Observations on the Zodiacal Light, tending to show its Connection with the 
Sun's Motion in Space. By H. SKEY. 
(With Illustrations.) 
[Read before the Otago Institute, 12th March, 1872.] 
Tue remarkable illumination in the heavens, known as the Zodiacal Light, 
is visible just after sunset, when the air is very clear, during the months of 
March and April, and again, just before sunrise, during the opposite months 
of September and October, and follows in a general direction the course of the 
ecliptic, or, according to Sir John Herschel, that of the sun’s equator. Its 
apparent angular extent from the sun at its base to the vertex of the cone of 
illumination varies from 40° to 50°, and sometimes even to 90°, with a 
breadth varying from 10° to 30°. It has been conjectured that it derives its 
form (that of a lenticularly formed envelope) by its rapid revolution with 
the sun on its axis, only the upward half of which we see at one time, the 
other half being below the horizon. 
An insuperable objection, however, to this explanation must at once 
present itself. If we see the upward half of this figure just after sunset, in 
March, what is there to prevent the other half from being seen during the 
same month in the mornings, just before sunrise? Why have we to wait till 
the opposite season ? 
It follows, therefore, that whatever may be the cause of this illuminated 
cone, it exists on one side only of the solar orb; and the next step is to 
account for its visibility at one time of the year only in the evenings, and at 
the opposite season only in the mornings. Let the accompanying figure (P1. 
XIV.) represent the earth’s annual motion along the ecliptic, the small arrows 
indicating the direction of its diurnal rotation ; then, as the Zodiacal Light 
during September is visible in the mornings, it follows that the direction of 
the cone must point towards some portion of the earth’s orbit lying between 
September and March. For reasons hereafter adduced, let us assume it as 
constantly extending towards the earth’s position early in December (as far as 
` «longitude is concerned), and examine the appearance it would present in- 
March, when the earth has arrived at a diametrically opposite part of its 
orbit. ; 
It will be seen on reference to the diagram that the Zodiacal Light can 
then only be visible in the evenings, just after sunset, when its extremely 
delicate illumination ceases to be overpowered by the direct solar light. 
In accounting physically for the existence of matter, or of a medium 
susceptible of illumination, on the one side only of the sun, let us consider the 
direction of the sun’s proper motion in space in connection with some inter- 
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