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.] 



The 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 1 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 (PI. 

 XIY.) represent the earth's annual motion along the ecliptic, the small arrows 

 indicating the direction of its diurnal rotation j 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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