Part 16

The great majority of comets are only to be seen with a telescope, and hardly a year passes without the detection of at least a few of these faint objects. The number of really brilliant comets that can be seen in a lifetime could, however, be counted on the fingers.

ENCKE’S COMET.

We have already alluded to a little body called Encke’s comet, which was discovered by an astronomer at Marseilles. It was in the year 1818 that he was scanning the heavens with a small telescope, when an object attracted his attention. It was not one of those grand long-tailed comets which every one notices; this body was so faint that it merely appeared as a very small cloud of light, and was recognized as a comet by the fact that it was moving. It happens that there are other bodies in the sky very like comets; we call them nebulæ, and we shall have something to say about them afterwards. But it is remarkable that just as a planet is liable to be mistaken for a star, so a comet is liable to be mistaken for a nebula. However, in each case the fact of its movement is the test by which the planet or the comet is at once detected. A nebula stays always in the same spot, like a star, while a comet is incessantly moving. In fact, with a telescope you can actually watch a comet stealing past the stars that lie near it. You know that an object a very long way off may appear to move slowly, though in reality it is moving very rapidly. Look at a steamer near the horizon at sea. In the course of a minute or two it will not appear to have shifted its position to any appreciable extent, but that is because it is far off. If you were near the ship, you would see that it was dashing along at the rate of perhaps fifteen or twenty miles an hour. In a similar manner the comet seems to move slowly, because it is at such a great distance. As a matter of fact it is moving faster at the time we see it than any steamer, faster than any express train, faster than any cannon-ball. There were special reasons why the movements of Encke’s comet should be watched with peculiar care, and the track which it pursued be ascertained. If you can observe a comet three times and measure its position in the sky, the movement of that comet is completely determined. Perhaps I should say would be determined if the comet were let alone, which, unfortunately, is not often the case. Indeed, you may remember how I told you some of the misadventures of this very comet when we were speaking about the planet Mercury. Encke’s comet comes round in a period of a little more than three years, and it gives us some curious information that has been ascertained during its journeys. One of the facts we have thus learned is so important that we cannot omit to notice it (Fig. 73).

At increasing heights above the earth’s surface there is gradually less and less air; until at last, at about 200 or 300 miles above the surface on which we dwell, there would be none. You might as well try to quench your thirst by drinking out of an empty cup as attempt to breathe in the open space which begins a few hundred miles aloft. In open space motion could take place quite freely. Down here the resistance of the air is a great impediment to movement, especially when very rapid. A heavy cannon-ball is checked and robbed of its pace by having to plough its way through our dense atmosphere. The motion is arrested in the same way, though not of course to the same degree, as if the cannon-ball had been fired into water. Unsubstantial objects are, of course, impeded by the air to a far greater extent than such heavy bodies as cannon-balls. You know that you cannot throw a handful of feathers across the road in the same way that you could throw a handful of gravel. The light feathers cannot force their way through the air so well as the pebbles. A body so flimsy as a comet would never be able to push its way through an atmosphere like ours; but out in empty space the comet meets with no resistance during the greater part of its path. Accordingly, though it has little more substance than a will-o’-the-wisp, the comet pursues its journey with as much resolute dignity as if it were made of cast iron. If in any part of its track the body should have to pierce its way through any material like even the thinnest possible air, then the unsubstantial nature of the cometary materials would be at once shown. The motion would be impeded, and the body’s path would be changed. In this way a comet may be made very instructive, for it will show whether space is really so empty as we sometimes suppose it to be. During the greater part of its course the flimsy little Encke tears along with such ease and speed that there seems to be nothing to impede it, and thus we learn that space is generally empty. However, when the comet begins to wheel around the sun, the freedom of its movements seems to receive a check. The unsubstantial object has to force its way with a difficulty that it did not experience so long as it was moving round the greater part of its orbit. We thus learn that there is a thin diffused atmosphere surrounding the sun. We cannot, indeed, say that it is like our air. Its composition is quite different, and almost the only way we know of its existence is by the evidence which this comet affords. In a former lecture I showed how Encke’s comet told us the mass of the planet Mercury. Now we see how the travels of the same body give us information about the sun himself. I ought, however, to add that some more recent observations seem not to have confirmed the belief that there is the resistance of the kind we have just been considering.

THE GREAT COMET OF HALLEY.

I dare say you would think it more interesting to talk about some big and bright comets rather than about objects so faint as that of Encke. It unfortunately happens that most of the fine comets pay our system only a single visit. There is only one of the really splendid objects of this kind that comes back to us with anything like regularity.

It was last seen in the year 1835, and I am glad to tell you that it is coming again; it is expected about the year 1910. You may ask, How can we feel sure that such a prediction as I have mentioned will turn out correctly? The fact is that this comet has been watched for a great many centuries. We find ancient records, some of them nearly 2000 years old, of the appearance of grand comets, and several of these are found to fit in with the supposition that there is a body which accomplishes its journey in a period of about seventy-five or seventy-six years. Of course there are thousands of other comets recorded in these old books as well; but what I mean is that among the records many are found which clearly indicate some successive returns of this particular body.

I will explain how the movements of this comet were discovered. There was a great astronomer called Halley, who lived two hundred years ago, and in the year 1682 he, like every one else, was looking with admiration at a splendid comet with a magnificent tail which adorned the sky in that year. At the observatories, of course, they diligently set down the positions of the comet, which they ascertained by carefully measuring it with telescopes. Halley first calculated the highway which this comet followed through the heavens, and then he looked at the list of old comets that had been seen before. He thus found that in 1607--that was, seventy-five years earlier--a great comet had also appeared, the path of which seemed much the same as that which he found for the body that he had himself observed. This was a remarkable fact, and it became still more significant when he discovered that seventy-six years earlier--namely, in 1531--another great comet had been recorded, which moved in a path also agreeing with those of 1607 and 1682. It then occurred to Halley that possibly these were not three different objects, but only different exhibitions of one and the same, which moved round in the period of seventy-five or seventy-six years.

There is a test which an astronomer can often apply in the proof of his theory, and it is a very severe test. He will not only show himself to be wrong if it fails, but he will also make himself somewhat ridiculous. Halley ventured to submit his reputation to this ordeal. He prophesied that the comet would appear again in another seventy-five or seventy-six years. He knew that he would, of course, be dead long before 1758 should arrive; but when he ventured to make the prediction, he said that he hoped posterity would not refuse to admit that this discovery had been made by an Englishman.

You can easily imagine that as 1758 drew near, great interest was excited among astronomers to see if the prediction of Halley would be fulfilled. We are accustomed in these days to find many astronomical events foretold with the same sort of punctuality as we expect in railway time-tables. The Nautical Almanac is full of such prophecies, and we find them universally fulfilled. Even now, however, we are not able to set forth our time-tables for comets with the same confidence that we show when issuing them for the sun, the moon, or the stars. How astonishing, then, must Halley’s prediction have seemed! Here was a vast comet which had to make a voyage through space to the extent of many hundreds of millions of miles. For three-quarters of a century it would be utterly invisible in the greatest telescopes, and the only way in which it could be perceived was by figures and calculations which enabled the mind’s eye to follow the hidden body all around its mysterious track. For fifty, or sixty, or seventy years nothing had been seen of the comet, nor, indeed, was anything expected to be seen of it; but as seventy-one, and seventy-two, and seventy-three years had passed, it was felt that the wanderer, though still unseen, must be rapidly drawing near. The problem was made more difficult for those skilful mathematicians who essayed to calculate it by the fact that the comet approached the thoroughfares where the planets circulate; and, of course, the flimsy object would be pulled hither and thither out of its path by the attractions of the weighty bodies. It was computed that the influence of Saturn alone was sufficient to delay the comet for more than three months, while it appeared that the attraction of Jupiter was potent enough to retard the expected event for a year and a half more. Was it not wonderful that mathematicians should be able to find out all these facts from merely knowing the track which the comet was expected to follow? Clairaut, who devoted himself to this problem, suggested that there might also be some disturbances from other causes of which he did not know, and that consequently the expected return of the comet might be a month wrong either way. Great indeed was the admiration in astronomical circles when, true to prediction, the comet blazed upon the world within the limits of time Clairaut had specified.

The remarkable fulfilment of this prophecy entitles us to speak with confidence about the past performances of this comet. Among all the apparitions of Halley’s comet for the last two thousand years, perhaps the most remarkable is that which took place in the year 1066. I am sure you will all remember this date in your English history; it was the year of the Conquest. In those days they did not understand astronomy as we understand it now; they used to think of a comet as a fearful portent of evil, sent to threaten some frightful calamity; such as a pestilence, a war, a famine, or something else equally disagreeable. Hence in the year of the Conquest the appearance of so terrific an object in the sky was a very significant omen. Attention was concentrated upon the spectacle, and a picture of Halley’s comet as it appeared to the somewhat terrified imaginations of the people of those days has been preserved. There is a celebrated tapestry at Bayeux on which historical incidents are represented by beautifully worked pictures. On this fabric we have a view of Halley’s comet in a quaint and rather ludicrous aspect. You will read of this comet also in the early pages of Tennyson’s “Harold.”

HOW THE TELEGRAPH IS USED FOR COMETS.

In these days the study of comets is prosecuted with energy. Over the world observatories are situated, and whenever a comet is discovered, tidings of the event are diffused among those likely to be interested. Suppose that one is discovered in the southern hemisphere, the astronomers then write to warn the northern observatories of the event. But comets often move faster than her Majesty’s mails, so that the telegraph has to be put into requisition. The kind of message is one which shall show the position and the movements of the body. It necessarily involves a good many figures and words, and consequently it is desirable to abbreviate as much as possible for the sake of economy. There is a further difficulty in using the telegraph, because the messages are not of an intelligible description to those not specially versed in astronomy. Skilful as the telegraph clerks are, they can hardly be expected to be familiar with the technicalities of astronomers. The clerk at the receiving end is handed a message which he does not understand very clearly. The clerk at the other end does not understand the message which is delivered to him, and between them it has happened that they have transformed the message into something which not only they do not understand, but which, unfortunately, nobody else can understand either. These difficulties have been surmounted by an agreement between astronomers, which is so simple and interesting that I must mention it.

The kind of message that expresses the place of a comet will consist of sentences something of this kind: “One hundred and twenty-three degrees and forty-five minutes.” Surely it would be an advantage to be able to replace all these words by a single word, particularly if by doing so the risk of error would be diminished. This is what the astronomers’ telegraphic arrangement enables them to accomplish. There is a certain excellent Dictionary known as Worcester’s. I am sure when Mr. Worcester arranged this work, he had not the slightest anticipation of an odd use to which it would occasionally be put. Every astronomer who is co-operating in the comet scheme must have a copy of the book. To send the message I have just referred to, he has to take up his Dictionary and look out page 123. Then he will count down the column until he comes to the forty-fifth word on that page, which he finds to be “constituent,” and according to this plan the message, or at least this part of it, is merely that one word, “constituent.” The astronomer who receives this message and wishes to interpret it takes up his copy of Worcester’s Dictionary and looks out for “constituent.” He sees that it is on page 123, and that it is the forty-fifth word down on that page; and therefore he knows that the interpretation of the message is to be one hundred and twenty-three degrees and forty-five minutes.

THE PARABOLA.

Generally speaking, great comets come to us once and are then never seen again. Such bodies do not move in closed ovals or ellipses, they follow another kind of curve, like that represented in Fig. 74. It is one that every boy ought to know. In fact, in one of his earliest accomplishments he learned how to make a parabola. It is true he did not call it by any name so fine as this, but every time a ball is thrown into the air it describes a part of the beautiful curve which geometers know by this word (Fig. 74). In fact, you could not throw a ball so that it should describe any other curve except a parabola. No boy could throw a stone in a truly horizontal line. It will always curve down a little, will always, in fact, be a portion of a parabola.

There are big parabolas and there are small ones. One of the shells which are thrown into a town when bombarded from a distance describes, as it rises and then slopes down again, part of a mighty parabola. So does a tennis ball thrown by the hand or struck by the racket; though here, indeed, I admit that a spin may be given to the ball which will somewhat detract from the simplicity of its movement. In playing baseball, a large part of the skill of the pitcher consists in throwing the ball in such a way that it shall not move in a parabola, but in some twisting curve by which he hopes to baffle his adversary. Setting aside these exceptions, and such another as the case of a body tossed straight up or dropped straight down, we may assert that the path of a projectile is a parabola.

There are some remarkable applications of the same curve for practical purposes. From our lighthouses we want to send beams off to sea, so as to guide ships into port. If we merely employed a lamp without concentrating its rays, we should have a very imperfect lighthouse, for the lamp scatters light about in all directions. Much of it goes straight up into the air, much of it would be directed inland; in fact, there is only an extremely small part of the entire number of rays that will naturally take the useful direction. We therefore require something round the lamp which shall catch the truant rays that are running away to idleness and loss, and shall concentrate them into the direction in which they will be useful to the mariner. An effective way of doing this is to furnish the lamp with a reflector. On its bright surface (Fig. 75) all the rays fall which would otherwise have gone astray, and each of them is properly redirected, where the sailors can see it. It is essential that the mirror shall do this work accurately, and this it will only do when it has been truly shaped so as to be a parabola.

You will remember, also, how I described to you the reflector which Herschel made for his great telescope. The shape of the mirror must be most accurately worked, and it, too, must have a parabola for its section; so that you see this curve is one of importance in a variety of ways.

But the grandest of all parabolas are those which the comets pursue. Unlike the ellipse, the parabola is an open curve; it has two branches stretching away and away forever, and always getting further apart. Of course, in the examples of this curve that I have given it is only a small part of the figure that is concerned. When you throw a stone it only describes that part of the parabola that lies between your hand and the spot where the stone hits the ground. It is just a part of the curve in the same way that a crescent may be a bit of a circle. It is to comets that we must look for the most complete illustration of the ample extent of a parabola.

The shape of this grand curve will explain why so many comets only appear to us once. It is quite clear that if you begin to run round a closed racecourse, you may, if you continue your career long enough, pass and repass the starting-post thousands of times. Thus, comets which move in ellipses, and are consequently tracing closed curves, will pass the earth times without number. For this reason we may see them over and over again, as we do Encke’s comet or Halley’s comet. But suppose you were travelling along a road which, no matter how it may turn, never leads again into itself, then it is quite plain that, even if you were to continue your journey forever, you can never twice pass the same house on the roadside. That is exactly the condition in which most of the comets are moving. Their orbits are parabolas which bend round the sun; and, generally speaking, the sun is very close to the turning-point. The earth is also, comparatively speaking, close to the sun; so that while the comet is in that neighborhood we can sometimes see it. We do not see the comet for a long time before it approaches the sun, or for a long time after it has passed the sun. All we know, therefore, of its journey is that the two ends of the parabola stretch on and on forever into space. The comet is first perceived coming in along one of these branches to whirl round the sun; and after doing so, it retreats along the other branch, and gradually sinks into the depths of space.

Why one of these mysterious wanderers should approach in such a hurry, and then why it should fly back again, can be partially explained without the aid of mathematics.

Let us suppose that, at a distance of thousands of millions of miles, there floated a mass of flimsy material resembling that from which comets are made. Notwithstanding its vast distance from the sun, the attraction of that great body will extend thither. It is true the pull of the sun on the comet will be of the feeblest and slightest description, on account of the enormously great distance. Still, the comet will respond in some degree, and will commence gradually to move in the direction in which the sun invites it. Perhaps centuries, or perhaps thousands, or even tens of thousands, of years will elapse before the object has gained the solar system. By that time its speed will be augmented to such a degree, that after a terrific whirl around the sun, it will at once fly off again along the other branch of the parabola. Perhaps you will wonder why it does not tumble straight into the sun. It would do so, no doubt, if it started at first from a position of rest; generally, however, the comet has a motion to begin with which would not be directed exactly to the luminary. This it is which causes the comet to miss actually hitting the sun.

It may also be difficult to understand why the sun does not keep the comet when at last it has arrived. Why should the wandering body be in such a hurry to recede? Surely it might be expected that the attraction of the sun ought to hold it. If something were to check the pace of the comet in its terrific dash round the sun, then, no doubt, the object would simply tumble down into the sun and be lost. The sun has, however, not time to pull in the comet when it comes up with a speed 20,000 times that of an express train. But the sun does succeed in altering the _direction_ of the motion of the comet, and the attraction has shown itself in that way.

I can illustrate what happens in this manner. Here is a heavy weight suspended from the ceiling by a wire; it hangs straight down, of course, and there it is kept by the pull of the earth. Supposing I draw the weight aside and allow it to swing to and fro, then the motion continues like the beat of a pendulum. The weight is always pulled down as near to the earth as possible, but when it gets to the lowest point, it does not stay there, it goes through that point, and rises up at the other side. The reason is that the weight has acquired speed by the time it reaches the lowest point; and that, in virtue of its speed, it passes through the position in which it would naturally rest, and actually ascends the other side in opposition to the earth’s pull, which is dragging it back all the time. This will illustrate how the comet can pass by and even recede from the body which is continually attracting it.

Just a few words of caution must be added. Suppose you had an ellipse so long that the comet would take thousands and thousands of years to complete a circuit, then the part of the ellipse in which the comet moves during the time when we can see it is so like a parabola, that we might possibly be mistaken in the matter. In fact, a geometer will tell us that if one end of an ellipse was to go further and further away, the end that stayed with us would gradually become more and more like this curve. Therefore, some of those comets which seem to move in parabolas may really be moving in extremely elongated ellipses, and thus, after excessively long periods of time, may come back to revisit us.