Art. XIV.—Notes on the reported Collision of Biela's Comet with the Earth's Atmosphere.
[Read before the Otago Institute, 11th June, 1874.]
With reference to a paper read before this Institute on 12th March, 1872, on the zodiacal light, * in which a theory is advanced tending to show that the periodic November meteors form part of that illuminated medium, thus forming a probable clue as to the real direction of solar motion in space, it will, I think, be interesting to review the published results which followed the actual collision of Biela's comet with the earth's atmosphere on the 27th of November following, and which confirms in a remarkable manner the hypothesis set forth in that paper of the action which would result from the collision of meteoric vapours with the earth's atmosphere; for it is now generally conceded that to Biela's comet is due the magnificent display of shooting-stars which, on the night of the 27th November, 1872, streamed into our terrestrial atmosphere.
[Footnote] * Trans. N.Z. Inst., V., app. p. lxiii.
This collision may, I think, be fairly considered to be one of the most remarkable astronomical occurrences of modern times, and appears to invalidate the generally received opinion as to the solidity of the matter of meteors, and consequently induces us to question the correctness of what is known as the “Meteoric Theory of the Sun,” which assumes the sun's light and heat as being sustained by its surface being bombarded by myriads of solid meteors, and the zodiacal light has been conjectured as being an illuminated shower, or rather tornado of stones, which are to furnish their equivalents of heat and light by the sudden check to their motion when they fall into it.
Now comets are known to be bodies of almost inconceivable tenuity, as proved by the immense perturbations they undergo by the attraction of the planets, without being able to induce any appreciable effect themselves on the orbits of the planets, or even of their satellites. Another proof of their tenuity is the fact of their not being well seen in telescopes of high magnifying power, and it is highly probable that a comet, when very close to the earth, cannot be seen at all by its ordinary illumination, for its light would be so much diffused, and subtend so large an angle, that its illumination would fail to be detected.
Immediately after the meteoric display alluded to it occurred to Klinker-fues to suggest to the Madras astronomer a search of the heavens near theta Centauri, or in direction diametrically opposite to the radiant point of the meteors (gamma Andromedœ) for the missing comet, and a cometic-looking object was actually noted by him, but not sufficiently near to that point to render it demonstrable that the object he saw was the truant comet or any part thereof. When, however, we consider the enormous perturbation the comet must have undergone by its proximity to the earth probably that difficulty might vanish. It has been calculated that the meteor stream of the 27th November was nearly twelve weeks behind the head of the comet. Now we can scarcely suppose that any detached collection of solids should be dragged after a comet, such would surely be contrary to all analogy. The conclusion presents itself that if a comet is so drawn out, either by the perturbations of the planets, or by a resisting medium, the parts so drawn out would be of greater tenuity than the comet itself.
We may, therefore, fairly infer that matter in a non-solid or vapourous form, when coming into collision with the earth's atmosphere at orbital velocities is competent to produce all the phenomena known as shooting stars and meteors.
At a future meeting I shall be prepared to give another paper embodying a series of actual observations “On the direction—the arc subtended—and times of visibility of the Zodiacal Light,” made since 1872, tending to confirm the theory that this medium has points of greatest elongation in a line coinciding with that of the sun's motion in space.