Part 16

288. _The Tails of Comets._--The _tails_ of brilliant comets are rapidly formed as the comet approaches the sun, their increase in length often being at the rate of several million miles a day. These appendages seem to be formed entirely out of the matter which is emitted from the nucleus in the luminous jets which are at first directed towards the sun. The tails of comets are, however, always directed away from the sun, as shown in Fig. 315.

It will be seen that the comet, as it approaches the sun, travels head foremost; but as it leaves the sun it goes tail foremost.

The apparent length of the tail of a comet depends partly upon its real length, partly upon the distance of the comet, and partly upon the direction of the axis of the tail with reference to the line of vision. The longer the tail, the nearer the comet; and the more nearly at right angles to the line of vision is the axis of the tail, the greater is the apparent length of the tail. In the majority of cases the tails of comets measure only a few degrees; but, in the case of many comets recorded in history, the tail has extended half way across the heavens.

The tail of a comet, when seen at all, is usually several million miles in length; and in some instances the tail is long enough to reach across the orbit of the earth, or twice as far as from the earth to the sun.

The tails of comets are apparently hollow, and are sometimes a million of miles in diameter. So great, however, is the tenuity of the matter in them, that the faintest stars are seen through it without any apparent obscuration. See Fig. 316, which is a view of the great comet of 1264.

The tails of comets are sometimes straight, as in Fig. 316, but usually more or less curved, as in Fig. 317, which is a view of _Donati's_ comet as it appeared at one time. The tail of a comet is occasionally divided into a number of streamers, as in Figs. 318 and 319. Fig. 318 is a view of the great comet of 1744, and Fig. 319 of the great comet of 1861. No. 1, in Fig. 320, is a view of the comet of 1577; No. 2, of the comet of 1680; and No. 3, of the comet of 1769.

Fig. 321 shows some of the forms which the imagination of a superstitious age saw depicted in comets, when these heavenly visitants were thought to be the forerunners of wars, pestilence, famine, and other dire calamities.

289. _Visibility of Comets._--Even the brightest comets are visible only a short time near their perihelion passage. When near the sun, they sometimes become very brilliant, and on rare occasions have been visible even at mid-day. It is seldom that a comet can be seen, even with a powerful telescope, during its perihelion passage, unless its perihelion is either inside of the earth's orbit, or but little outside of it.

Motion and Origin of Comets.

290. _Recognition of a Telescopic Comet._--It is impossible to distinguish telescopic comets by their appearance from another class of heavenly bodies known as _nebulæ_. Such comets can be recognized only by their motion. Thus, in Fig. 322, the upper and lower bodies look exactly alike; but the upper one is found to remain stationary, while the lower one moves across the field of view. The upper one is thus shown to be a nebula, and the lower one a comet.

291. _Orbits of Comets._--All comets are found to move in _very eccentric ellipses_, in _parabolas_, or in _hyperbolas_.

Since an ellipse is a _closed_ curve (48), all comets that move in ellipses, no matter how eccentric, are permanent members of the solar system, and will return to the sun at intervals of greater or less length, according to the size of the ellipses and the rate of the comet's motion.

Parabolas and hyperbolas being _open_ curves (48), comets that move in either of these orbits are only temporary members of our solar system. After passing the sun, they move off into space, never to return, unless deflected hither by the action of some heavenly body which they pass in their journey.

Since a comet is visible only while it is near the sun, it is impossible to tell, by the form of the portion of the orbit which it describes during the period of its visibility, whether it is a part of a very elongated ellipse, a parabola, or a hyperbola. Thus in Fig. 323 are shown two orbits, one of which is a very elongated ellipse, and the other a parabola. The part _ab_, in each case, is the portion of the orbit described by the comet during its visibility. While describing the dotted portions of the orbit, the comet is invisible. Now it is impossible to distinguish the form of the visible portion in the two orbits. The same would be true were one of the orbits a hyperbola.

Whether a comet will describe an ellipse, a parabola, or a hyperbola, can be determined only by its _velocity_, taken in connection with its _distance from the sun_. Were a comet ninety-two and a half million miles from the sun, moving away from the sun at the rate of twenty-six miles a second, it would have just the velocity necessary to describe a _parabola_. Were it moving with a greater velocity, it would necessarily describe a _hyperbola_, and, with a less velocity, an _ellipse_. So, at any distance from the sun, there is a certain velocity which would cause a comet to describe a parabola; while a greater velocity would cause it to describe a hyperbola, and a less velocity to describe an ellipse. If the comet is moving in an ellipse, the less its velocity, the less the eccentricity of its orbit: hence, in order to determine the form of the orbit of any comet, it is only necessary to ascertain its distance from the sun, and its velocity at any given time.

Comets move in every direction in their orbits, and these orbits have every conceivable inclination to the ecliptic.

292. _Periodic Comets._--There are quite a number of comets which are known to be _periodic_, returning to the sun at regular intervals in elliptic orbits. Some of these have been observed at several returns, so that their period has been determined with great certainty. In the case of others the periodicity is inferred from the fact that the velocity fell so far short of the parabolic limit that the comet must move in an ellipse. The number of known periodic comets is increasing every year, three having been added to the list in 1881.

The velocity of most comets is so near the parabolic limit that it is not possible to decide, from observations, whether it falls short of it, or exceeds it. In the case of a few comets the observations indicate a minute excess of velocity; but this cannot be confidently asserted. It is not, therefore, absolutely certain that any known comet revolves in a hyperbolic orbit; and thus it is possible that all comets belong to our system, and will ultimately return to it. It is, however, certain, that, in the majority of cases, the return will be delayed for many centuries, and perhaps for many thousand years.

293. _Origin of Comets._--It is now generally believed that the original home of the comets is in the stellar spaces outside of our solar system, and that they are drawn towards the sun, one by one, in the long lapse of ages. Were the sun unaccompanied by planets, or were the planets immovable, a comet thus drawn in would whirl around the sun in a parabolic orbit, and leave it again never to return, unless its path were again deflected by its approach to some star. But, when a comet is moving in a parabola, the slightest _retardation_ would change its orbit to an ellipse, and the slightest _acceleration_ into a hyperbola. Owing to the motion of the several planets in their orbits, the velocity of a comet would be changed on passing each of them. Whether its velocity would be accelerated or retarded, would depend upon the way in which it passed. Were the comet accelerated by the action of the planets, on its passage through our system, more than it was retarded by them, it would leave the system with a more than parabolic orbit, and would therefore move in a hyperbola. Were it, on the contrary, retarded more than accelerated by the action of the planets, its velocity would be reduced, so that the comet would move in a more or less elongated ellipse, and thus become a permanent member of the solar system.

In the majority of cases the retardation would be so slight that it could not be detected by the most delicate observation, and the comet would return to the sun only after the expiration of tens or hundreds of thousands of years; but, were the comet to pass very near one of the larger planets, the retardation might be sufficient to cause the comet to revolve in an elliptical orbit of quite a short period. The orbit of a comet thus captured by a planet would have its aphelion point near the orbit of the planet which captured it. Now, it happens that each of the larger planets has a family of comets whose aphelia are about its own distance from the sun. It is therefore probable that these comets have been captured by the action of these planets. As might be expected from the gigantic size of Jupiter, the Jovian family of comets is the largest. The orbits of several of the comets of this group are shown in Fig. 324.

294. _Number of Comets._--The number of comets recorded as visible to the naked eye since the birth of Christ is about five hundred, while about two hundred telescopic comets have been observed since the invention of the telescope. The total number of comets observed since the Christian era is therefore about seven hundred. It is certain, however, that only an insignificant fraction of all existing comets have ever been observed. Since they can be seen only when near their perihelion, and since it is probable that the period of most of those which have been observed is reckoned by thousands of years (if, indeed, they ever return at all), our observations must be continued for many thousand years before we have seen all which come within range of our telescopes. Besides, as already stated (289), a comet can seldom be seen unless its perihelion is either inside the orbit of the earth, or but little outside of it; and it is probable that the perihelia of the great majority of comets are beyond this limit of visibility.

Remarkable Comets.

295. _The Comet of 1680._--The great comet of 1680, shown in Fig. 320, is one of the most celebrated on record. It was by his study of its motions that Newton proved the orbit of a comet to be one of the conic sections, and therefore that these bodies move under the influence of gravity. This comet descended almost in a direct line to the sun, passing nearer to that luminary than any comet before known. Newton estimated, that, at its perihelion point, it was exposed to a temperature two thousand times that of red-hot iron. During its perihelion passage it was exceedingly brilliant. Halley suspected that this comet had a period of five hundred and seventy-five years, and that its first recorded appearance was in 43 B.C., its third in 1106, and its fourth in 1680. If this is its real period, it will return in 2255. The comet of 43 B.C. made its appearance just after the assassination of Julius Cæsar. The Romans called it the _Julian Star_, and regarded it as a celestial chariot sent to convey the soul of Cæsar to the skies. It was seen two or three hours before sunset, and continued visible for eight successive days. The great comet of 1106 was described as an object of terrific splendor, and was visible in close proximity to the sun. The comet of 1680 has become celebrated, not only on account of its great brilliance, and on account of Newton's investigation of its orbit, but also on account of the speculation of the theologian Whiston in regard to it. He accepted five hundred and seventy-five years as its period, and calculated that one of its earlier apparitions must have occurred at the date of the flood, which he supposed to have been caused by its near approach to the earth; and he imagined that the earth is doomed to be destroyed by fire on some future encounter with this comet.

296. _The Comet of 1811._--The great comet of 1811, a view of which is given in Fig. 325, is, perhaps, the most remarkable comet on record. It was visible for nearly seventeen months, and was very brilliant, although at its perihelion passage it was over a hundred million miles from the sun. Its tail was a hundred and twenty million miles in length, and several million miles through. It has been calculated that its aphelion point is about two hundred times as far from the sun as its perihelion point, or some seven times the distance of Neptune from the sun. Its period is estimated at about three thousand years. It was an object of superstitious terror, especially in the East. The Russians regarded it as presaging Napoleon's great and fatal war with Russia.

297. _Halley's Comet._--Halley's comet has become one of the most celebrated of modern times. It is the first comet whose return was both predicted and observed. It made its appearance in 1682. Halley computed its orbit, and compared it with those of previous comets, whose orbits he also computed from recorded observations. He found that it coincided so exactly with that of the comet observed by Kepler in 1607, that there could be no doubt of the identity of the two orbits. So close were they together, that, were they both drawn in the heavens, the naked eye would almost see them joined into one line. There could therefore be no doubt that the comet of 1682 was the same that had appeared in 1607, and that it moved in an elliptic orbit, with a period of about seventy-five years. He found that this comet had previously appeared in 1531 and in 1456; and he predicted that it would return about 1758. Its actual return was retarded somewhat by the action of the planets on it in its passage through the solar system. It, however, appeared again in 1759, and a third time in 1835. Its next appearance will be about 1911. The orbit of this comet is shown in Fig. 326. Fig. 327 shows the comet as it appeared to the naked eye, and in a telescope of moderate power, in 1835. This comet appears to be growing less brilliant. In 1456 it appeared as a comet of great splendor; and coming as it did in a very superstitious age, soon after the fall of Constantinople, and during the threatened invasion of Europe by the Turks, it caused great alarm. Fig. 328 shows the changes undergone by the nucleus of this comet during its perihelion passage in 1835.

298. _Encke's Comet._--This telescopic comet, two views of which are given in Figs. 329 and 330, appeared in 1818. Encke computed its orbit, and found it to lie wholly within the orbit of Jupiter (Fig. 324), and the period to be about three years and a third. By comparing the intervals between the successive returns of this comet, it has been ascertained that its orbit is continually growing smaller and smaller. To account for the retardation of this comet, Olbers announced his celebrated hypothesis, that the celestial spaces are filled with a subtile _resisting medium_. This hypothesis was adopted by Encke, and has been accepted by certain other astronomers; but it has by no means gained universal assent.

299. _Biela's Comet._--This comet appeared in 1826, and was found to have a period of about six years and two thirds. On its return in 1845, it met with a singular, and as yet unexplained, accident, which has rendered the otherwise rather insignificant comet famous. In November and December of that year it was observed as usual, without any thing remarkable about it; but, in January of the following year, it was found to have been divided into two distinct parts, so as to appear as two comets instead of one. The two parts were at first of very unequal brightness; but, during the following month, the smaller of the two increased in brilliancy until it equalled its companion; it then grew fainter till it entirely disappeared, a month before its companion. The two parts were about two hundred thousand miles apart. Fig. 331 shows these two parts as they appeared on the 19th of February, and Fig. 332 as they appeared on the 21st of February. On its return in 1852, the comets were found still to be double; but the two components were now about a million and a half miles apart. They are shown in Fig. 333 as they appeared at this time. Sometimes one of the parts appeared the brighter, and sometimes the other; so that it was impossible to decide which was really the principal comet. The two portions passed out of view in September, and have not been seen since; although in 1872 the position of the comet would have been especially favorable for observation. The comet appears to have become completely broken up.

300. _The Comet of 1843._--The great comet of 1843, a view of which is given in Fig. 334, was favorably situated for observation only in southern latitudes. It was exceedingly brilliant, and was easily seen in full daylight, in close proximity to the sun. The apparent length of its tail was sixty-five degrees, and its real length a hundred and fifty million miles, or nearly twice the distance from the earth to the sun. This comet is especially remarkable on account of its near approach to the sun. At the time of its perihelion passage the distance of the comet from the photosphere of the sun was less than one-fourteenth of the diameter of the sun. This distance was only one-half that of the comet of 1680 when at its perihelion. When at perihelion, this comet was plunging through the sun's outer atmosphere at the rate of one million, two hundred and eighty thousand miles an hour. It passed half way round the sun in the space of _two hours_, and its tail was whirled round through a hundred and eighty degrees in that brief time. As the tail extended almost double the earth's distance from the sun, the end of the tail must have traversed in two hours a space nearly equal to the circumference of the earth's orbit,--a distance which the earth, moving at the rate of about twenty miles a second, is a _whole year_ in passing. It is almost impossible to suppose that the matter forming this tail remained the same throughout this tremendous sweep.

301. _Donati's Comet._--The great comet of 1858, known as _Donati's_ comet, was one of the most magnificent of modern times. When at its brightest it was only about fifty million miles from the earth. Its tail was then more than fifty million miles long. Had the comet at this time been directly between the earth and sun, the earth must have passed through its tail; but this did not occur. The orbit of this comet was found to be decidedly elliptic, with a period of about two thousand years. This comet is especially celebrated on account of the careful telescopic observations of its nucleus and coma at the time of its perihelion passage. Attention has already been called (287) to the changes it underwent at that time. Its tail was curved, and of a curious feather-like form, as shown in Fig. 335. At times it developed lateral streamers, as shown in Fig. 336. Fig. 337 shows the head of the comet as it was seen by Bond of the Harvard Observatory, whose delineations of this comet have been justly celebrated.

302. _The Comet of 1861._--The great comet of 1861 is remarkable for its great brilliancy, for its peculiar fan-shaped tail, and for the probable passage of the earth through its tail. Sir John Herschel declared that it far exceeded in brilliancy any comet he had ever seen, not excepting those of 1811 and 1858. Secchi found its tail to be a hundred and eighteen degrees in length, the largest but one on record. Fig. 338 shows this comet as it appeared at one time. Fig. 339 shows the position of the earth at _E_, in the tail of this comet, on the 30th of June, 1861. Fig. 340 shows the probable passage of the earth through the tail of the comet on that date. As the tail of a comet doubtless consists of something much less dense than our atmosphere, it is not surprising that no noticeable effect was produced upon us by the encounter, if it occurred.

303. _Coggia's Comet._--This comet, which appeared in 1874, looked very large, because it came very near the earth. It was not at all brilliant. Its nucleus was carefully studied, and was found to develop a series of envelops similar to those of Donati's comet. Figs. 341 and 342 are two views of the head of this comet. Fig. 343 shows the system of envelops that were developed during its perihelion passage.

304. _The Comet of June, 1881._--This comet, though far from being one of the largest of modern times, was still very brilliant. It will ever be memorable as the first brilliant comet which has admitted of careful examination with the spectroscope.

Connection between Meteors and Comets.

305. _Shooting-Stars._--On watching the heavens any clear night, we frequently see an appearance as of a star shooting rapidly through a short space in the sky, and then suddenly disappearing. Three or four such _shooting-stars_ may, on the average, be observed in the course of an hour. They are usually seen only a second or two; but they sometimes move slowly, and are visible much longer. These stars begin to be visible at an average height of about seventy-five miles, and they disappear at an average height of about fifty miles. They are occasionally seen as high as a hundred and fifty miles, and continue to be visible till within thirty miles of the earth. Their visible paths vary from ten to a hundred miles in length, though they are occasionally two hundred or three hundred miles long. Their average velocity, relatively to the earth's surface, varies from ten to forty-five miles a second.

The average number of shooting-stars visible to the naked eye at any one place is estimated at about _a thousand an hour_; and the average number large enough to be visible to the naked eye, that traverse the atmosphere daily, is estimated at _over eight millions_. The number of telescopic shooting-stars would of course be much greater.

Occasionally, shooting-stars leave behind them a trail of light which lasts for several seconds. These trails are sometimes straight, as shown in Fig. 344, and sometimes curved, as in Figs. 345 and 346. They often disappear like trails of smoke, as shown in Fig. 347.

Shooting-stars are seen to move in all directions through the heavens. Their apparent paths are, however, generally inclined downward, though sometimes upward; and after midnight they come in the greatest numbers from that quarter of the heavens toward which the earth is moving in its journey around the sun.

306. _Meteors._--Occasionally these bodies are brilliant enough to illuminate the whole heavens. They are then called _meteors_, although this term is equally applicable to ordinary shooting-stars. Such a meteor is shown in Fig. 348.

Sometimes these brilliant meteors are seen to explode, as shown in Fig. 349; and the explosion is accompanied with a loud detonation, like the discharge of cannon.