THE ZODIACAL LIGHT. 



By the Late F. W. HENKEL, B.A., F.R.A.S. 



Astronomers are generally agreed to consider 

 this phenomenon as falling within the purview of 

 their science, though there have not been wanting 

 some who, by a wide extension of the term 

 Meteorology — t« /ueTeupa," things aloft" — have rather 

 included it under the latter heading. Inhabitants of 

 towns, " those in populous cities pent," are not 

 often favoured with a view of its delicate luminosity, 

 the multitude of artificial illuminations completely 

 veiling its feebler intensity. But in tropical regions 

 it is a fairly regular phenomenon and, according to 

 Humboldt and other observers, its brilliancy often 

 greatly surpasses that of the Milky Way. 



In our latitudes, the Zodiacal Light is best seen in 

 the evenings of February and March, being then 

 nearly perpendicular to the western horizon ; in the 

 autumn mornings it is to be similarly seen in the 

 east before sunrise. As seen on a late February 

 evening it has the appearance of a cone or lens- 

 shaped beam of light reaching from the horizon 

 towards the zenith, following generally the course of 

 the ecliptic (or perhaps that of the Sun's equator), 

 whence its name, the Zodiac being that part of the 

 sky within which are performed the apparent move- 

 ments of the Sun, Moon, and the principal planets. 

 This cone has its base at the horizon, and its vertex 

 is at a distance from the Sun's position varying from 

 50° to 90°, the breadth of the base being not less 

 than 8° or more than 30°. Cassini and Mairan 

 observed it, at times, not less than 100° from the 

 Sun's place, and occasionally even further, showing 

 its extension in space beyond the Earth's orbit. 

 Humboldt and Brorsen seem to have been 

 amongst the first to observe a second light in the 

 East at the same time that there was a principal 

 light in the West, a narrow band of fainter 

 luminosity uniting the two, whilst Flammarion says 

 that in the equatorial regions of the Earth the 

 conical form disappears with the last trace of twilight, 

 and by " night there is seen a luminous band forming 

 a more or less complete circle in the sky, sometimes 

 stretching from West to East, the parts nearest the 

 Sun being the most brilliant, other regions less so, 

 the whole of a pure white tint." In the region of 

 the sky exactly opposite the Sun's place there is 

 often seen a patch of several degrees diameter, more 

 luminous than the surrounding portion, and this is 

 known as the " Gegenschein," or counterglow. 

 Whilst under the Tropics the light is of pure white 

 colour, in our latitudes it is more commonly of a 

 reddish tint, especially at its base ; this is, however, 

 probably due to the last traces of twilight. 

 Humboldt, however, says ("Cosmos," Vol.1): " I have 

 occasionally perceived not exactly a reddish colora- 



tion, nor the lower part darkened, nor even a 

 scintillation such as Mairan asserts he has seen, but 

 a sort of tremulous shivering of the light." 



It is perhaps a little surprising that the zodiacal 

 light seems to have escaped the notice of the ancient 

 astronomers, unless we may suppose that a reference 

 to the " trabes " by Pliny, in his " Natural History," 

 may be so interpreted ; but this is more probably an 

 allusion to the aurora. Kepler and Descartes make 

 obscure allusions which may be interpreted to 

 indicate their familiarity with it ; but the earliest 

 authentic mention of the light occurs in the works 

 of Childrey, Chaplain to Lord Henry Somerset, 

 who, in his " Natural History of England " (1659) 

 and his " Britannia Baconica " (1661), was the first 

 to draw the attention of his contemporaries to the 

 remarkable observations made by him during several 

 previous years, in the evenings of February and 

 March, after sunset. About twenty years later, 

 Dominique Cassini gave considerable attention to 

 the phenomena as seen in Central Europe, and 

 studied them " in all their bearings with regard to 

 space," formulating a theory of their origin which 

 differs but little from that generally admitted by 

 present-day astronomers. He considered that the 

 light was produced by a ring of small planetary 

 bodies revolving in orbits nearly in the plane of the 

 Ecliptic and reflecting the light of the Sun. He 

 even thought that the fall of meteoric stones or 

 bolides might be due to the passage of our Earth 

 through this ring. The principal difference between 

 this and later views consists in regarding the in- 

 numerable small particles as rather forming a thin, 

 flat sheet partly lying between the Earth's orbit and 

 the Sun and partly beyond it, the light being due to 

 reflection mainly, but perhaps, as we shall see later, 

 to a small extent intrinsic also. It is by no means 

 impossible that to the action of the denser part of this 

 layer of "meteoric dust " within the orbit of Mercury 

 may be due the unexplained motion of the peri- 

 helion of that planet's orbit as detected by Leverrier. 

 Sir John Herschel says : " It may be conjectured 

 to be no other than the denser part of that medium 

 which, as we have reason to believe, resists the 

 motion of comets, loaded, perhaps, with the actual 

 materials of the tails of millions of those bodies of 

 which they have been stripped in their successive 

 perihelion passages." 



Liais and Mayer thought that to this layer might 

 be referred the maintenance of the Sun's light and 

 heat, " the meteoric theory of solar radiant energy." 

 Materials must be continually falling upon the Sun's 

 surface, and the "arrested motion" transformed into 

 heat, thus making up for that lost by radiation, so 



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