MOTION, DIRECTION OF. We have inserted this article, not for the sake of rectilinear, but of circular motion, the consideration of which is apt to cause some embarrassment to the beginner. In motion along a given right line there can be but two directions, in one or other of which the course must be ; and these two directions are oppo site to one another. But in circular or other rotatory motion, all imaginable directions are taken in the course of a revolution, and whatever linear direction the moving body takes at any one point, it has the opposite direction at the opposite point. Still, however, there are two ways of moving on a circle : the motion may either be from c to A through B, or from A to c through B. These are called, some what improperly, different directions of motion.
If two bodies be moving over two circles, they are said to move in the same direction when, two radii being taken in the same direction, the linear directions of motion are the same, as n n and q n. Thus
care must be taken not to compare two circular motions by positions which belong to radii in opposite directions. If, for instance, the directions of motion be A B 0 and q v s (which are the same), and if at the same time the two bodies be at n and s, their linear directions of motion are opposite, though according to the definition their circular motions are in the same direction. Thus, in the case of the moon, and her revolution round her own axis [Moosr], the middle point of the visible moon is moving round the moon's axis in a direction opposite to the orbital motion of the moon ; but the radius of that middle point is opposite in direction to the line joining the centres of the earth and moon ; so that the direction of revolution of the moon's rotation is the same as that of the orbital rotation.