Ecliptic - Definition and Overview

The plane of the Ecliptic is well seen in this picture from the 1994 lunar prospecting Clementine spacecraft. Clementine's camera reveals (from right to left) the Moon lit by Earthshine, the Sun's glare rising over the Moon's dark limb, and the planets Saturn, Mars and Mercury (the three dots at lower left).

The ecliptic is the geometric plane that contains the orbit of the Earth. The orbits of most planets in the Solar System lie very close to it. Seen from the Earth, this is a bisecting great circle, superimposed upon the celestial sphere, which contains the different points of the Sun's path, relative to the background stars, over the course of a year. The zodiac also lies along the ecliptic plane. The ecliptic plane is inclined by ~23.5°, with respect to the celestial equator; a result of axial tilt. The orbital plane of the Moon is inclined by ~5°, with respect to the ecliptic.

Because there are ~365.25 days in a year and 360 degrees in a circle, the Sun appears to move along the ecliptic at a rate of about 1° per day. This motion is from west to east, in opposition to the apparent east-west movement of the celestial sphere.

The ecliptic and the celestial equator intersect at two points, directly opposite one another. These are the equinoxes and when the Sun appears at these points, day and night are each about 12 hours long at all locations on Earth.

The point on the ecliptic that is farthest north of the celestial equator is called the summer solstice in the northern hemisphere, and the winter solstice in the southern hemisphere. When the Sun is farthest south of the celestial equator the reverse is true.

If the Moon crosses the ecliptic (such points of crossing are nodes) during new moon or full moon, an eclipse will occur.