Page:Popular Science Monthly Volume 4.djvu/307

Rh 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