Digitale Ausgabe

“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 resultof his observations of the Zodiacal Light in the seas of China andJapan, the conjecture of a second radiating ring of light having arelation to the moon. This conjecture is founded upon the extra-ordinary spectacle of the Zodiacal Light simultaneously observedat both east and west horizons from eleven to one o’clock, duringseveral days in succession. As I observed something analogousfifty-two years ago in the Southern Ocean during the voyage offorty days from Callao, in Peru, to the port of Acapulco in Mexico,and have given only a very brief account of it in the astronomicalpart of my Cosmos, it may not be uninteresting to the members ofthe Academy if I laid before them an extract from my FrenchJournal, written at sea, relating to this phenomenon, which hithertohas not formed the subject of any detailed remarks. The ZodiacalLight, and the difficult question whether we ought to attribute toa physical cause existing beyond our atmosphere the remarkablevariations of light which it undergoes, while in tropical nights thesmallest stars exhibit the same brightness to the naked eye, were

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* Monatsbericht der Kön. Preuss. Akademie der Wissenschaften, Juli, 1855.|17| subjects which engaged my attention during a period of five yearsupon great heights among the Cordilleras, in the extensive plainsor Llanos, at sea, and on both sides of the equator, as will be seenby reference to my partly published correspondence with Olbers(Cosmos, vol. i. p. 412.) From my ship-journal I extract thefollowing observations, extending from the 14th to the 19th ofMarch, 1803, between north latitude 12° 9′ and 15° 20′, andchronometrical longitude 104° 27′ and 105° 46′ west of Paris. “On the 17th and 18th of March the Zodiacal Light, the baseof which appeared to rest upon the sun, shone with a brightnesswhich I had never seen on any former occasion of the approachof the vernal equinox. The luminous pyramid terminated be-tween Aldebaran and the Pleiades at an apparent altitude of39° 5′ measured above the sea-horizon, which was still sufficientlyvisible. The vertex was somewhat inclined towards the north;and the direction of the brightest part appeared by the compass tobe west-north-west. What has struck me most during thisvoyage, is the great regularity with which, during five or sixnights in succession, the brightness of the Zodiacal Light progres-sively increased and diminished. Its existence was hardly dis-cernible during the first three-quarters of an hour after sunset,although the darkness was sufficiently great to render visible thestars of the fourth and fifth magnitude; but after 7h 15m the lumi-nous spindle appeared at once in all its beauty. Its colour wasnot white, like that of the milky way, but a reddish yellow, asDominique Cassini assures us he had seen it in Europe. Verysmall clouds, situated accidentally towards the horizon, reflectedupon the reddish ground a lively blue light. One would almostsuppose he saw a second sunset in the west. About ten o’clockthe light entirely disappeared; at midnight I perceived only afeeble trace of it, although the celestial vault still exhibited thesame degree of transparency. While the light was very bright inthe west, we constantly perceived in the east (and this is beyonddoubt a very striking phenomenon) a whitish light, which wasalso of a pyramidal form. The latter augmented the brightnessof the sky in a very striking manner. Even the sailors were de-lighted with this double light in the west and the east; and I aminclined to think that this white light in the east was the reflexionof the real Zodiacal Light at setting. Both also disappeared at thesame time. Analogous reflexions frequently present themselves inour climates at sunset, but I should never have imagined that thebrightness of the zodiacal light could be sufficiently strong torepeat itself by the simple reflexion of the rays. All these lumi-nous appearances were almost the same from the 14th to the 19thof March. We did not see the Zodiacal Light on the 20th and21st of March, although the nights were beautiful in the highestdegree.” These are the words of my ship-journal, containing my obser-vations, and also the thoughts which they suggested at the time tomy mind. It was in reference to what I had written down in an |18| unpublished ship-journal on the occasion of a voyage in theSouthern Ocean, about the beginning of the present century, that,five years previous to the publication of the interesting observa-tions of the Rev. Mr. George Jones, I made the following state-ment in the astronomical part of Cosmos:— “On the whole the variations of the Zodiacal Light appear tome to depend upon variations inherent in the phenomenon, uponthe greater or less intensity of the luminous processes going on inthe ring. This is proved by my observations in the SouthernOcean, which indicated an opposite light in the heavens similar tothat seen at sunset.” (Cosmos, vol. iii. p. 589.) I may remark further that I have been surprised at the in-creased brightness of the Zodiacal Light upon ascending to greataltitudes. This was observable upon the lofty peaks of theCordilleras 10,000 or 12,000 feet high; also in Mexico, in Jan-uary 1804, at altitudes of only 7000 feet; and from the Cloisterof Mount Cenis, where I remained with Gay Lussac severalnights (in March 1805) at an altitude of 6350 feet, for the purposeof determining the intensity of the magnetic force during veryintense cold, and ascertaining the quantity of oxygen contained inthe atmosphere. It was, consequently, seen both in tropical and intemperate latitudes. But the variations in the brightness of thephenomenon cannot, according to my experience, be accounted forsolely by the constitution of our atmosphere. There remains muchstill to be observed relative to this subject. The following is a copy of the letter to which Baron Humboldtrefers:— (From the Rev. George Jones, U.S.N. to the Editor of the AstronomicalJournal, 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 theglobe, I had excellent opportunities for observation of the ZodiacalLight. This light, you know, appears to the best advantage withinthe tropics, where it stretches upward to a great elevation, and isa remarkable object; but it is also very desirable to observe it inhigh latitudes; and in this also I was favoured, as our voyagingextended from 41° north to 53° south latitude; and in some in-stances our transitions, for weeks together, were very rapid, thusgiving me opportunities for observing whether any parallax couldbe made or not. “I was also fortunate enough to be twice near the latitude of23° 28′ north, when the sun was at the opposite solstice, in whichposition the observer has the ecliptic, at midnight, at right angleswith his horizon, and bearing east and west. Whether the lattercircumstance affected the result or not I cannot say; but I thenhad the extraordinary spectacle of the Zodiacal Light, simul-taneously at both east and west horizons, from 11 to 1 o’clock, forseveral nights in succession. “In the first part of our cruise my observations were of a |19| desultory character; but I soon began to see the necessity of greatprecision, and accordingly constructed star-charts from a celestialglobe (a small but excellent one) that happened to be on board,which charts I afterwards had cut in wood at Canton; and thus Iwas furnished with materials for accurately recording all thechanges of this phenomenon, not only in successive nights, butalso in the successive hours of the same night. My rule was, todraw on my charts the boundaries of the Zodiacal Light as ex-hibited among the stars, with such annotations as the case re-quired; then, again, do the same after an interval of an hour orhalf-hour; and so to continue, generally as long as the boundariescould be made out reliably; then, if the morning admitted it, toresume observations at the earliest possible hour, and so to proceeduntil the dawn. I have thus, in many instances, observations forevery hour of the night. “At an early period I began to query whether the moon, whennear its full, might not give a Zodiacal Light: and at last, when Ihad gained more experience in observing, and in the peculiar cha-racter of this light, I was able to get, at different periods, fourteenreliable observations of what I think must be considered a Zodi-acal Light produced by the moon. I have also two records of adistinct Zodiacal Light produced by the joint action of the sun andmoon, i. e. at the hour when the moon, then near its first quar-tering, was about 65° above the western horizon; the reflexionfrom the combined light of the sun and moon being sufficient tooverpower the moonlight proper, and thus to produce a decidedstream of light in the sky within the Zodiacal-Light boundaries.The latter of these observations was the more remarkable, inas-much as the moon was then without the boundary of this jointreflected light. “You will excuse my prolixity in stating these varieties ofobservation, for the conclusion from all the data in my possessionis a startling one. It seems to me that these data can be explainedonly by the supposition of a nebulous ring with the earth for itscentre, and lying within the orbit of the moon. This conclusionseems to evolve itself, — 1st, from the simultaneous midnight eastand west observations, which preclude the possibility of a ringaround the sun within the earth’s orbit; 2dly, from the greathourly lateral changes (often semi-hourly) in the boundaries of theLight, caused by the observer’s change of place, in that time, asregards the ecliptic or axis of the Zodiacal Light, which lateralchange in the Light is too great to allow of our considering it at adistance of 170,000,000 of miles, as its lower end would be, neardawn, 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 itis. That it is a ring, the unbroken continuity of my observationssatisfactorily determines. For more than two years, I never failedto see this Light, evening and morning, when the moon and cloudsdid not interfere; and, except one evening, I have continuousrecords of this kind. |20| “I could get no parallax; but. on the contrary, as we wentsouth, the boundaries of the Zodiacal Light changed with us tothe south among the stars; and so, vice versâ, towards the north,caused, doubtless, by the ring’s presenting new portions of its widereflecting surface to the sun’s light. “George Jones, “Chaplain U.S. Navy.” Brooklyn, May 17, 1855.