Popular Science Monthly/Volume 4/January 1874/Past and Future of a Constellation
|PAST AND FUTURE OF A CONSTELLATION.|
TRANSLATED FROM THE FRENCH, BY J. FITZGERALD, A. M.
THE notions hitherto entertained as to the stars and the heavens are destined to undergo a complete revolution. There are no fixed stars. Each one of those distant suns, naming in infinitude, is swept along in a stupendous movement which the imagination can hardly conceive. Notwithstanding the countless millions of miles of space between them and us, making them appear to us only as luminous points, whereas they are as great as our own sun, and thousands and millions of times greater than the earth, still, by means of the telescope and computation, astronomers have been able to come at them, and to demonstrate that they are all moving in every possible direction. The heavens are no longer motionless, nor can the constellations any longer be regarded as the symbol of the unchangeable. Take, for instance, Ursa Major, or the Great Bear, the first of the constellations to be observed and named. Who is there that has not taken that figure as the enduring symbol of the preestablished harmony, the unalterable duration of the firmament? Well, that ancient constellation will be destroyed. Each one of the stars which constitute it is endowed with a movement of its own. The result is that, in course of time, the form of Ursa Major will be changed. It now somewhat resembles in outline a wagon, and hence its popular title everywhere of car, or wain, while the learned have given it the name of the Bear, that being the only animal known to the ancients as living in polar regions. As every one knows, the four stars arranged in the form of a quadrilateral are* supposed to represent the four wheels, and the three stars in the front of the figure three horses. But the proper movement of the separate stars will alter this arrangement: it will bring the foremost horse to a point back of where he now is, while the other two will move onward. As for the two hinder wheels, they will proceed in contrary directions. When we know the annual value of the displacement of each of these seven stars, we can calculate their future relative positions. This I have done, and I here lay before the reader the curious results of my calculations.
In order to get an exact account of the difference in the form of this constellation, which will be observable at a given time, let us first portray its present state.
The Arabs gave these seven famous stars names which are sometimes applied to them still. Beginning with the hind off-wheel, and then taking in the order indicated by the Greek letters (β, γ, δ, ε, ζ, η) the other wheels and the horses, the Arab names are as follows: Dubhe, Merak, Phegda, Megrez, Alioth, Mizar, and Ackaïr. The last is the name given to the foremost horse. Persons possessed of good visual powers can discern above the second horse, Mizar, a small star which is called the Postilion. But these names are seldom employed in our times, the usual custom being to designate the seven principal stars of Ursa Major by the first seven letters of the Greek alphabet, as shown in the diagram. All these stars are of the second magnitude, except Delta (Fig. 1), which is of the third.
In the diagram, the arrows show the mean direction in which each of the seven stars moves. It will be seen that, of the seven, the first and the last, Alpha and Eta, are moving in a direction contrary to that of the other five. It must be added that they have not all the same velocity. Eta, for example, moves rapidly; Epsilon slowly; and so on with the others.
The quantity of their annual proper movements in right ascension, and in polar distance, is given for each of the seven stars in the following table:
|R. A.||P. D.|
In consequence of these proper movements, the relative distances between the stars of the constellation are ever changing. But, as this change only amounts to a few seconds in a century, many centuries must elapse before it is perceptible to the naked eye. Our human generations, our dynasties, nay, even our nations, are not sufficiently long-lived to measure this change.
We have here to deal with astronomical quantities, and, to appreciate these, we must choose terms which correspond with them. The earth has but one measure of time that can be employed here, viz., its great year, the precession of its equinoxes—a slow revolution of the globe, which is completed in more than 25,000 years. A period like that might serve as a basis of measurement in geology and in sidereal astronomy. Taking, then, four of these periods—in round numbers 100,000 years—we ought, after that length of time, to have a sensible difference in the aspect of the heavens; and, in fact, on computation, I find that in this interval—which in the history of the stars is but a brief span—all the present constellations will be broken up.
In Fig. 2 I give the geometrical results of my calculations as to the proper movements of the stars in Ursa Major. Here is to be seen the shape which that constellation will wear 100,000 years hence. There is nothing like a wagon in this new figure. Alpha has moved downward and ranged itself on the right of Beta, and both of these lie on one line with Gamma, and even with Eta. Delta, Epsilon, and Zeta, are seen ranged on another line. If, in that distant epoch, the languages of terrestrial man shall still give to this constellation the title of the Wain, no one will be able to understand why. In considering what a mighty change it is destined to undergo in the future, the question arises, How long has it worn the shape in which it is familiar to us, and how did it look ages ago? One hundred thousand years ago there were, as yet, in all probability, no human beings on the earth, and the antediluvian monsters were the only creatures that could then view the starry sky.
Still, some of the older planets—Mars, Jupiter, Saturn, Uranus, and Neptune—were, no doubt, inhabited even then; and, as the heavens have the same appearance when viewed from them as from the earth, the dwellers in those worlds saw Ursa Major as it appeared in those days. All that we have to do, in order to find the position of each of the seven stars 100,000 years ago, is to move them back from their present positions as far as they were moved forward in our second diagram. The result is a figure having no resemblance to either of the others. It is a sort of cross, with Beta at the point of intersection, Alpha marking the extremity of the left arm, and Gamma that of the right; Beta the top, Epsilon, Zeta, and Eta, the stem. Eta was, properly speaking, not yet in association with the other six. For the rest, on analyzing the movement of these stars, we see that five of them, Beta, Gamma, Delta, Epsilon, and Zeta, are associated together by a common tie. They are a group of friends: they move on with one accord, and, as will be seen, maintain the same relative position to one another. Alpha and Eta are only intruders, and, though they happen just at present to be with the group, they really have nothing to do with it. Look at Fig. 2: Alpha, which is evermoving toward the right, will, in time, quit the group for good. On the other hand, Fig. 3 shows Eta coming in on the left; previously that star had no relation at all to the five.
The remarks just made with regard to the secular transformation of Ursa Major might be applied to all the other constellations. We have selected that one, because it is the best known of all, and one of the most characteristic. To sum up: we find that a knowledge of the proper movements of the stars completely transforms our common notions about the fixity of the heavens. The stars move in all directions through the endless realms of space, and as the aspect of the heavens changes, so does the constitution of the universe also change from age to age, undergoing innumerable metamorphoses.—Revue Scientifique