We call the stars _fixed_, but motion and life are necessary to all things. Besides the motion in the line of sight described already, there is motion in every other conceivable direction. We knew Sirius moved before we had found the cause. We know that our sun moves back and forth in his easy bed one-half his vast diameter, as the larger planets combine their influence on one side or the other. The sun has another movement. We find the stars in Hercules gradually spreading from each other. Hercules's brawny limbs grow brawnier every century. There can be but one cause: we are approaching that quarter of the heavens. (See [Symbol], Fig. 72.) We are even [Page 227] able to compute the velocity of our approach; it is four miles a second. The stars in the opposite quarter of the heavens in Argo are drawing nearer together. This movement would have no effect on the apparent place of the stars at either pole, if they were all equally distant; but it must greatly extend or contract the apparent space between them, since they are situated at various distances. Independent of this, the stars themselves are all in motion, but so vast is the distance from which we observe them that it has taken an accumulation of centuries before they could be made measurable. A train going forty miles an hour, seen from a distance of two miles, almost seems to stand still. Arcturus moves through space three times as fast as the earth, but it takes a century to appear to move the eighth part of the diameter of the moon. There is a star in the Hunting Dogs, known as 1830 Groombridge, which has a velocity beyond what all the attraction of the matter of the known universe could give it. By the year 9000 it may be in Berenice's Hair. Some stars have a common movement, being evidently related together. A large proportion of the brighter stars between Aldebaran and the Pleiades have a common motion eastward of about ten seconds a century. All the angles marked by a, b, g, ch Orionis will be altered in different directions; l is moving toward g. l and e will appear as a double star. In A.D. 50,000 Procyon will be nearer ch Orionis than Rigel now is, and Sirius will be in line with a and ch Orionis. All the stars of the Great Dipper, except Benetnasch and Dubhe, have a common motion somewhat in the direction [Page 228] of Thuban (Fig. 67), while the two named have a motion nearly opposite. In 36,000 years the end of the Dipper will have fallen out so that it will hold no water, and the handle will be broken square off at Mizar. "The Southern Cross," says Humboldt, "will not always keep its characteristic form, for its four stars travel in different directions with unequal velocities. At the present time it is not known how many myriads of years must elapse before its entire dislocation."