44 



THE ZODIACAL LIGHT. 



stances where vulgar sense sees nothing but dy- 

 ing sparks in the clear vault of heaven, and in the 

 black stone that falls from the crackling cloud 

 the crude product ofsome vt^ild force of nature. 

 If the crowd of shooting asteroids, upon 

 which we have paused so long with pleasure, 

 be assimilated in some respects, in their small 

 masses and in the variety of their orbits, with 

 comets, they are still essentially distinguish- 

 ed from these bodies in this — that we first be- 

 come aware of their existence almost in the 

 moment of their destruction, when fettered by 

 the earth they become luminous, and ignite. 

 But to embrace everything that belongs to our 

 solar system, which has now become so com- 

 plex, so rich in variety of forms, by the discov- 

 ery of the telescopic planets, of the inner com- 

 ets of short period, and the meteoric asteroids, 

 we have still to speak particularly of the ring 

 of Zodiacal Light, to which we have already 

 alluded incidentally oftener than once. He who 

 has lived for years in the zone of the palms, 

 retains a delightful recollection of the mild ra- 

 diance with which the zodiacal light, rising like 

 a pyramid from the horizon, illumines a portion 

 of the unvarying length of the tropical night. 

 I have seen it occasionally more intensely lu- 

 minous than the milky way in Saggitarius ; and 

 that not only in the thin and dry atmosphere of 

 the summits of the Andes, at the height of 

 twelve or fourteen thousand feet above the lev- 

 el of the sea, but also in the boundless grassy 

 plains (Llanos) of Venezuel-a, as well as on the 

 coasts of the ocean under the ever-serene sky 

 of Cumana. Of most peculiar beauty was the 

 phenomenon, when small fleecy clouds appear- 

 ed projected upon the light, and stood out pic- 

 turesquely from the luminous back-ground. A 

 leaf of my journal, during the sea voyage from 

 Lima to the western coast of Mexico, preserves 

 the memorial of this air-picture : " For the last 

 three or four nights (between 10° and 14° N. 

 lat.) I see the zodiacal light with a splendour 

 such as I have never observed before. In this 

 part of the Pacific, judging from the brilliancy 

 of the stars, and the distinctness of the nebulae, 

 the transparency of the air is wonderfully great. 

 From the 14th to the 19th of March, very reg- 

 ularly for three-quarters of an hour after the 

 disc of the sun has dipped into the sea, there is 

 no trace of the zodiacal light, although it is by 

 this time completely dark ; but, an hour after 

 sun-set, it suddenly becomes visible, of great 

 brilliancy, between Aldebaran and the Pleiades ; 

 and on the 18th of March having an altitude of 

 39° 5'. Long narrow stripes of cloud show 

 themselves, scattered over the beautiful blue, 

 and deep on the horizon in front of a kind of 

 yellow screen. The higher clouds are play- 

 ing from time to time with variegated tints. It 

 seems as if the sun were setting for the second 

 time. On this side of the vault of heaven, the 

 brilliancy of the night appears to be increased, 

 almost as it is in the first quarter of the moon. 

 Towards ten o'clock, the zodiacal light, in this 

 part of the Pacific, was usually extremely faint ; 

 about midnight I could merely perceive a trace 

 of it. On the 16th of March, when the phe- 

 nomenon presented itself in its greatest splen- 

 dour, there was a counter-blush of mild light 

 apparent in the east." In our misty northern 

 temperate zone, as it is called, the zodiacal 



light is only to be distinctly seen in the early 

 spring, after the evening twilight, in the west- 

 ern, and towards the end of autumn before the 

 morning twilight, in the eastern horizon. 



It is difficult to comprehend how a natural 

 phenomenon, so remarkable as the zodiacal 

 light, should only first have attracted the atten- 

 tion of natural philosophers and astronomers 

 about the middle of the 17th century, and how 

 it could have escaped the observant Arabians 

 in Ancient Bactria, on the Euphrates, and in 

 the south of Spain. The tardy observation of 

 the nebulae in Andromeda and Orion, first de- 

 scribed by Simon Marius and Huygens, excites 

 almost equal astonishment. The first distinct 

 description of the zodiacal light is contained in 

 Childrey's Britannia Baconica("), of the year 

 1661 ; the first observation upon it may have 

 been made two or three years earlier ; but 

 Dominic Cassini has the indisputable merit of 

 having, in the spring of 1683, investigated the 

 phenomenon in all its relations in space. The 

 luminous appearance which he observed in 

 1668, at Bologna, and which was seen at the 

 same time in Persia by the celebrated travel- 

 ler, Chardin, (the court- astrologers of Ispahan 

 called this light, which they had never seen 

 before, nyzek, or little lance,) was not, as has 

 been frequently said("), the zodiacal light, but 

 the monstrous tail of a comet, whose head was 

 hidden amidst the vapours of the horizon, and 

 which, in point of length and appearance, pre- 

 sented many points of resemblance to the great 

 comet of 1843. It might be maintained, with 

 no slight show of probability, that the remark- 

 able light, rising pyramidally from the earth, 

 which was seen in the eastern sky for forty 

 nights in succession, on the lofty plateau of 

 Mexico in 1509, was the zodiacal light. I find 

 this phenomenon mentioned in an ancient Az- 

 tekan manuscript (Codex Telleriano-Remensis) 

 of the Royal Library at Paris(^3). 



The Zodiacal Light, of primeval antiquity, 

 doubtless, though first discovered in Europe by 

 Childery and Cassini, is not the luminous at- 

 mosphere of the sun itself; for this, from me- 

 chanical laws, cannot be more oblate than in 

 the ratio of two to three, and not more dilated 

 than 9-20ths of Mercury's distance. The same 

 laws determine that, in the case of a revolving 

 planetary body, the height or distance of the 

 extreme limits of its' atmosphere — the point, 

 namely, where gravity and the centrifugal force 

 are in equilibrium — is that alone in which a 

 satellite can revolve around this in the same 

 time as the primary rotates upon its axis("). 

 Such a limitation of the sun's atmosphere in its 

 present concentrated state, comes to be more 

 particularly remarkable when we compare the 

 central body of our system with the nucleus of 

 other nebulous stars. Herschel discovered 

 many in which the semidiameter of the burr 

 which surrounds the star appears under an an- 

 gle of 150". Assuming a parallax which does 

 not quite reach 1", we find the outermost neb- 

 ulous layer of such a star 150 times farther 

 from its centre than the earth is distant from 

 the sun. Were the nebulous star in the place 

 of our sun, consequently, its atmosphere would 

 not merely include the orbit of Uranus, but 

 would extend 8 times beyond it("). 



With the narrow limits of the sun's atmo- 



