <d>elevation<\d> 
observation, - he can hardly fail to see the phenomenon in the 
most marked degree. A beginner must be especially cautioned 
not to begin looking too soon in the twilight to discern the 
"Sun's atmosphere" under the idea haply, of catching it before 
all traces of the sun's light on the horizon are completely gone; 
and he should also be forewarned of the immense influence which 
climate and geographical position have on the visibility, and 
apparently on the form and size of the phenomenon. Thus in 
56[degree]. N. Lat. & still less further North, even were the elongation 
of the light E. & W. equal in every respect, it would still never 
appear equally visible and would but seldom be seen either 
way. In the summer the twilight would render the sight 
impossible, and in winter the sun's path is too low & oblique. 
In the spring evenings* the light would be well seen, because 
then the twilight is of a moderate length, and the Zodiacal 
light rises at an angle to the horizon of 30 degrees greater 
than the equator, and therefore does not set till long after the 
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Manuscript details

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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