Many persons seem to despair of gaining any real knowledge of astronomy, merely because they are not versed in recognising the constellations. For instance, they will say:—"What is the use of my reading anything about the subject? Why, I believe I couldn't even point out the Great Bear, were I asked to do so!" But if such persons will only consider for a moment that what we call the Great Bear has no existence in fact, they need not be at all disheartened. Could we but view this familiar constellation from a different position in space, we should perhaps be quite unable to recognise it. Mountain masses, for instance, when seen from new directions, are often unrecognisable.
It took, as we have seen, a very long time for men to acknowledge the immense distances of the stars from our earth. Their seeming unchangeableness of position was, as we have seen, largely responsible for the idea that the earth was immovable in space. It is a wonder that the Copernican system ever gained the day in the face of this apparent fixity of the stars. As time went on, it became indeed necessary to accord to these objects an almost inconceivable distance, in order to account for the fact that they remained apparently quite undisplaced, notwithstanding the journey of millions of miles which the earth was now acknowledged to make each year around the sun. In the face of the gradual and immense improvement in telescopes, this apparent immobility of the stars was, however, not destined to last. The first ascertained displacement of a star, namely that of 61 Cygni, noted by Bessel in the year 1838, definitely proved to men the truth of the Copernican system. Since then some forty more stars have been found to show similar tiny displacements. We are, therefore, in possession of the fact, that the actual distances of a few out of the great host can be calculated.
To mention some of these. The nearest star to the earth, so far as we yet know, is Alpha Centauri, which is distant from us about 25 billions of miles. The light from this star, travelling at the stupendous rate of about 186,000 miles per second, takes about 4¼ years to reach our earth, or, to speak astronomically, Alpha Centauri is about 4¼ "light years" distant from us. Sirius—the brightest star in the whole sky—is at twice this distance, i.e. about 8½ light years. Vega is about 30 light years distant from us, Capella about 32, and Arcturus about 100.
The displacements, consequent on the earth's movement, have, however, plainly nothing to say to any real movements on the part of the stars themselves. The old idea was that the stars were absolutely fixed; hence arose the term "fixed stars"—a term which, though inaccurate, has not yet been entirely banished from the astronomical vocabulary. But careful observations extending over a number of years have shown slight changes of position among these bodies; and such alterations cannot be ascribed to the revolution of the earth in its orbit, for they appear to take place in every direction. These evidences of movement are known as "proper motions," that is to say, actual motions in space proper to the stars themselves. Stars which are comparatively near to us show, as a rule, greater proper motions than those which are farther off. It must not, however, be concluded that these proper motions are of any very noticeable amounts. They are, as a matter of fact, merely upon the same apparently minute scale as other changes in the heavens; and would largely remain unnoticed were it not for the great precision of modern astronomical instruments.
One of the swiftest moving of the stars is a star of the sixth magnitude in the constellation of the Great Bear; which is known as "1830 Groombridge," because this was the number assigned to it in a catalogue of stars made by an astronomer of that name. It is popularly known as the "Runaway Star," a name given to it by Professor Newcomb. Its speed is estimated to be at least 138 miles per second. It may be actually moving at a much greater rate, for it is possible that we see its path somewhat foreshortened.
A still greater proper motion—the greatest, in fact, known—is that of an eighth magnitude star in the southern hemisphere, in the constellation of Pictor. Nothing, indeed, better shows the enormous distance of the stars from us, and the consequent inability of even such rapid movements to alter the appearance of the sky during the course of ages, than the fact that it would take more than two centuries for the star in question to change its position in the sky by a space equal to the apparent diameter of the moon; a statement which is equivalent to saying that, were it possible to see this star with the naked eye, which it is not, at least twenty-five years would have to elapse before one would notice that it had changed its place at all!
Both the stars just mentioned are very faint. That in Pictor is, as has been said, not visible to the naked eye. It appears besides to be a very small body, for Sir David Gill finds a parallax which makes it only as far off from us as Sirius. The Groombridge star, too, is just about the limit of ordinary visibility. It is, indeed, a curious fact that the fainter stars seem, on the average, to be moving more rapidly than the brighter.
Investigations into proper motions lead us to think that every one of the stars must be moving in space in some particular direction. To take a few of the best known. Sirius and Vega are both approaching our system at a rate of about 10 miles per second, Arcturus at about 5 miles per second, while Capella is receding from us at about 15 miles per second. Of the twin brethren, Castor and Pollux, Castor is moving away from us at about 4½ miles per second, while Pollux is coming towards us at about 33 miles per second.
Much of our knowledge of proper motions has been obtained indirectly by means of the spectroscope, on the Doppler principle already treated of, by which we are enabled to ascertain whether a source from which light is coming is approaching or receding.
The sun being, after all, a mere star, it will appear only natural for it also to have a proper motion of its own. This is indeed the case; and it is rushing along in space at a rate of between ten and twelve miles per second, carrying with it its whole family of planets and satellites, of comets and meteors. The direction in which it is advancing is towards a point in the constellation of Lyra, not far from its chief star Vega. This is shown by the fact that the stars about the region in question appear to be opening out slightly, while those in the contrary portion of the sky appear similarly to be closing together.