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.
