CHAPTER VII.

VISITORS.

We come next to the very largest members of our Solar System.

From time to time in past days--and days not very long past either--people were startled by the sight of a long-tailed star, moving quickly across the sky, called a comet. We see such long-tailed stars still, now and then; but their appearance no longer startles us.

It is hardly surprising, however, that fears were once felt. The great size and brilliancy of some of these comets naturally caused large ideas to be held as to their weight, and the general uncertainty about their movements naturally added to the mysterious notions afloat with respect to their power of doing harm.

A collision between the earth and a comet seemed no unlikely event; and if it happened--what then? Why, then, of course, the earth would be overpowered, crushed, burnt up, destroyed. So convinced were many on this point that the sight of a comet and the dread of the coming “end of the world” were fast bound together in their minds.

Even when astronomers began to understand the paths of some of the comets, and to foretell their return at certain dates, the old fear was not quickly laid to rest. So late as the beginning of the present century, astronomers having told of an approaching comet, other people added the tidings of an approaching collision. “If a collision, then the end of the world,” was the cry; and one worthy family, living and keeping a shop in a well-known town on the south coast of England, packed up and fled to America--doubtless under full belief that the destruction of the Old World would not include the destruction of the New.

The nature of these singular bodies is somewhat better known in the present day; yet even now, among all the members of the Solar System, they are perhaps the ones about which we have most to learn. The nucleus, or bright and star-like spot, which, with the surrounding coma or “hair,” we sometimes call the “head” of the comet, is the densest and heaviest part of the whole. The comets are of immense size, sometimes actually filling more space than the sun himself, and their tails stream often for millions of miles behind them; nevertheless, they appear to be among the lightest of the members of the Solar System.

This excessive lightness greatly lessens the comet’s power of harm-doing. In the rebound from all the old exaggerated fears, men laughed at the notion of so light and delicate a substance working any injury whatever, and even declared that a collision might take place without people on earth being aware of the fact. It is now felt that we really know too little about the nature of the said substance to be able to say what might or might not be the result of a collision. A certain amount of injury to the surface of the earth might possibly take place. But of the “end of the world,” as likely to be brought about by any comet in existence, we may safely banish all idea.

The word “comet” means “a hairy body,” the name having been given from the hairy appearance of the light around the nucleus. About seventeen hundred different comets have been seen at different times by men--some large, some small; some visible to the naked eye, but most of them only visible through telescopes. These hundreds are, there is no doubt, but a very small number out of the myriads ranging through the heavens.

If you were seated in a little boat in mid-ocean, counting the number of fishes which in one hour passed near enough in the clear water for your sight to reach them, you might fairly conclude, even if you did not know the fact, that for every single fish which you could see, there were tens of thousands which you could not see.

Reasoning thus about the comets, as we watch them from our earth-boat in the ocean of space, we feel little doubt that for each one which we can see, millions pass to and fro beyond reach of our vision. Indeed, so long ago as the days of Kepler, that great astronomer gave it as his belief that the comets in the Solar System, large and small, were as plentiful as the fishes in the sea. And all that modern astronomers can discover only tends to strengthen this view.

Why should the comets be called “visitors?” I call them so simply because many of them _are_ visitors. Some, it is true, belong to the Solar System. But even in their case, strong doubts are felt whether they were not once visitors from a distance, caught in the first instance by the attraction of one of the larger planets, and retained thenceforward, for a time at least, by the strong attraction of the sun.

[Illustration: PASSAGE OF THE EARTH AND THE MOON THROUGH THE TAIL OF A COMET.]

Every comet, like every planet, has his own orbit or pathway in the heavens, though the kind of orbit varies with different comets. There are, first, those comets which travel round and round the sun in “closed orbits”--that is, in a ring with joined ends. Only the ring is always oval, not round. There are, secondly, those which travel in an orbit which _may_ be closed; but if so, the oval is so long and narrow, and the farther closed end is at so great a distance, that we can not speak certainly. There are, thirdly, those which decidedly are mere visitors. They come from the far-off star-depths, flash once with their brilliant trains of light through our busy Solar System, causing some little excitement by the way, and go off in another direction, never to return.

Only a small part of the orbits of these comets can be seen from earth; but by careful attention astronomers learn something of the shape of the curve in which they travel. It is in that way possible to calculate, sometimes certainly, and sometimes uncertainly, whether a comet may be expected to return, or whether we have seen him for the first and the last time. By looking at _part_ of a curve, the rest of which is hidden from us, we are able to judge whether that part belongs to a circle or an oval, or whether the two ends pass away in different directions, and do not join.

The comets, whether members of our family circle or visitors from a distance, are altogether very perplexing. They are often extremely large, yet they are always extremely light. They reflect the sun’s brightness like a planet, yet in some measure they seem to shine by their own light, like a star. They obey the attraction of the sun, yet he appears to have a singular power of driving the comets’ tails away from himself.

For, however rapidly the comet may be rushing round the sun, and however long the tail may be, it is almost always found to stream in an opposite direction from the sun. An exception to this rule was seen in the case of a certain comet with two tails, one of which did actually point towards the sun; but the inner tail may have been only a “jet” of unusual length, like in kind to the smaller jets often thus poured out from the nucleus.

Very curious changes take place in comets as they journey, especially as they come near the sun. One was seen in the course of a few days to lose all his hair, and also his tail. Another was seen to break into two pieces, both of which pieces at last disappeared. Sometimes the one tail divides into two tails.

Traveling, as the comets do, from intense cold into burning heat, they are very much affected by the violent change of climate. For the paths of the comets are such long ovals, or ellipses, that, while they approach the sun very closely in one part of their “year,” they travel to enormous distances in the other part.

“Halley’s Comet,” which takes seventy-six of our years to journey round the sun, comes nearer to him than Venus, and goes farther away from him than Neptune. As this comet draws gradually closer, he has to make up for the added pull of the sun’s increasing attraction by rushing onward with greater and greater rapidity, till he whirls madly past the sun, and then, with slowly slackening speed, journeys farther and farther away, creeps at length lazily round the farther end of his orbit in the chill, dark, neighborhood of Neptune, and once more travels towards the sun with growing haste.

“Encke’s Comet” has a year of only three and a half of our years, so he may be said to live quite in our midst. But many comets travel much farther away than the one named after Halley. It is calculated of some that, if they ever return at all, it can not be for many hundreds of years.

[Illustration: PASSAGE OF THE COMET OF 1843 CLOSE TO THE SUN (FEBRUARY 27TH, 10 HOURS, 29 MINUTES.)]

“Newton’s Comet,” seen about two centuries ago, has a journey to perform of such length that he is not expected again to appear for several thousand years. Yet, at the nearest point in his orbit, he approached the sun so closely, that the heat which he endured was about two thousand times that of red-hot iron. Changes were seen to be taking place in his shape, as he drew near to the sun, and disappeared. Four days he was hidden in the sun’s rays. He vanished, with a tail streaming millions of miles behind him. He made his appearance again with a tail streaming millions of miles in front of him. But how this wonderful movement took place is beyond man’s power to explain.

The comet of 1843 was one of the most attractive seen during the present century. It was first observed in March, and it appeared with a suddenness which had quite a startling effect. It was an imposing object in the southern regions. The comet was seen in Italy on the 28th of February; at Washington, on the 6th of March; at Oporto, on the 14th; but owing to unfavorable weather, it was not visible in England, or any of the northern countries of Europe, previous to the 17th. A little after sunset on that day the tail was observed in the western sky, but the head had already sunk below the horizon. The whole of the comet appeared on the following evenings for a short time, for it was traveling away from the sun with great velocity, having doubled the solar orb before it became visible; and about the beginning of April it finally disappeared.

The appearance of this startling stranger, as observed at Washington, is thus described by Lieutenant Maury, of the Hydrographical Office in that city: “On Monday morning, March 6th, our attention was called to a paragraph in the newspapers, stating that a comet was visible near the sun at midday with the naked eye. The sky was clear; but not being able to discover any thing with the unassisted eye, recourse was had to the telescope, but with no better success. About sunset in the evening, the examination was renewed with great diligence, but to no purpose. The last faint streak of day gilded the west; beautiful and delicate fleeces of cloud curtained the bed of the sun; the upper sky was studded with stars, and all hopes of seeing the comet that evening had vanished. Soon after we had retired, the officer of the watch announced its appearance in the west. The phenomenon was sublime and beautiful. The needle was greatly agitated, and a strongly marked pencil of light was streaming up from the path of the sun in an oblique direction to the southward and eastward; its edges were parallel. It was 30° long. Stars could be seen twinkling through it, and no doubt was at first entertained that this was the tail of the comet.”

The tail, as seen in northerly countries, spread over an arc of the heavens of about 40°; but in southern latitudes it extended to from 60° to 70°. It had an absolute length of two hundred millions of miles; so that, had it been coiled around the earth like a serpent, it would have girdled it eight thousand times at the equator. This comet approached still nearer the sun than that of Newton, and must therefore have been exposed to a heat of greater intensity. Its center is computed to have been within a hundred thousand miles of the solar surface; and according to Sir John Herschel’s calculations, it was then exposed to a heat equal to that which would be received by an equal portion of the earth’s surface, if it were subject to the influence of forty-seven thousand suns, placed at the common distance of the actual sun. It is difficult to conceive how a flimsy substance in such circumstances could escape being entirely dissipated. But such was its velocity that it wheeled round the sun in less than two hours.

The general question of the probability and the consequences of a collision with a comet may be legitimately entertained. With reference to the first point, it can not be denied that collision is possible; but, at the same time, it is so extremely improbable that it may be safely dismissed from apprehension. The fact of such an event not having been experienced in the known course of terrestrial history is surely some guarantee against its occurrence. Another may be found in the small volume of the earth and of comets when compared with the immensity of space in which they move. According to the well-understood principles of probabilities, Arago has calculated that, upon the appearance of a new comet, the odds are as 281,000,000 to 1, that it will not strike against our globe. But even supposing collision to occur, all that we know of the constitution of comets justifies the conclusion that the encounter would involve no terrestrial convulsion, nor any result incompatible with full security to life and happiness.

So much for the largest members of our circle--largest, though lightest; members some, visitors others. Now we turn to the smallest.