페이지 이미지
PDF
ePub

puzzled by it, was enabled to explain even that apparently slight change in the position of the stars. He found that it was not owing to the change of the place of the earth, but to a slight oscillation of the earth which produced a trifling change in the position of the glass.

THE THEORY OF LIGHT.

Bradley observed another change—a change produced by aberration of light; and this leads me to another subject that is, the attempt to discover a unit of measurement with regard to these fixed stars; because, such is the enormous distance of these stars, that we find a difficulty in expressing them by numerals in common use. If we take for unit one, the length of the earth’s orbit—that is, if we were to take 190,000,000 of miles as standing for one, such is the enormous distance required to be measured and expressed in astronomical observations, that even this unit would be insufficient for the purpose. We should soon find ourselves dealing with numbers so inconceivably great, that they would utterly confuse us. We must, therefore, have some other unit of measure; and the velocity of light was therefore fixed upon as this unit of measurement.-Mr. Molesworth explained the astronomical observations which led to the discovery that light travelled about the rate of 12,000,000 miles a minute, and then proceeded :-Viewing these objects by a new standard of measurement, star 61 Cygni—that is, star number 61 in the cluster of the Swan-was selected for measurement. This star was found to be at such a distance, that light, travelling at this rate of 12,000,000 of miles á minute, would take nearly ten years to travel from that almost nearest of the fixed stars, to our earth. That great point being effected, the next step I shall have to call attention to, is the circumstance that several new stars have, at different times, made their appearance. Tycho Brahe, the

great Danish astronomer, observed one of great brightness. | Again, other stars which have occupied conspicuous places

in the heavens for a long time, have suddenly disappeared, and, so far as we know, never re-apeared.

HERSCHEL'S SOUNDINGS. Now we come to consider the soundings of the universe, made by Herschel, and which show to us, more than anything else, the enormous extent of the universe to which we belong. You have all observed, no doubt, what is called the Milky Way in the heavens-a broad line of milkwhite light extending over a great part of the heavens. That light is produced by an enormous number of stars, which abound in that portion of the heavens more than in any other. Wherever you turn the telescope, you find that the number of stars in that direction is enormous; and for every star you see with the naked eyė, you may see thousands and hundreds of thousands by means of the telescope When David, in boldly figurative language, spoke of the stars being as great in number as the sands on the sea-shore, he little knew how very near to the truth that statement was. However, Herschel's object was to ascertain the relative distances at which these stars are situated from us, and to penetrate, if possible, to the outer regions of the universe to which we belong. Now, the principle upon which he proceeded was this :—He took a telescope of the largest class, covered up à part of the object glass, and fixed it upon a certain quarter of the heavens. He looked through that. He then enlarged the aperture, still keeping the telescope fixed upon that point, and now he saw many stars coming to view which he had not formerly seen ; but still behind a sort of luminous haze. He enlarged the aperture again, and then saw that the luminous haze became in its turn myriads of stars. But behind that again, there was another luminous haze ; and so he went on increasing the aperture till he reached a point at which stars in great multitude were clearly seen through the object glass, and beyond which there was no longer the luminous haze--nothing but the deep black vault of heaven. He thus conceived that his glass had penetrated through the whole universe in that directionthat he had reached the outermost limits—and so far fathomed the deepest depths of the universe. Then, tuming his glass to another quarter of the heavens, he took similar soundings in that direction, and fathomed the heavens in all quarters. He was thus enabled to form some idea of the general shape of our universe. But even there Herschel did not stop. He calculated, with regard to many of those stars which he had observed, that they were at such distances that light, travelling at 12,000,000 of miles a minute, would take no less than 2,000 years to reach our sun and our system. He went beyond this. Not only did

he probe our system, but he endeavoured to ascertain if there might not be something beyond it. He observed in the heavens, apparently outside our system altogether, certain small patches of light. Turning his most powerful telescope upon these patches, he was enabled at last, as the astronomers say, to resolve them into stars. He thus discovered outside our universe, lying at an enormous distance away from us, so many island universes, as it were, scattered through the immensity of space by which we are surrounded. Not content with these observations, he endeavoured to form a judgment with respect to the distances of these island universes, and he came to the conclusion that they were situated at such distances from us, that light, travelling, as I have told you, at the rate of 12,000,000 miles a minute, would take no less a period than 120,000 years to reach our earth; so that the light which came to his eyes from these systems, showed him not the systems as they exist now, but as they were existing 120,000 years ago; and yet, probably, even that long period has scarcely wrought any change at all in the apparent conformation of these systems. For, great as that time seems to us, we shall find that as millions of miles are as nothing when we are dealing with the astronomy of space, so do 60,000 or 120,000 years sink into insignificance when we have to deal with the astronomy of time.

MOTIONS OF THE STARS,

Mr. Molesworth commenced his second lecture by pointing out the fact that not only were these apparently fixed stars in motion, but in rapid motion; and then proceeded to consider the question of these motions. He said-We know that a heavenly body may have two kinds of motion, and that most, if not all, of the bodies of our system have, in fact, two different motions: a motion of rotationthat is, a motion of spinning like a top, by which on our earth is produced the day and night; and a forward motion in space--a revolution round the sun, by which we have the change of seasons; so that when I enter on the motions of the fixed stars, I have two questions to ask :-First. Do they rotate ? Second. Do they move forward in space ?

I have already pointed out to you the analogy which exists between the sun and the fixed stars, leaving us to suppose the stars are suns, and, like our own sun, surrounded by their attendant planets. There have been observed upon the surface of the sun certain spots; by these spots we are able to trace with considerable accuracy the movements of the sun. These spots pass across the disc and disappear, and then reappear on the same place as that from which they apparently set out; leaving us to suppose that as planets have their motions of rotation, so the sun, too, spins round, and has a rotary motion of its own.

We find that it goes round and round at such a rate that it performe one revolution round its axis in (if I recollect right) 25 days, 8 hours, and 9 minutes ; I am not quite certain as to these figures, but you may take it as a fact that we know the period of a revolution within a moment. Having found that the sun revolves about its axis, and believing the stars to be suns, we are led by analogy to suppose that they too will probably have the same amount of rotation which we find our sun to have. And now the question comes-Are there any facts with regard to stars, confirming this idea ? There are. The brightness of stars varies considerably. There are stars of the first magnitude which pass to the third magnitude; then, after an interval, become stars of the first magnitude; and then again stars of the third magnitude, and so on. And this occurs periodi. cally. We call these stars, therefore, periodic stars. What is the explanation of this? The probable explanation is, that these stars have spots on their disc like spots upon our sun; and the consequence is, that on one side there is much less brightness than on the other side, and that is the reason why they become more bright and less bright, leaving us to suppose that they have the same motions as our sun. So, by means of these spots, we are not only able to ascertain the fact of their rotation, but even the time of their rotation; and this is all the evidence we have. But there are other stars, the great majority, in which this does not take place, and in regard to which we are not able to prove any rotation. But if we prove that some have a motion of rotation, we may fairly infer that all have, and that the only reason why we are not able to observe the fact is, that some have very large spots, while others have no spots at all, or very small spots. For instanceif our sun were at the same distance from us as these fixed stars are, it would not present to us first a brighter side and then a darker side; but the difference between the two sides would be so small that we should not be able to discover

any

evidence of rotation; and this may be the case in the majority of stars. But as we have some evidence to lead us to suppose that many of these stars do rotate as the sun rotątes, we think it is only a fair extension of the evidence to suppose that all stars have a movement of rotation. We then come, in the next place, to inquire-As the stars have a movement of rotation, have they also a forward movement in space ? On that subject we have light thrown by a discovery made by Herschel in his researches after parallax. On examining the heavens, he found a multitude of what he called double stars, and then multiple stars. It was found that when the telescope was turned upon these stars, which seemed to be isolated ones, they were really double stars, at apparently slight, but really immense, distances. And it was found that there were not only two, but three, four, five, and even greater numbers; in fact, vast numbers of these multiple stars were discovered. Herschel argued on the discovery in this way :-“There is a great difference between the sizes of these stars. I presume this circumstance is not owing to any real difference between the stars, but to the circumstance of their greater distance from each other. I find, for instance, two stars together at certain intervals, but one of these stars is only half as large as the other. I conclude that the smaller one is twice as far away as the larger. If this is the case, then the earth's motion round the sun will have the effect of causing them to appear to approach or recede from one another, and thus I shall be able to discover something as to their distances.” So to work he went with those spider lines I spoke to you of in my last lecture. First of all, he arranged them so as to pass exactly through both of these multiple stars; then he brought them parallel in order that he might measure accurately the distances between them. He made these observations with regard to many stars; and by these observations, followed up night after night, for a long period, he soon found that not only movement existed among them, but different movements to what he had anticipated. Instead of a movement of approach or retirement, he found a motion of revolution taking place between these stars. They were evidently revolving one about the other. He came to see clearly and unquestionably that these two stars were, astronomically speaking, near to

« 이전계속 »