52 



KNOWLEDGE. 



[Februaby 1, 1897. 



of the zodiacal light sloping upwards from the sun's place. 

 The boundaries are generally ill-defined, and the apex has 

 usually to be imagined more or less, so exceedingly delicate 

 is the glow towards the extremity of the cone. Guillemin, 

 in " The Heavens,'' has a very delicate plate of it, as seen 

 in Japan. In this picture it looks fainter than it is some- 

 times seen in England. 



For all practical purposes the axis coincides with the 

 plane of the ecliptic. 



An open horizon has a good deal to do with its visi- 

 bility. For years I looked carefully for it in England 

 without success, and I think the reason was that trees 

 and other prominent objects, dark against the western 

 sky, must have put my eye out of focus for the faint light. 

 There are some who have not the power or capacity for 

 this class of seeing, and I have before now, when in 

 southern latitudes, pointed it out, quite blazing, to a 

 friend, who, however, could make nothing of it, although 

 it was literally staring him in the face. 



Humboldt, in his " Cosmos," has some most interesting 

 remarks on the subject, the more so as they are based on 

 personal observation and considerable literary research. 



My first observation of zodiacal light was in Natal, where 

 it was at once palpable. It was in the dry, clear air of 

 the uplands of South Africa, however, that it was seen to 

 the greatest perfection ; while in the Transvaal and in 

 Bechuanalaud the light was almost blazing — at least, it 

 seemed so to an eye accustomed to astronomical phenomena, 

 not to bonfires. Unless one has been in these circum- 

 stances of latitude and climate, it is impossible to imagine 

 ■what an exquisitely delicate and ethereal object the light is. 

 For there one has great advantages, notably in the trans- 

 parency of the atmosphere, and the fact that the light is 

 perpendicular, or nearly so, to the horizon. It is extremely 

 difficult, when it slopes at the angle it does in England, to 

 judge of the relative sharpness of the margins: a point 

 which I have studied of late in connection with Dr. 

 Veeder's theory of its duplicity. At Gibraltar, whence I 

 write, it is often most beautifully seen as a whole, and 

 Humboldt refers to its greater intensity in Spain, on the 

 coast of Valencia, etc. From om- coign of vantage we 

 have the privilege of looking across the Bay of (.iibraltar, 

 with the range of hills beyond Algeciras in the distance, 

 and, except for occasional shipping, this view is free from 

 artificial illumination or glare. 



Humboldt thinks that the zodiacal light was imknown 

 to, or at least not mentioned by, the ancients. The word 

 tnthcs, occui-ring in Pliny, which some consider means the 

 phenomenon in question, refers to meteors, bolides, and 

 comets. According to Humboldt, it is first explicitly 

 described by Childrey, chaplain to Lord Henry Somerset, 

 in his "Britannia Baconia." 



Dom. Cassini was the first to investigate the pheno- 

 menon in its relations in space, in 1683. Cassini, 

 Humboldt says, maintained that a phenomenon observed 

 by him at Bologna, also by Chardin in Persia, in 1608, 

 was the zodiacal light, but that really this was the 

 tail of the comet of 1683, whose head was at the time 

 below the horizon. This calls to mind the great comet of 

 1882, which I first saw after perihelion in the beautiful sky 

 of Natal about 4 .^^.m. The head was below the horizon, 

 and the tail sloped upwards for many degrees — a most 

 extraordinary vision. Never can that supremely beautiful 

 sight be forgotten, as the comet majestically rose and 

 displayed himself, surrounded by a gauze veil more ethereal 

 than any bride's. 



But no one could for a moment mistake the comet of 

 1882 for the zodiacal light. On the contrary, the view of 

 the great comet of 1880, which I also got in Natal, might 



have been easily confounded with it, on account of its 

 exceeding faintness and delicacy. 



Humboldt considers that a remarkable light seen in 

 1509 for forty nights consecutively, in the elevated plains 

 of Mexico, was the zodiacal light. He found a notice of 

 this light in an ancient Aztec MS., preserved in the Royal 

 Library of Paris. 



Humboldt proceeds to inquire into the nature of the 

 zodiacal light, and considers " a very compressed annulus 

 of nebulous matter revolving freely in space, between the 

 orbits of Venus and Mars, as the material cause' of it. 

 Coming to more modem times. Sir J. Herschel thinks it is 

 " that medium which resists the motion of comets, loaded, 

 perhaps, with milhons of tails of these bodies." Still later, 

 the idea is that it consists of myriads of small meteors, 

 each revolving round the sun in its own path, the light 

 reflected from which impresses itself on our eyes as 

 the zodiacal Ught. This is the opinion of Prof. Pickering, 

 who, in Vol. XIX. of " The Annals of Harvard College 

 Observatory," has discussed many observations with his 

 usual acumen. 



It would seem certain from the observed angular distance 

 of the apex from the sun that these meteors extend beyond 

 the earth's orbit ; and as the light lies in, or very close to, 

 the ecliptic, we must be perpetually cutting through them 

 as we revolve round the sun. Do we see in our atmosphere 

 the falling meteors which have originally formed a portion 

 of the zodiacal light '? We are of opinion this question 

 can only be settled when that of the relation of comets to 

 meteors is settled, and we know what a dubious matter 

 that is in its purely physical aspect. 



Dr. Veeder, of New York, considers that the zodiacal 

 light " does not conform to the plane of the earth's orbit, 

 but to that of the equator of the body which it surrounds, 

 which in this case is the sun itself. As viewed from the 

 earth, these coronal extensions are at times foreshortened, 

 and at times opened out, so as to become more plainly 

 visible. In the spring months, the south pole of the sun 

 is inclined towards the earth, so that the latter is almost 

 exactly in the heliocentric zenith of the southern sunspot 

 belt and coronal extension. Consequently the particles 

 composing this extension are in a direct line between sun 

 and earth, and, shining as they do by reflected light, like 

 the new moon, they become almost invisible. Coincidently 

 the coronal extension overlying the northern sunspot belt 

 is opened out to its widest extent, acd reflects more light 

 earthward than at any other time. Hence, if these exten- 

 sions become visible as the zodiacal light, the southern 

 edge at this season should be the more sharply defined, and 

 more exactly included within the plane of the ecliptic, 

 because of the lack of illumination described ; and the 

 northern edge, on the other hand, should shade ofi' very 

 gradually, departing more widely from the plane of the 

 ecliptic, and this is precisely what has been fomid to be 

 the case." 



My own observations made at Gibraltar most certainly 

 support this \'iew.* It has also been observed by Cassini 

 and others. Dr. Veeder further observes that it is probable 

 " that these coronal extensions serve as conductors of 

 electrical impulses," and are therefore connected with the 

 appearance of auroriv. 



The question of the variability of the zodiacal light has 

 been much discussed. Humboldt says : " I have occasionally 

 been astonished, in the tropical climates of South America, 

 to observe the variable intensity of the zodiacal light. 

 As I passed the nights, during many months, in the open 

 ail-, on the shores of rivers and llanos, I enjoyed ample 



* See English Mec/tanic, Ko. 1025. 



