﻿Mr. R. A. Proctor on the Zodiacal Light. 55 



The setting of the zodiacal light when the western half is 

 visible, and the rising of the light when the eastern half is 

 visible, take place quite regularly, and in a manner precisely 

 corresponding with what would be observed if the zodiacal light 

 were a distant object like a planet, a star, or a portion of the 

 Milky Way. 



Now these circumstances at once enable us to reject the theory 

 that the zodiacal light is a terrestrial appendage, by which I 

 understand for the moment an object lying within the earth's 

 atmosphere. For there can be no question whatever that if any 

 definite portion of our atmosphere were rendered luminous in 

 any way, that portion would either occupy an unchanged posi- 

 tion or would shift according to the laws regulating the process 

 of illumination, or according to the winds or other like terres- 

 trial causes. Now that on any given occasion such causes 

 might so operate as to give the illuminated air the appearance 

 of rising or setting, as celestial objects do (that is, not merely 

 rising or setting, but rising or setting along declination-paral- 

 lels), is quite possible, however unlikely. To take an illustrative 

 instance : a balloon seen at any one instant between an observer 

 and the sun might be carried by the winds so as to continue 

 between him and the sun, even until the hour of sunset. Bat 

 to suppose that night after night at any station a relation so 

 peculiar would characterize the illuminated air, is like sup- 

 posing that a balloon, started day after day from a given place, 

 would day after day fulfil the condition considered above. This 

 is obviously incredible. But even if it were credible, it would 

 be insufficient, since the region of our atmosphere which would 

 have to be illuminated, in order to account for the zodiacal light 

 as seen in one place, would, as seen from other stations, present 

 an appearance wholly different from that of the zodiacal light. 

 In fact, if the former place were in England, the zodiacal light 

 would actually be in the zenith of places 900 miles or so west or 

 east of England. 



Next we have the normal aspect of the zodiacal light in dif- 

 ferent latitudes to consider. Now we have the most positive 

 assurances from astronomers of eminence that the zodiacal light, 

 wherever seen, occupies ordinarily precisely those regions of the 

 heavens corresponding to the theory that it is too far from the 

 earth to have an appreciable parallactic displacement. We have 

 the evidence of practised astronomers like the Astronomer Royal 

 for Scotland, Captain Jacob, and others ; and all the evidence we 

 have points to the conclusion that the zodiacal light, as seen in 

 the tropics, extends at any moment over those same parts of the 

 stellar heavens which it illuminates as seen from our northern 

 stand-point. The limits of the light may seem greater in those 



