Since the sun moves in the ecliptic, it is in the last-named sense the point of intersection of the ecliptic and the celestial equator.
The axis of the ecliptic is a line perpendicular to this plane.
The laws of motion of the ecliptic and equator are stated in the article Precession Of The Equinoxes.
The plane of the ecliptic is that plane in or near which the centre of gravity of the earth and moon.
The vernal equinox is taken as the initial point on the sphere from which co-ordinates are measured in the equatorial and ecliptic systems. Referring to fig.
The obliquity of the ecliptic is the angle which its plane makes with that of the equator.
The chief conclusions of astronomers concerning the .spherical figure and dimensions of the earth, its relation to the heavenly bodies, and the great circles of the globe - the equator, the ecliptic and the tropics - were considered as well established.
Conceiving the line NN' to be that of the nodes at any time, and the earth and lunar orbit to be moving in the direction of the straight arrows, the earth will be on one side of the ecliptic from M2 to M5, and on the other side from M6 to M 1, intersecting it at the nodes.
The ecliptic itself is the great circle in which this plane meets the celestial sphere.
The motion of the ecliptic produces a secular variation in the obliquity which is now diminishing by an amount nearly equal to the entire motion of the ecliptic itself.