DIRECT MOTION, in astronomy, the apparent motion of a body .of the solar system on the celestial sphere in the direction from west to east; so called because this is the usual direction of revolution and rotation of the heavenly bodies.
In astronomy, we are principally concerned with the orientation of points on a sphere - the so-called celestial sphere - with regard to certain planes and points within the sphere; this subject is treated in the article Astronomy (Spherical).
But on the celestial sphere the great circles of these two planes are coincident, so that this distinction is not necessary (see Astronomy: Spherical).
Zones similar to those already drawn out for the celestial sphere were first introduced by the Pythagoreans.
The line of the equinoxes is the imaginary diameter of the celestial sphere which joins them.