Not only do many stars revolve in orbits about near-by companions, but all the stars, without exception, are independently in motion. They appear to be travelling through space in many different directions, each following its own chosen way without regard to the others, and each moving at its own gait. These movements of the stars are called proper motions. The direction of the sun's proper motion is, roughly speaking, northward, and it travels at the rate of twelve or fourteen miles per second, carrying the earth and the other planets along with it. Some stars have a much greater speed than the sun, and some a less speed. As we have said, these motions are in many different directions, and no attempt to discover any common law underlying them has been entirely successful, although it has been found that in some parts of the sky a certain number of stars appear to be travelling along nearly parallel paths, like flocks of migrating birds. In recent years some indications have been found of the possible existence of two great general currents of movement, almost directly opposed to each other, part of the stars following one current and part the other. But no indication has been discovered of the existence of any common centre of motion. Several relatively near-by stars appear to be moving in the same direction as the sun. Stars that are closely grouped together, like the cluster of the Pleiades, seem to share a common motion of translation through space. We have already remarked that when stars are found to be moving toward or away from the sun, spectroscopic observation of the shifting of their lines gives a means of calculating their velocity. In other cases, the velocity across the line of sight can be calculated if we know the distance of the stars concerned. One interesting result of the fact that the earth goes along with the sun in its flight is that the orbit of the earth cannot be a closed curve, but must have the form of a spiral in space. In consequence of this we are continually advancing, at the rate of at least 400,000,000 miles per year, toward the northern quarter of the sky. The path pursued by the sun appears to be straight, although it may, in fact, be a curve so large that we are unable in the course of a lifetime, or many lifetimes, to detect its departure from a direct line. At any rate we know that, as the earth accompanies the sun, we are continually moving into new regions of space.