Astronomy for Everybody/Part 5/Chapter 1

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Comets differ from the heavenly bodies which we have hitherto studied in their peculiar aspects, their eccentric orbits, and the rarity of their appearance. Some mystery still surrounds the question of their constitution, but this does not detract from the interest of the phenomena which they present. When one of these objects is carefully examined we find it to embody three features which, however, are not separate and distinct, but merge into each other.

First we have what, to the naked eye, appears to be a star of greater or less brilliancy. This is called the nucleus of the comet.

Surrounding the nucleus is a cloudy nebulous mass, like a little bunch of fog, shading off very gradually toward the edge, so that we cannot exactly define its boundary. This is called the coma (Latin for hair). Nucleus and coma together are called the head of the comet, which looks like a star shining through a patch of mist or fog.

Stretching away from the comet is the tail, which may be of almost any length. In small comets the tail may be ever so short, while in the greatest it stretches over a long arc of the heavens. It is narrow and bright near the head of the comet and grows wider and more diffuse as it ​recedes from the head. It is therefore always more or less fan-shaped. Toward the end it fades away so gradually that it is impossible to say how far the eye can trace it.

Comets differ enormously in brightness, and, notwith standing the splendid aspect which the brighter ones assume, the great majority of these objects are quite invisible to the naked eye. Such are called telescopic comets. There is, however, no broad distinction to be drawn between a telescopic comet and a bright one, there being a regular range of brightness from the faintest of these objects to the most brilliant. Sometimes a telescopic comet has no visible tail; this, however, is the case only when the object is extremely faint. Sometimes, also, the nucleus is almost wholly wanting. In such a case all that can be seen is a small hairy mass, like a very thin cloud, which may be a little brighter in the centre.

From the historical records it would appear that from twenty to thirty comets visible to the naked eye generally appear in the course of a century. But when the telescope was employed in sweeping the heavens it was found that these objects were more numerous than had been supposed. Quite a number are now found every year by diligent observers. Doubtless the number depends very largely on accident, as well as on the skill applied in the search. Sometimes the same comet will be found independently by several observers. The credit is then given to the one who first accurately fixes the position of the comet at a given time, and telegraphs the fact to an observatory.

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Soon after the invention of the telescope it was found that comets resembled the planets in moving in orbits around the sun. Sir Isaac Newton showed that their motions were ruled by the sun's gravitation in the same way as the motions of the planets. The great difference was that, instead of the orbits being nearly circular, like those of the planets, they were so elongated that, in most cases, it could not be determined where the aphelion, or farther end, was. As many of our readers may desire an exact statement of the nature of cometary orbits, and the laws governing them, we shall enter into some
Fig. 45.—Parabolic Orbit of a Comet. explanations of the subject.

It was shown by Newton that a body moving under the influence of the sun's attraction would always describe a conic section. This curve is of three kinds, an ellipse, a parabola, and a hyperbola. The first, as we all know, is a closed curve returning into itself. But the parabola and the hyperbola are not such; each of them extends out without end in two branches. In the case of the parabola these two branches approach more nearly to having the same direction as we get out farther, but in the case of the hyperbola they always diverge from each other.

​Having these curves in mind, let us imagine the earth to leave us hanging in space at some point of its orbit, our planet pursuing its course without us, until, at the end of a year, it returns to pick us up again. During the interval of its absence we, hanging in midspace, amuse ourselves by firing off balls to perform their revolutions around the sun like little planets. The result will be that all the balls we send off with a velocity less than that of the earth, that is to say, less than eighteen and six tenths miles per second, will move around the sun in closed orbits, smaller than the orbit of the earth, no matter what direction we send them in. A very simple and curious law is that these orbits will always have the same period if the velocity is the same. All the balls sent with the velocity of the earth will be one year in making their revolution and will, therefore, come together, at the point from which they started, at the same moment. If the velocity exceeds eighteen and six tenths miles a second, the orbit will be larger than that of the earth and the period of revolution will be longer the greater the velocity. With a speed exceeding about twenty-six miles a second, the attraction of the sun could never hold in the ball, which would fly away for good in one of the branches of a hyperbola. This would happen no matter in what direction we threw the object. There is, therefore, at every distance from the sun, a certain limiting velocity which, if a comet exceeds, it will fly off from the sun never to return; while, if it falls short, it will be sure to get back at some time.

The nearer we are to the sun, the greater is this limit​ing velocity. It varies inversely as the square root of the distance from the sun, hence, four times away from the sun, it is only half as great. The rule for finding the limiting velocity at any point in space is very simple. It is to take the speed of a planet passing through that point in a circular orbit, and multiply it by the square root of 2. This is 1.414. . . .

It follows that if the astronomer, by means of his observations, can find the velocity with which a comet is passing a known point of its orbit, he can determine the distance to which it will fly from the sun and the period of its return. By a careful comparison of observation made during the whole period of visibility of the comet he can generally reach a definite conclusion on the subject.

It is a curious fact that no comet has yet been seen of which the speed certainly exceeds the limit which we have described. It is true that, in many cases, a slight excess has been calculated from the observations, but this excess was no greater than might result from the necessary errors of observations on bodies of this kind. Commonly the speed is so near the limit that it is impossible to say whether it exceeds it or not. It is then certain that the comet will fly out to an immense distance, not returning for hundreds, thousands, or tens of thousands of years. There are also cases in which the speed of the comet is found to be less than the limit by a considerable amount. Such comets complete their revolutions in shorter periods and are called periodic comets.

So far as we know, the history of the motion of the ​large majority of the comets is this. They appear to us as if falling toward the sun from some great distance, we know not what. If a comet fell exactly toward the sun, it would fall into it, but this is a case which has not been known to occur and which, for reasons to be explained later, cannot be expected ever to occur. As it approaches the sun, it acquires greater and greater velocity, speeds around the central body in a great curve, and, by the centrifugal force thus generated, flies off again, returning to the abyss of space nearly in the direction from which it came.

Owing to the faintness of these objects they are visible, even in powerful telescopes, only in that part of their orbit which is comparatively near the sun. This is what makes it so difficult in many cases to determine the exact period of a comet which has only been seen once.

The first of these objects which was found to return in a regular period is celebrated in the history of astronomy under the name of Halley's comet. It appeared in August, 1682, and was observed for about a month, when it disappeared from view. Halley was able, from the observations made upon it, to compute the position of the orbit. He found that the latter was in the same position as that of a bright comet observed by Kepler in 1607.

It did not seem at all likely that two comets should move precisely in the same orbit. Halley therefore judged that the real orbit was an ellipse, and that the comet had a period of about seventy-five years. If this ​were the case, it should have been visible at intervals of about seventy-five years in the past.

So he subtracted this period from the several dates in order to determine whether any comets were recorded. Subtracting seventy-five from 1607 we have 1532. He found that a comet had actually appeared in 1531, which he had reason to believe was moving in the same orbit. Again subtracting seventy-five from this year we have the year 1456. A comet really did appear in 1456, which spread such horror throughout Christendom that Pope Calixtus III ordered prayers to be offered for protection against the comet as well as against the Turks, who were at war against Europe. It is probable that the myth of "the Pope's Bull against the comet" refers to this circumstance.

Other possible appearances of the comet were found in past history, but Halley was not able to identify the comet with exactness, owing to the absence of any precise description of the body. But the four well-observed dates, 1456, 1531, 1607, and 1682, afforded ample ground for predicting that the comet would again return to the sun about 1758. Clairaut, one of the most eminent mathematicians then in France, was able to calculate what effect would be produced by the action of Jupiter and Saturn on the period of the comet. He found that this action would so delay its return that it would not reach perihelion until the spring of 1759. It appeared according to the prediction, and actually passed perihelion on March twelfth of that year.

The next predicted return was in 1835. Several ​mathematicians now made computations of the effect of the planets in changing its period. So exact was their work that two of them hit the time within five days: Professor Rosenberger assigned November eleventh as the date of return, and Pontécoulant predicted it for November thirteenth. It actually passed perihelion on November sixteenth. After being observed for several months it disappeared from view and has not since been seen. But so exact is astronomical science that an astronomer could, at any time during the intervening interval, have pointed his telescope exactly at the object, after making the necessary calculations to determine its position.

Its next return is now approaching, but the exact date has not yet been computed. It will probably be some time between 1910 and 1912.

The most striking discovery of a comet after Halley announced the one which bears his name, was made by the French astronomer Lexell, in June, 1770. The object soon became visible to the naked eye. On laying down the orbit in which it moved, it was found, to the surprise of astronomers, that the orbit was an ellipse, with a period of only about six years. Its return was, therefore, confidently predicted, but it never reappeared. The cause was, however, speedily discovered. When it returned at the end of six years, it was on the opposite side of the sun, and therefore could not be seen. Passing out to complete its revolution, it was found by calculation that it must have gone into the immediate neighbourhood ​of the planet Jupiter, which, by its powerful attraction, started the comet off into some new orbit, so that it never again came within reach of the telescope. This, also, explained why the comet had not been seen before. Three years before Lexell found it, it had come from the neighbourhood of the planet Jupiter, which had thrown it into an orbit different from its former one. Thus the giant planet of our system had, so to speak, given the comet a pull about 1767 so that it should pass into the immediate neighbourhood of the sun, and having allowed it to make two revolutions around the sun, again encountered it in 1779, and gave it a new swing off, no one knows where. Since that time twenty or thirty comets, found to be periodic, have been observed, most, but not all of them, at two or more returns.

The most remarkable fact brought out by the study of these objects has been that they do not appear to be of seemingly infinite duration, like the planets, but are, as a general rule, subject to dissolution and decay, like living beings. The most curious case of a comet being completely disintegrated is that of Biela's comet. This was first observed in 1772, but was not known to be periodic. It was again seen in 1805, and again the astronomer did not notice the identity of the orbit in which it was moving with that of the comet of 1772. In 1826 it was discovered a third time, and now, on computing the orbit by the improved methods which had been invented, its identity with the former comets was brought out. The time of revolution was fixed at six and two thirds years. It should, therefore, appear in 1832 and 1839. But on ​these returns the earth was not in a position to admit of its being seen. Toward the end of 1845 it again appeared and was observed in November and December. In January, 1846, as it came nearer to the earth and sun, it was found to have separated into two distinct bodies. At first the smaller of these was quite faint, but it seemed to increase in brightness until it became equal to the other.

The next return was in 1852. The two bodies were then found to be far more widely separated than before. In 1846 their distance apart was about two hundred thousand miles; in 1852 more than a million miles. The last observations were made in September, 1852. Although since that time the comet should have completed seven revolutions, it has never again been seen. From the former returns it was possible to compute the position where it should appear with a good deal of precision, and from its non-appearance we conclude that it has been completely disintegrated. We shall, in the next chapter, learn a little more about the matter which composed it.

Two or three comets have disappeared in the same way. They were observed for one or more revolutions, growing fainter and more attenuated on each occasion, and finally became completely invisible.

Of the periodic comets the one that is most frequently and regularly observed bears the name of Encke, the German astronomer who first accurately determined its motion. Its first discovery was made in 1786, but, as was often the case then, its orbit could not at first be ​determined. It was again seen in 1795 by Miss Caroline Herschel. It was found again in 1805 and 1818. Not until the latter date was the accurate orbit determined, and then the periodic character of the comet and its identity with the comet observed in previous years was established.

Encke now found the period to be about three years and one hundred and ten days, varying a little according to the attraction of the planets, especially of Jupiter. In recent times it has been observed somewhere at almost every return. Its last return was in September, 1901.

What has given this comet its celebrity is the theory of Encke that its orbit was continually becoming smaller, probably through its motion being resisted by some medium surrounding the sun. A number of able mathematicians have investigated this subject on the various returns of the comet. Sometimes there appears to be evidence of a retardation, like that found by Encke, and sometimes not. The question is, therefore, still in an unsettled condition. The computations are so intricate and difficult, and, indeed, the whole problem of the motion of a comet under the influence of the planets is so complicated, that it is almost impossible to secure a solution which can be guaranteed as absolutely correct.

A remarkable case, in which a new comet was made a member of the solar system, occurred in the years 1886-1889. In the latter year a comet was observed by Brooks of Geneva, New York, which proved to be revolving in ​an orbit with a period of only seven years. As it was quite bright, the question arose why it had never been observed before. This question was soon answered by the discovery that in the year 1886 the comet had passed close to Jupiter. The attraction of the planet had so changed its course as to throw the comet into the orbit which it now describes. Several other periodic comets pass so near to Jupiter that there is little doubt that they were brought into the system in this way.

The question therefore arises whether this may not be true of all periodic comets. This question must be answered in the negative, because Halley's comet does not pass near any planet. The same is true of Encke's comet, which does not come near enough to the orbit of Jupiter to have been drawn into its present orbit. Without the action of that planet, so far as we know, these comets always have been members of the system.

It was supposed, until a recent time, that comets might come into the solar system from the vast spaces between the stars. This view, however, seems to be set aside by the fact that no comet has been proved to move with a much higher speed than it would get by falling to the sun from a distance, which, though far outside the solar system, is much less than the distance of the stars. We shall see hereafter that the sun itself is in motion through space. Even if we grant that comets come from space far outside the solar sj^stem, the fact that we have just cited still shows that they partook of the motion of the ​sun and solar system through space while tliey were still outside that system.

The view which now seems established by a study of the whole subject is that these objects have their regular orbits, differing from those of the planets in their great eccentricities. Their periods of revolution are generally thousands, and sometimes tens of thousands, and even hundreds of thousands of years. During this long interval they fly out to an enormous distance beyond the confines of the system. If, as they return to the sun, they chance to pass very near a planet, two things may happen: Either the comet may be given an additional swing that will accelerate its speed, throw it out to a greater distance than it ever had before or possibly to a distance from which it can never return, or the speed may be retarded and the comet made to move in a smaller orbit. Thus it is that we have comets of so many different periods. If comets come from the regions of the fixed stars, there is no reason why the motion of one might not be directly toward the sun, so that it would fall into our central luminary. But such an occurrence is hardly possible when the comet belongs to our system, because one of these bodies nearing an orbit passing through the sun would have fallen into the sun on its first round, long ages ago, and never could have a chance to fall in again.

The very bright comets which appear from time to time are of the greatest interest to every beholder. It is purely a matter of chance, so far as our knowledge ex​

​tends, when one shall appear. Of what are called great comets, there were, five or six during the nineteenth century. The most remarkable and brilliant of all appeared in 1858, and bears the name of Donati, its discoverer, an astronomer of Florence, Italy. Its history will show the changes through which such a body goes. It was first seen on June second, but was then only a faint nebulosity, visible in the telescope like a minute white cloud in the heavens. No tail was then visible, nor was there the slightest indication of what the little cloud would grow into until the middle of August. Then a small tail gradually began to form. Early in September the object became visible to the naked eye. From that time it increased at an extraordinary rate, growing larger and more conspicuous night after night. Its motions were such that it seemed to move but little for the period of a whole month, floating in the western sky night after night. It attained its greatest brilliancy about October tenth. Careful drawings of it were made from time to time by George P. Bond, of the Harvard Observatory. We give two of these, one a naked eye view, the other a telescopic one showing what the head of the comet looked like. After October tenth it rapidly faded away. It soon travelled toward the south, and passed below our horizon, but was followed by observers in the southern hemisphere until March, 1859. Before the comet had passed out of sight, computers began to calculate its orbit. It was soon found not to move in an exact parabola, but in a very elongated ellipse. The period was not far from nineteen hundred years, but ​

​may have been a hundred years more or less than this. It must therefore have been visible at its preceding return sometime in the first century before Christ, but there is no record by which it could be identified. It may be expected again in the thirty-eighth or thirty-ninth century.

A very remarkable case of several comets moving in very nearly the same orbit is afforded by the comets of 1843, 1880, and 1882. The first of these was one of the most memorable comets on record, as it passed so near the sun as almost to graze the surface. In fact, it must have passed quite through the outer portions of the solar corona. It came into view with remarkable suddenness in the neighbourhood of the sun, about the end of February. It was visible in full daylight. By a singular coincidence it appeared shortly after the well-known prediction of Miller that the end of the world was to come in the year 1843. Those who had been alarmed by this prediction saw in the comet an omen of the approaching catastrophe.

The comet disappeared from view in April, so that the time of observation was rather short. The period of revolution now became a subject of interest. It was found, however, that its orbit did not differ sensibly from the parabola. But the time of observation was so brief that any estimate of the period would be somewhat uncertain. All that could be said was that the comet would not return for several centuries.

Great, therefore, was the surprise when, thirty-seven years later, a comet was seen by observers in the southern ​

​hemisphere and found to be moving in almost the same orbit. The first sign which it gave of its approach was its long tail rising above the horizon. This was seen in the Argentine Republic, at the Cape of Good Hope, and in Australia. Not until the fourth of February did the head become visible. It swept around the sun, again passed to the south, and disappeared without observers in the northern hemisphere seeing it.

The question now arose whether this could possibly be the same comet that had appeared in 1843. Previously it had been supposed that when two such bodies moved in the same orbit with a long interval between they must be the same. In the present case, however, the hypothesis of identity seemed to be incompatible with the observations. The question was set at rest by the appearance in 1882 of a third comet moving in about the same orbit. This certainly could not be a return of the comet which had appeared a little more than two years before. The remarkable spectacle was therefore offered of three bright comets all moving in the same orbit at varying intervals of time. Possibly there were more even than these three, for, in 1680, a comet had passed very near the sun. Its orbit, however, was somewhat different from those of the three comets already mentioned.

The most probable explanation of the case seems to be that these comets were parts of some nebulous mass which gradually broke up, its different members pursuing their courses independently. The result would be that, for many ages, the objects would all continue in nearly the same orbit.

​Besides these, brilliant comets appeared in 1859, 1860, and 1881. How long we may have to wait for another no one can say. It is probable that Halley's comet, when it appears eight or ten years hence, will at least be visible to the naked eye, but no one can predict even its apparent brightness. At its return in 1835 it was so small an affair that it was difficult to explain the excitement it caused in 1456 and later, except by supposing a great diminution in the dimensions, at least of its tail.

The question of the exact nature of a comet is still in doubt. In the case of large and bright comets, it is possible that the nucleus may be a solid body, though probably much smaller than it looks. Some light on the question is thrown by an observation, which is unique, made at the Cape of Good Hope when the great comet of 1882 made a transit across the sun's disk, as Mercury and Venus are sometimes known to do. Unfortunately, astronomers generally were not prepared for such a phenomenon, as the comet had been visible only in the southern hemisphere, and the transit occurred only a week or two after its first discovery. Hence it happened that the Cape Observatory was the only one at which an observation of the greatest interest in astronomy could be made; and here the circumstances were extremely unfavourable. The sun was about to set behind Table Mountain as the comet approached it. By careful watching, two of the astronomers, Messrs. Finlay and Elkin, were enabled to keep sight of the comet until it actually disap​peared at the limb of the sun. This happened fifteen minutes before the sun disappeared from view. During this time, if the nucleus were a solid body, it ought to have been seen as a black spot projected against the sun. Nothing of the sort could be made out. The conclusion is either that the substance of the comet was transparent to the sun's rays, or that the solid nucleus was too small to be distinguished under the circumstances. Unfortunately, owing to the low altitude of the sun and the bad condition of the air, it was impossible to be quite sure how small the nucleus must have been to be invisible. It seemed certain, however, that the solid portion, if any such the comet had, was much smaller than the apparent nucleus as seen in the telescope.

There seems also to be some reason for suspecting that a comet is nothing but a collection of meteoric matter, consisting perhaps of separate objects, of sizes ranging anywhere from that of grains of sand to masses as large as the meteorites which sometimes fall from the sky. The question then is to explain how the parts are kept together through so many revolutions of the comet. The changes of shape which the nucleus often undergoes as it is passing near to the sun seem to show that this hypothesis may be near the truth.

The spectra of those comets whose light has been analysed by the spectroscope are remarkable in showing that this light is not merely reflected sunlight. The principal feature is three bright bands, which bear a striking resemblance to those given by the compounds of carbon and hydrogen. Taking this fact by itself, the ​conclusion would be that the comet is a glowing gas, shining as incandescent gases do in our chemical laboratories. That such should be the case and the whole case seems impossible for two reasons. The comet cannot be hot enough to glow; and its light fades out to nothing as it recedes from the sun. The most likely conclusion seems to be that the action of the sun's rays causes a glow through some process which has not yet been made clear to us.

What seems certain is that the matter of which a bright comet is composed is volatile. When a bright comet is carefully scrutinised with a telescope, masses of vapour can be seen from time to time slowly rising from its head in the direction of the sun, then spreading out and moving away from the sun so as to form the tail. The latter is not an appendage which the comet carries as animals carry their tails, but is like a stream of smoke issuing from a chimney.

It frequently happens that when a comet is first discovered it has no tail at all. The latter begins to form when the sun is approached. The nearer the comet approaches the sun, and the greater the heat to which it is exposed, the more rapidly the tail develops. All this shows that the matter which composes a great comet is, in part volatile. When warmed by the heat of the sun it begins to evaporate, just as water would under the same circumstances. The steam or vapour thus arising is repelled by the sun, so as to form a stream of matter issuing from the comet.