Attempt to apply instrumental measurement to the Zodiacal Light

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                                and appearance of the Zodiacal light. A larger portion of the land=
=scape has therefore been introduced, than would otherwise have been 
altogether appropriate in Astronomical drawings. 
A more important addition is however the insertion in 
the margin of the circles of Right Ascension & Declination, which 
show what particular projection has been employed, and serve 
to identify the stars, fix the latitude in which the observations 
were made, the time at the instant, and to give an idea of 
the dimensions of the body under discussion, and the region of 
the sky in which it is found. To represent <s>the [text?]<\s> in true perspec=
=tive, or as the Eastern or Western portion  of the sky would be 
thrown on paper by the camera lucida, the horizon is defined 
by a straight line in the picture, and the East or West point 
must lie in the middle of that line; then the eye of the spec=
=tator being directed towards it as such, the equator being a great 
circle will be represented by a straight line drawn through 
that point, and rising at an angle to the horizon equal to 
the latitude of the place, and the Meridian lines or the paral=
lels of Right Ascension, being also great circles, must be 
expressed by straight lines crossing the equator at right angles: 
on the other hand, the parallels of Declination being small 
circles will appear as conoidal curves. 
A great circle becomes a straight line on the picture, 
since it is a plane passing through the eye, and the common 
section of this plane with the plane of the picture, is a straight 
line. A small circle is a conoidal curve on the picture, because 
a small circle is seen as a cone of which the apex is at the 
eye, and the common section of this cone with the plane 
of the picture, is a conic section. The form of the conic section 
will vary as the inclination of the cone's axis to the plane 
of the picture varies. 
In all the drawings given herewith, the  line of sight 
is seldom directed to the East or West point, but 

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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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