Art. XV.—On the Zodiacal Light, as seen in Southern Latitudes.
[Read before the Otago Institute, 14th September, 1874.]
Not only has the science of astronomy received a most remarkable impetus by the use of the spectroscope, but that of meteorology has been extended likewise, so that from treating only of mundane or terrestrial results—as the weather—it now claims within its domain the magnificent phenomena of solar atmospheric action, the physical formation of stars, comets, and meteors, and even to the bounds of the known universe it ascertains without a doubt the gaseous constitution of some of those distant nebulæ and the remarkable changes which they undergo.
Intimately connected with these subjects may be considered the investigations on the nature of the zodiacal light. In a previous paper * on this subject a theory was advanced in which the elongation of the medium of this remarkable illumination was attributed to the action of an interstellar and resisting medium upon the heat-repelled constituents of the solar atmosphere, thus causing a portion thereof to drift in rear of the sun's proper motion, whereby it would cease to retain the diurnal rotation and tend towards an orbital motion; and the times of visibility of this light in the northern hemisphere, as given by Herschel, were adduced as leading to the conclusion that in the month of November the earth must pass near if not actually through it annually, the November meteors being instanced as the result of the collision. A diagram was also given † explaining its appearance as seen from the northern hemisphere.
In November, 1872, Mr. Webb gave an interesting account of the most recent observations on this light, ‡ stating that it was visible in some parts of the earth nearly all the year round. The accounts therein given do not, however, furnish us with the amount of elongation, nor mention whether that elongation is constant, nor yet inform us whether it is visible in the mornings and evenings of the same day, and the time of year that it is invisible; and as these data are necessary in order to ascertain the true shape of the zodiacal envelope I therefore commenced tabulating what could be observed on these points in our southern latitude from 1872, the results of which are appended:—
1872, July 2nd—An illumination along the ecliptic was observed in the western part of the sky at 7.30 p.m. It was nearly as bright as all but the brightest portion of the Milky Way, which was also very brilliantly shining. The air was very transparent and the hygrometer showed saturation. The
[Footnote] * Trans. N.Z. Inst, Vol. V., App. p. xliii.
[Footnote] † Vol. V., pl. XIV.
[Footnote] ‡ Vol. V., App. p. xlvii.
light subtended an arc of 40° from the base to the vertex. It was also seen on the 5th after twilight and on the 26th, at 7.30, it subtended an arc of 45°.
August 1st—It was very clearly visible at 8 p.m., subtending an arc of nearly 60°; also on the 26th, when the air was so transparent that small stars could be seen very near to the horizon. The length of arc being about 45°.
September 1st—It was also observed. The angle subtended is less, but the vertex now reaches nearly to the Milky Way; also on the 19th and very distinctly on the 27th, its vertex now reaches the Milky Way in the constellation Scorpio.
October 2nd, 4th, 5th, and 21st, the same light was visible for a short time after sunset, but its vertex soon sets at this season and ceases to be seen in the evenings. This light was never seen in the mornings before twilight and sunrise in any of the preceding months, although looked for under favourable circumstances, but it begins to be faintly seen in the mornings in the beginning of March, and on the 31st it was very distinct, extending along the ecliptic, subtending an arc of 30° at 3 a.m., and of 45° at 4 a.m., reaching nearly to the Milky Way and pointing to Antares. During April and May it is still seen, but in June it becomes so faint as to be doubtful. It began to be faintly seen, however, in the western sky at the end of June in the ecliptic pointing and extending near to Spica Virginis and was seen every favourable evening in July reaching to Spica Virginis. In August it extended beyond that star reaching nearly to Antares, subtending on the 23rd an arc of 50° at 8.30 p.m. In September it had the same appearance as in 1872, extending to Antares.
During the years 1873 and 1874 it also presented the same appearance at those times.
A summary of these observations shows that in S. Lat. 46° this illumination begins to be visible in the evenings after twilight, and when the moon is absent at the end of June; early in August it attains its greatest elongation and brightness, forming quite a feature in celestial scenery; it then gradually decreases till the end of October, when it ceases to appear in the evenings, but can be seen in the mornings in March, April, and May, and very faintly in June. Its inclination to the plane of the ecliptic appears to be nearly four degrees, the advancing cone seen in this latitude lying on the south side of the ecliptic.
It always points towards the same constellation, Scorpio, whether seen in mornings or evenings, showing that it is that part only of the zodiacal envelope which is on one side of the solar orb which can be seen in this latitude, and that that part is in advance of the solar motion in space.
The colour of the illumination as seen in south latitude is invariably clear
white, while the colour of that part of the envelope seen in north latitude, and which is in the rear of the sun's motion pointing towards the constellation Taurus, if projected on the ecliptic, is said to be a characteristic rose-red. This indicates a difference of constitution suggesting a spectroscopic examination.
The vertex of this light appears to be projected by parallax on different parts of the ecliptic by the earth's motion in its orbit, thus appearing to prove that its shape is elongated, and not circular, for if circular, then it would subtend a similar arc all the year round.
It regularly decreases in altitude in the evenings by an amount corresponding to the angular motion of the earth on its axis, and when seen in the mornings it increases in the same ratio, from the first appearance of the apex above the horizon to such a time as it ceases to be seen from its delicate illumination being overpowered by the solar glare, which fact appears quite sufficient to prove its extra-terrestrial origin.
From the constancy of its appearance at the above seasons it does not appear to be of an intermittent nature, such as we might expect if produced electrically, or if it was of a similar nature to our auroras.
Observations on zodiacal light, comets, and meteors, are fraught with renewed interest now that such stupendous commotions are known to occur near the sun's surface, especially after the connection apparently established between the highly attenuated material of Biela's comet with the meteoric display of 27th November, 1872; and it is to the meteorological changes of matter in the sun's neighbourhood through all its possible states from gaseous to vapourous—liquid and solid, and again from solid to liquid—vapourous and gaseous, that we may probably look as furnishing a clue to one of the most important problems in modern physics, namely, explaining the action of those laws of attraction to a centre, and of heat repulsion from a centre, which appears to characterise all cosmical aggregations of matter.