Attempt to apply instrumental measurement to the Zodiacal Light

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                                of mere ideas, should not be allowed to form the data in this par=
=ticular branch of Astronomy any longer. 
The general results to be deduced from the data given 
in the Table, are, 
1<sup>st<\sup>. that the Zodiacal Light is a body of a lenticular form, 
spread out nearly in the plane of the Earth's orbit, and ex=
=tending almost equally from the Sun in every direction. Were 
the ordinary European observations made about the time of 
the spring equinox, the only ones existing, - we should merely 
be entitled to <s>conclude<\s> infer the existence of a one lobed projection 
from the Sun; but when we combine therewith the Cape 
Observations, we find that the body is seen all through the 
year and on both sides of the Sun, of pretty nearly the same 
size and shape, viz; a curvilinear-sided wedge, in which the 
light continually increases from the borders towards the centre 
of the base, or the actual position of the Sun; appearances which 
can only be satisfied by a lenticular body seen in section. 
2<sup>nd<\sup>. The Zdiacal Light is proved to be excentrically disposed 
about the sun,* by the elongations observed East and West on 
the same day being different; showing indeed at various 
times an excentricity of from 1/20 to 1/10. 
3<sup>rd<\sup>. The Zodiacal Light may also be considered to rotate about 
the Sun, and to be brighter in some parts than in others; 
because, it is observed to be of different lengths and degrees 
of brightness at corresponding periods in successive years. 
For although such an effect may be produced by a periodical 
alteration in the size & general lustre of the body, still the supposi=
=tion of such a rapid material change in so large a member 
of the Solar system, is extremely improbable, whilst this body's 
revolving may be held to be necessary according to the prin=
=ciples of gravitation, for otherwise the component particles 
would speedily fall into the sun; and, that some portions 
are brighter than others, follows partly as a consequence of the 
                            
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Manuscript details

Author
Charles Piazzi Smyth
Reference
AP/30/18
Series
AP
Date
1840
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Attempt to apply instrumental measurement to the Zodiacal Light , 1840. From The Royal Society, AP/30/18

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