Notes on the Zodiacal Light.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 5, 1872, Page xlvii

Notes on the Zodiacal Light. By J. S. Webb. [Read before the Otago Institute, 19th November, 1872.] Having recently met with an account by Signor Respighi of some spectroscopic observations of the Zodiacal Light, I felt interested to ascertain how far the facts indicated are compatible with the theory broached by Mr. Skey, in the paper he read at our meeting in March last (see preceding article). Looking for other information on the subject to assist the inquiry, I was surprised to find how little was to be obtained. This being so, I have thought that I should render what I have to say more interesting by prefacing it with a general account of this interesting and ill-understood phenomenon. The account of the Zodiacal Light given by Sir John Herschel is substantially the same as that to be found in Mr. Skey's paper. It has remained unchanged throughout the successive editions of his “Outlines of Astronomy,” although some interesting additions to our knowledge of the subject have been made in the meantime. I think Mr. Skey has been somewhat misled by this, as he lays stress on the fact that the Zodiacal Light is, as stated by Herschel, only visible about the vernal and autumnal equinoxes, and for a few weeks before and after those dates, whilst in point of fact it is visible all the year round, or nearly so. This error does not, as it appears to me, invalidate Mr. Skey's theory, but a knowledge of it would probably have led him to alter his diagram (Pl. XIV.) somewhat, and to avoid some of the remarks he has been led to make. After a search through all the books accessible here which were likely to afford any information, I found the best account of the Zodiacal Light where I least expected it, namely in the introductory notes to Keith Johnston's “School Atlas of Astronomy.” These notes are by Mr. J. R. Hind, and I

cannot do better than quote what he says on this subject [Extract read; Keith Johnston's Atlas of Astronomy,” page 5.] I have brought with me the illustration referred to by Mr. Hind which, judging from my own observation and from the accounts of others, conveys a fair representation of the phenomenon. With regard to the time of the year during which the Zodiacal Light is visible in England, the record of the Rev. T. W. Webb in “Nature,” 8th February, 1872 (vol. v., p. 285), of the latest observations there, of which I have seen an acount, corroborates what Mr. Hind says on the subject. Of the only observation of the Zodiacal Light which I have had the opportunity of making in this hemisphere, I can only speak from memory. It was during the winter months, and the apex of the cone of light, which was on that occasion defined with more than usual clearness, was near one of the brighter stars in the constellation Leo. The sun was at the time far below the horizon, and the distance of the apex from the horizon was fully 40°. A reference to Mr. Skey's diagram will show that if the figure he has given as an approximation to that of the Zodiacal Light is to be taken as an essential detail of his hypothesis, this observation, and indeed all observations during our winter months, decidedly invalidate it. As I have already said, this particular detail does not appear to be essential to the theory, although it renders it desirable that the manner in which it has been expressed should be revised. The general form of the envelope from which we derive the Zodiacal Light may be somewhat as Mr. Skey has supposed, but it is necessary to admit of a very considerable extension in all directions from the sun in and near the plane of his equator, in order to account for its visibility throughout the year. Various hypotheses as to the constitution of this solar envelope have been put forward. Sir John Herschel speaks of it in his “Outlines of Astronomy,” paragraphs 897 and 898. Becquerel, in a recent work, gives the prevalent opinion among French physicists as follows:—“Many explanations of this phenomenon have been offered, the most probable being that which considers it due to a group of bodies which form, as it were, a zone around the sun of solid asteroids, widely separated from one another, but occupying an enormous space, in the midst of which the earth is plunged; aerolites and shooting stars will then be but isolated bodies belonging to this group, which, drawn within the sphere of the earth's activity, fall upon its surface. According to this hypothesis, the Zodiacal Light will be due to reflected solar light, and the absence of polarization which has been observed in it is a result of the light being reflected in all possible planes from the variously presented surfaces of this multitude of bodies.”—(Becquerel, La Lumière ses Causes et ses Effets, Tome I., p. 7, 1867.) Mr. Skey, as you are aware,

has suggested that the bodies which yield us this mysterious light are wholly gaseous, and form the source of our November meteors. M. Liais, to whose previous communication Signor Respighi refers in his letter to the Comptes Rendus de L'Académie des Sciences, Tome 74, p. 514, 19th Feb., 1872, has long held the opinion that the Solar Corona and the Zodiacal Light are phenomena intimately connected with one another. He endeavours to reconcile the different results they give when examined by the polariscope by suggesting that the bodies which, when seen as the Zodiacal Light, reflect the solar rays from their surfaces, when they approach the sun so closely as to form part of the corona are rendered gaseous and incandescent by the excessive temperature to which they are subjected. (Comptes Rendus, Tome 74, p. 263, 22nd Jan., 1872.) His observations serve to connect the Zodiacal Light with that of the Corona; those of Signor Respighi and others demonstrate an intimate likeness between the former and the light of the Aurora, whilst others, still more numerous, have shown independently that at least a part of the Coronal Light is identical in its character with that of the Aurora. I shall not attempt a technical account of these. Plate IX., in Schellen's “Spectrum Analysis,” English Translation, 1872, exhibits very clearly the coincidence between certain lines observed in the spectrum of the Corona with those which are peculiar to the Aurora. One of these is the “line in the green,” observed by Signor Respighi in the spectrum of the Zodiacal Light. Recent observations of the Aurora have shown that it also yields a faint, almost continuous, spectrum situated similarly to that which Respighi and Liais describe as belonging to the Zodiacal Light. Further, the eclipse observations of December last have shown definitely that besides the lines shown in the plate just referred to, the Corona also yields a continuous spectrum, a part only of which is due to reflected solar light. We must conclude from all this that, though these three phenomena are far from being shown to be identical in character, and although it is not probable that they are so, they have at least one character in common. Though suspected before it is only now that this has been positively demonstrated. It appears then that the revelations of the spectroscope as to the nature of the Zodiacal Light do not invalidate Mr. Skey's hypothesis that it is yielded by gaseous bodies driven off from the sun during solar cyclones. They nevertheless show that this is not a complete explanation of the phenomenon. The argument for the existence of solid or liquid reflecting bodies in the envelope which the non-polarization of the light affords is in the meantime unanswerable, and is supported by the results of spectroscopic observation. But, so far as our knowledge will carry us, we must assume that attenuated gaseous matter also exists in the region of the Zodiacal Light, and that at least a part of the light we see is caused by electric action upon such matter.