﻿Mr. R. A. Proctor on the Zodiacal Light. 53 



medium which surrounds the sun may have been sufficient in 

 many cases to change the paths of comets which would otherwise 

 have passed away to other stars, into periodical orbits about the sun. 



Roundhay Vicarage, 

 December 1, 18/0. 



Errata in former paper. 

 Page 183, line 20, for directions read their directions of motion when at 

 a great distance from the sun. 

 Page 189. line 20, for 1" read 0"05. 



IX. Note on the Zodiacal Light. 

 By Richard A. Proctor, B.A.* 



IT cannot but be regarded as a remarkable circumstance that 

 the nature of the zodiacal light should in the present state 

 of astronomy continue to be a quastio vexata. I do not here 

 refer to the physical constitution of this object, respecting which 

 we may possibly be unable for many years to form a satisfactory 

 theory, but to the determination of the actual position of the 

 zodiacal light in space. Astronomers have been able to deter- 

 mine from geometrical considerations the paths of such objects 

 as comets and meteors ; it would therefore seem that the posi- 

 tion of such an object as the zodiacal light ought ere this to 

 have been determined. 



Yet it must be admitted that there are peculiar difficulties 

 in this problem. We can reason respecting the distance and 

 motion of a comet, because we know that our observations are 

 made on one and the same bod} r , whose motions are in accord- 

 ance with the laws of gravity. It is otherwise with respect to 

 the zodiacal light. We see a certain glow or radiance occupy- 

 ing a definite position with respect to the horizon and to the 

 celestial circles ; but we have no means of ascertaining whether 

 the objects from which that radiance proceeds are the same at 

 any one time as at any other, or indeed (as will presently appear) 

 whether a single one of the constituents forming the zodiacal 

 gleam at one season is present within the same region of the 

 solar system at another. 



The geometrical conditions applicable to the zodiacal light 

 are, however, too definite to admit of question ; in other words, 

 the path to be followed in seeking for a theory of this object is 

 unmistakable. Hitherto, so far as I am aware, that path has 

 not been traced out far enough for the attainment of definite 

 views, the perplexities which presently surround us as we follow 

 it having seemed perhaps to render further research hopeless. 



It happens, however, not unfrequently that the very difficul- 



* From the Monthly Notices of the Royal Astronomical Society, No- 

 vember 11, 18/0. 



