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



latitudes than in ours; but the axis of the conoidal gleam is 

 situated precisely as with us. 



Now it seems wholly unquestionable that this quality of the 

 light should dispose at once and for ever of the theory that the 

 zodiacal light is due to the existence of a ring of matter around 

 the earth. 



Let it be remembered that there is only one way in which the 

 ordinary aspect of the zodiacal light can at all be interpreted on 

 such an hypothesis. If there were a ring of meteorites as far 

 from us as the moon is, then undoubtedly there would be a 

 gleam in the west after sunset, and in the east before sunrise, in 

 the position where we see the zodiacal light. And further, the 

 individual meteorites producing any portion of that gleam would 

 undoubtedly rise and set much as the zodiacal light is observed 

 to do. But there would also be a gleam, and a much brighter 

 gleam, in the south. The meteorites rising and setting would 

 turn only a small portion of their illuminated faces towards us, 

 those in the south (on or close by the ecliptic) would be " full/' 

 so to speak, and their combined lustre would be proportionately 

 more considerable. Now supposing the ring exactly coincident 

 with the ecliptic, the earth's shadow would fall on the part due 

 south. But the width of this shadow would (on the supposi- 

 tion we are considering) be relatively small. At midnight, in 

 our latitudes, we should undoubtedly, on this supposition, see 

 two arms of light extending from the eastern and western hori- 

 zon along the ecliptic, each growing brighter and brighter 

 towards the south ; and a relatively narrow black rift would lie 

 between the bright extremities of these arms. It is no theory 

 that this would be the case, but a simple deduction from the 

 most obvious geometrical laws. 



If, then, we are to have a ring round the earth, it must lie 

 far within the moon's orbit, so that the earth's shadow may be 

 wide enough to cover the meteorites along the whole of that 

 long arc which under ordinary circumstances is undoubtedly un- 

 illuminated. The earth's shadow cannot be more than 8000 

 miles across anywhere, and we must have our ring at such a 

 distance that this width of 8000 miles may correspond to (or 

 subtend) that wide arc of darkness actually observed under or- 

 dinary circumstances. (It is absolutely essential that ordinary 

 circumstances should be accounted for; only when this has 

 been done need we begin to inquire into extraordinary circum- 

 stances.) 



Now we need not leave our own latitudes to decide how far 

 off the ring should be to account for the apparent dimensions of 

 the zodiacal light, because on the theory that the earth's shadow 

 falling on a ring of some sort defines the limits of visibility of 



