On certain Appearances connected with the Zodiacal Light.

By Baron Humboldt.

Monatsbericht der Kön. Preuss. Akademie der Wissenschaften, Juli, 1855.

"In Gould's valuable American Astronomical Journal (No. lxxxiv., May 26, 1855), there appears a letter from the Rev. Mr. Jones, chaplain of the frigate Mississippi, containing, as the result of his observations of the Zodiacal Light in the seas of China and Japan, the conjecture of a second radiating ring of light having a relation to the moon.

This conjecture is founded upon the extraordinary spectacle of the Zodiacal Light simultaneously observed at both east and west horizons from eleven to one o'clock, during several days in succession.

As I observed something analogous fifty-two years ago in the Southern Ocean during the voyage of forty days from Callao, in Peru, to the port of Acapulco in Mexico, and have given only a very brief account of it in the astronomical part of my Cosmos, it may not be uninteresting to the members of the Academy if I laid before them an extract from my French Journal, written at sea, relating to this phenomenon, which hitherto has not formed the subject of any detailed remarks.

The Zodiacal Light, and the difficult question whether we ought to attribute to a physical cause existing beyond our atmosphere the remarkable variations of light which it undergoes, while in tropical nights the smallest stars exhibit the same brightness to the naked eye, were subjects which engaged my attention during a period of five years upon great heights among the Cordilleras, in the extensive plains or Llanos, at sea, and on both sides of the equator, as will be seen by reference to my partly published correspondence with Olbers (Cosmos, vol. i. p. 412.)

From my ship-journal I extract the following observations, extending from the 14th to the 19th of March, 1803, between north latitude 12° 9' and 15° 20', and chronometrical longitude 104° 27' and 105° 46' west of Paris.

"On the 17th and 18th of March the Zodiacal Light, the base of which appeared to rest upon the sun, shone with a brightness which I had never seen on any former occasion of the approach of the vernal equinox.

The luminous pyramid terminated between Aldebaran and the Pleiades at an apparent altitude of 39° 5' measured above the sea-horizon, which was still sufficiently visible.

The vertex was somewhat inclined towards the north; and the direction of the brightest part appeared by the compass to be west-north-west.

What has struck me most during this voyage, is the great regularity with which, during five or six nights in succession, the brightness of the Zodiacal Light progressively increased and diminished.

Its existence was hardly discernible during the first three-quarters of an hour after sunset, although the darkness was sufficiently great to render visible the stars of the fourth and fifth magnitude; but after 7h 15m the luminous spindle appeared at once in all its beauty.

Its colour was not white, like that of the milky way, but a reddish yellow, as Dominique Cassini assures us he had seen it in Europe.

Very small clouds, situated accidentally towards the horizon, reflected upon the reddish ground a lively blue light.

One would almost suppose he saw a second sunset in the west.

About ten o'clock the light entirely disappeared; at midnight I perceived only a feeble trace of it, although the celestial vault still exhibited the same degree of transparency.

While the light was very bright in the west, we constantly perceived in the east (and this is beyond doubt a very striking phenomenon) a whitish light, which was also of a pyramidal form.

The latter augmented the brightness of the sky in a very striking manner.

Even the sailors were delighted with this double light in the west and the east; and I am inclined to think that this white light in the east was the reflexion of the real Zodiacal Light at setting.

Both also disappeared at the same time.

Analogous reflexions frequently present themselves in our climates at sunset, but I should never have imagined that the brightness of the zodiacal light could be sufficiently strong to repeat itself by the simple reflexion of the rays.

All these luminous appearances were almost the same from the 14th to the 19th of March.

We did not see the Zodiacal Light on the 20th and 21st of March, although the nights were beautiful in the highest degree."

These are the words of my ship-journal, containing my observations, and also the thoughts which they suggested at the time to my mind.

It was in reference to what I had written down in an unpublished ship-journal on the occasion of a voyage in the Southern Ocean, about the beginning of the present century, that, five years previous to the publication of the interesting observations of the Rev. Mr. George Jones, I made the following statement in the astronomical part of Cosmos:--

"On the whole the variations of the Zodiacal Light appear to me to depend upon variations inherent in the phenomenon, upon the greater or less intensity of the luminous processes going on in the ring.

This is proved by my observations in the Southern Ocean, which indicated an opposite light in the heavens similar to that seen at sunset." (Cosmos, vol. iii. p. 589.)

I may remark further that I have been surprised at the increased brightness of the Zodiacal Light upon ascending to great altitudes.

This was observable upon the lofty peaks of the Cordilleras 10,000 or 12,000 feet high; also in Mexico, in January 1804, at altitudes of only 7000 feet; and from the Cloister of Mount Cenis, where I remained with Gay Lussac several nights (in March 1805) at an altitude of 6350 feet, for the purpose of determining the intensity of the magnetic force during very intense cold, and ascertaining the quantity of oxygen contained in the atmosphere.

It was, consequently, seen both in tropical and in temperate latitudes.

But the variations in the brightness of the phenomenon cannot, according to my experience, be accounted for solely by the constitution of our atmosphere.

There remains much still to be observed relative to this subject.

The following is a copy of the letter to which Baron Humboldt refers:--

(From the Rev. George Jones, U.S.N. to the Editor of the Astronomical Journal, Cambridge, U.S.)

"In my recent cruise in the U.S. steam-frigate Mississippi, chiefly in the China and Japan seas, but taking us also around the globe, I had excellent opportunities for observation of the Zodiacal Light.

This light, you know, appears to the best advantage within the tropics, where it stretches upward to a great elevation, and is a remarkable object; but it is also very desirable to observe it in high latitudes; and in this also I was favoured, as our voyaging extended from 41° north to 53° south latitude; and in some instances our transitions, for weeks together, were very rapid, thus giving me opportunities for observing whether any parallax could be made or not.

"I was also fortunate enough to be twice near the latitude of 23° 28' north, when the sun was at the opposite solstice, in which position the observer has the ecliptic, at midnight, at right angles with his horizon, and bearing east and west.

Whether the latter circumstance affected the result or not I cannot say; but I then had the extraordinary spectacle of the Zodiacal Light, simultaneously at both east and west horizons, from 11 to 1 o'clock, for several nights in succession.

"In the first part of our cruise my observations were of a desultory character; but I soon began to see the necessity of great precision, and accordingly constructed star-charts from a celestial globe (a small but excellent one) that happened to be on board, which charts I afterwards had cut in wood at Canton; and thus I was furnished with materials for accurately recording all the changes of this phenomenon, not only in successive nights, but also in the successive hours of the same night.

My rule was, to draw on my charts the boundaries of the Zodiacal Light as exhibited among the stars, with such annotations as the case required; then, again, do the same after an interval of an hour or half-hour; and so to continue, generally as long as the boundaries could be made out reliably; then, if the morning admitted it, to resume observations at the earliest possible hour, and so to proceed until the dawn.

I have thus, in many instances, observations for every hour of the night.

"At an early period I began to query whether the moon, when near its full, might not give a Zodiacal Light: and at last, when I had gained more experience in observing, and in the peculiar character of this light, I was able to get, at different periods, fourteen reliable observations of what I think must be considered a Zodiacal Light produced by the moon. I have also two records of a distinct Zodiacal Light produced by the joint action of the sun and moon, i. e. at the hour when the moon, then near its first quartering, was about 65° above the western horizon; the reflexion from the combined light of the sun and moon being sufficient to overpower the moonlight proper, and thus to produce a decided stream of light in the sky within the Zodiacal-Light boundaries.

The latter of these observations was the more remarkable, inasmuch as the moon was then without the boundary of this joint reflected light.

"You will excuse my prolixity in stating these varieties of observation, for the conclusion from all the data in my possession is a startling one.

It seems to me that these data can be explained only by the supposition of a nebulous ring with the earth for its centre, and lying within the orbit of the moon.

This conclusion seems to evolve itself, -- 1st, from the simultaneous midnight east and west observations, which preclude the possibility of a ring around the sun within the earth's orbit; 2dly, from the great hourly lateral changes (often semi-hourly) in the boundaries of the Light, caused by the observer's change of place, in that time, as regards the ecliptic or axis of the Zodiacal Light, which lateral change in the Light is too great to allow of our considering it at a distance of 170,000,000 of miles, as its lower end would be, near dawn, if it is a ring around the sun and beyond the earth's orbit; and 3dly, from the moon's Zodiacal Light, if real, which I think it is.

That it is a ring, the unbroken continuity of my observations satisfactorily determines.

For more than two years, I never failed to see this Light, evening and morning, when the moon and clouds did not interfere; and, except one evening, I have continuous records of this kind.

"I could get no parallax; but.

on the contrary, as we went south, the boundaries of the Zodiacal Light changed with us to the south among the stars; and so, vice versa, towards the north, caused, doubtless, by the ring's presenting new portions of its wide reflecting surface to the sun's light.

"George Jones, Chaplain U.S. Navy."

Brooklyn, May 17, 1855.