Astronomy for Everybody/Part 6/Chapter 5

If I were asked what is the greatest fact that the intellect of man has ever brought to light I should say it was this:

Through all human history, nay, so far as we can discover, from the infancy of time, our solar system—sun, planets, and moons—has been flying through space toward the constellation Lyra with a speed of which we have no example on earth. To form a conception of this fact the reader has only to look at the beautiful Lyra and reflect that for every second that the clock tells off, we are ten miles nearer to that constellation. Every day that we live we are nearer to it by almost, perhaps quite, a million of miles. For every sentence that we utter, for every step that we take in the streets we are miles nearer to this star. We approached it by tens of thousands of miles while the writer has been penning these lines, and the reader has been carried nearer by a thousand miles while perusing them. This has been going on through all human history, and we have reason to believe that it will remain true for our remotest posterity. One of the greatest problems of astronomy is, when and how did this journey begin and when and how will it end? Before this question our science stands dumb. The astronomer can tell no more about the beginning or the end of the ​journey than can the untutored child. He can only impress upon the mind of his followers the magnitude of the problem.

Nothing can give us a better conception of the enormous distance of the stars than the reflection that notwithstanding the rapid motion, carrying us unceasingly forward through all the ages that the human race has existed on earth, ordinary observation would fail to show any change in the appearance of the constellation toward which we are travelling. From what we know of the distance of Vega we have reason to suppose that our solar system will not reach the region in which that star is now situated until the end of a period ranging somewhere between half a million and a million of years from the present time.

It does not follow, however, that our posterity, if any such shall then live on the earth, will find Vega when they arrive at its present place. It also is going on its own journey and is passing away from its present location almost as rapidly as we are approaching it.

What is true of our sun and of Vega is true, so far as we know, of every star in the heavens. Each of these bodies is flying straight ahead through space like a ball shot out from a cannon, with a speed which in most cases is almost inconceivable. It would be a very slow moving star of which the velocity did not exceed that of a cannon shot. In the great majority of cases it ranges from five to thirty miles per second—frequently more than fifty miles. Indeed there are two stars, of which Arcturus is one, whose speed we have reason to believe approaches ​two hundred miles a second. These motions of the stars are called their proper motions.

We have described the proper motions as so many miles per second. But owing to the enormous distance of the stars, rapid as the proper motions are in reality, they seem slow indeed when we observe them. So slow are they that if Ptolemy should come to life after his sleep of nearly eighteen hundred years, and be asked to compare the heavens as they are now with those of his time, he would not be able to see the slightest difference in the configuration of a single constellation. Even to the oldest Assyrian priests, the constellation Lyra and the star Vega looked exactly as they do to us to-day, notwithstanding the immeasurable distance by which we have approached them.

To resuscitate an inhabitant of the ancient world who would be able to perceive any change, we should have to go back four thousand years perhaps, to the time of Job, and we should have to take one of the swiftest moving stars in the heavens, Arcturus. Bringing Job to life and showing him the constellation Bootes, of which Arcturus is the brightest star, he would perceive the latter to have moved through about half of the distance in the accompanying diagram between the stars marked "1" and "2."

In considering these motions, the most natural thought to present itself is that the stars are describing vastly extended orbits around some centre, as the planets are moving round the sun, and that the motions we see are simply the motions in these orbits. But the facts do not support this view. The most refined observations yet made do not show the slightest curvature in the path of ​any star. Every one seems to be going straight ahead on its own account, never swerving to the right or left. It does not seem possible to admit the existence of bodies large and massive enough to control such rapid motions. A body massive enough to attract Arcturus from its head-

long course would throw all that part of the universe in which we live into disorder. The problem where the rapidly moving stars came from and whither they are going is therefore for us insoluble. What makes the case yet more difficult is that different stars move in different directions, without any seeming order, so that one motion seems to have no connection with another, unless in a few very rare cases.