The ecliptic is the apparent path that the Sun traces out in the sky during the year. As it appears to move in the sky in relation to the stars, the apparent path aligns with the planets throughout the course of the year. It is the intersection of a spherical surface, the celestial sphere, with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun.
The ecliptic plane should be distinguished from the invariable plane. This is the average plane of the orbits of the planets. More precisely, it is perpendicular to the vector sum of the angular momenta of all planetary orbital planes, to which Jupiter is the main contributor. The present ecliptic plane is inclined to the invariable ecliptic plane by about 1.5°.
The name ecliptic is derived from being the place where eclipses occur.
Because Earth's rotational axis is not perpendicular to its orbital plane, Earth's equatorial plane is not coplanar with the ecliptic plane, but is inclined to it by an angle of about 23.4°, which is known as the obliquity of the ecliptic. If the equator is projected outward to the celestial sphere, forming the celestial equator, it crosses the ecliptic at two points known as the equinoxes. The Sun, in its apparent motion along the ecliptic, crosses the celestial equator at these points, one from south to north, the other from north to south. The crossing from south to north is known as the vernal equinox, also known as the first point of Aries and the ascending node of the ecliptic on the celestial equator. The crossing from north to south is the autumnal equinox or descending node. The orientation of Earth's axis and equator are not fixed in space, but rotate about the poles of the ecliptic with a period of about 26,000 years, a process known as lunisolar precession, as it is due mostly to the gravitational effect of the Moon and Sun on Earth's equatorial bulge. Likewise, the ecliptic itself is not fixed. The gravitational perturbations of the other bodies of the Solar System cause a much smaller motion of the plane of Earth's orbit, and hence of the ecliptic, known as planetary precession. The combined action of these two motions is called general precession, and changes the position of the equinoxes by about 50 arc seconds (about 0.014°) per year.
Most of the major bodies of the Solar System orbit the Sun in nearly the same plane. This is likely due to the way in which the Solar System formed from a protoplanetary disk. Probably the closest current representation of the disk is known as the invariable plane of the Solar System. Earth's orbit, and hence, the ecliptic, is inclined a little more than 1° to the invariable plane, and the other major planets are also within about 6° of it. Because of this, most Solar System bodies appear very close to the ecliptic in the sky. The ecliptic is well defined by the motion of the Sun. The invariable plane is defined by the angular momentum of the entire Solar System, essentially the summation of all of the orbital motions and rotations of all the bodies of the system, a somewhat uncertain value that requires precise knowledge of every object in the system. For these reasons, the ecliptic is used as the reference plane of the Solar System out of convenience.
Ecliptic Facts for Kids. Kiddle Encyclopedia.