« السابقةمتابعة »
On Nature's alps I stand,
COMETARY ASTRONOMY. Tails of Comets. The tail of a comet is a continuation of the envelope, which, after nearly encompassing the hemisphere of the nucleus that is next to the sun, diverges to a greater or less extent in the opposite direction. There are some comets which have no tails, and these, it may be observed, are those whose periods of revolution are short, and in their physical constitution, mere collections of nebulous matter.
Comets bave no tails till they approach the sun : at the first the tail is scarcely perceptible; as the comet advances, the tail increases in length, till having wound its way round the solar orb, it diminishes in length as its distance from the sun increases, and, ultimately, disappears. The tail is always turned from the sun, whether the comet be approaching or retreating, and there can be no doubt but that this direction of it is produced by the solar action. According to some philosophers, the rays of light push off the matter of the nebulous envelope, after it has been raised by an approximation to the sun; according to others, the tail is produced by an unknown power of repulsion in the sun; others again argue, that the tail is raised by the attraction of the sun, in the same way that the tides of the ocean are raised by the moon; another opinion is, that there is a very dense atmosphere about the sun, in which the very rare matter of a comet's tail ascends as smoke does in the terrestrial atmosphere; others again attempt to explain the phenomenon on electrical principles, and assert, that just as bodies charged with one kind of electricity repel each other, so the tails of comets have a natural tendency to avoid or fly from the sun.
Without entering on the vain fancies, and unphilosophical reasonings connected with this branch of our subject, we conceive that the first mentioned hypothesis is an approximation to the truth, namely, that the solar rays impel or push off the nebulous matter raised about the nucleus into the form of a streaming tail; this nebulous matter may be supposed to be generated by the unequal action of the sun on the comet,--an unequal action, caused by its varying its distance so much from the centre of its motion; when near its aphelion it would bave its physical constitution considerably changed,- vapours and exhalations would be generated and accumulated in great quantities : loaded with these vapours, it would return towards the sun, the influences of which would elevate them to a considerable height above the nucleus; as it approached nearer, and felt more powerfully the solar energy, these accumulated vapours would be impelled in the direction in which light, heat, and other emanations, proceed from the sun, --that is, in right lines; so that if a comet were urging its course direct to the centre, the tail would be in
exactly the direction in which rays of light proceed from the sun.
But the path of a comet is curvilinear, and this figure of its course will account for a phenomenon observed in its tail, that it is not exactly opposite to the sun, and is also curved at the extremity; the nebulous projection is acted upon by two forces,-the impulsion of the solar rays, and the centrifugal principle by which a comet describes a curvilineal path ; it must also be observed, that the nucleus, or more dense part, is especially influenced by the sun's attraction, and that the motion of the tail is governed by the attraction of the nucleus. The curve-like form in the tail is most conspicuous when the comet is nearest the sun, which then moves quickest, consequently, the tail is behind that place to which a line would be directed if drawn from the sun to the comet.
In the tails of some comets there have been observed a dark line, proceeding to some distance from the nucleus; in others, this part has been distinguished for its superior brilliancy; such were the appearances observed in the comets of 1577 and 1618, while the comet of 1744 was observed to assume both of these appearances at different times. Some comets have exhibited corruscations in their tails, which singular appearance was observed in the comets of 1680; that of 1744 sparkled forth, or vibrated luminous particles. These remarkable phenomena might have been owing to physical changes in these bodies, though, doubtless, much is to be attributed to the state of the terrestrial atmosphere.
The tails of comets vary very considerably in their forms, --some straight, others curved, some whose sides are parallel, some expanding like a fan, others bifurcated. In the comet of 1744 the tail was divided into two branches, one part was 7° or 8° long, and the other 24°. In the comet of 1769 a stream appeared on each side of the tail, proceeding from the nucleus; the sides of these streams, or lesser tails, were parallel to each other, and between them could be distinguished a darkish space, through which telescopic stars could be distinctly perceived. An appearance, of a similar nature, was observed in the comet of 1811. In the second comet of 1825 the tail was bifid, being divided into two great branches, going off immediately from the nucleus at an angle of 45° with each other. (See plate, Second comet of 1825.)
Though the fact of the rotation of comets is uncertain, that of the rotation of the tail is sufficiently proved, at least, in one instance, that of the comet of 1825,one of the streams of the tail returned to the same form and position within a certain time, from whence it was concluded, that the period of rotation was twenty hours and a half; it may be suggested that this rotation of the tail might also have been accompanied by a rotation of the comet, of which the axis of the tail might have been the axis of revolution.
The tails of comets are hollow, which idea is suggested by the nature of the solar action, which can only affect those parts more immediately exposed to its influence, namely, that hemisphere of the comet which is on the side next the sun, the plane of which may be considered as the frustrum of a hollow cone; the appearance of the tail warrants this conclusion, the upper and under sides are better defined, and more brilliant than the middle,
as on the supposition of their being hollow, more rays would be seen at the edges where the eye passes through tbem obliquely, than in the middle, which is the plane of direct vision. In the comet of 1769, just referred to, there can be no doubt but that there were two of these hollow cones,—the one within the other; the outer one very thin, and only seen at the edges, where it appeared as two streams of light.
Often-so much I loved to trace
Nay, oft, so passionate my chase
Should scape me in the farthest night-
To visit distant climes of light;
Exulting out, when on my sight
“ Loves of the Angels.”