Ecliptic

The ecliptic is the apparent path of the Sun across the sky over the course of a year as seen from Earth, or equivalently the projection of the plane of Earth’s orbit around the Sun onto the celestial sphere. The ecliptic plane is the plane of Earth’s orbital motion around the Sun, and its intersection with the celestial sphere defines the ecliptic as a great circle.

The ecliptic is inclined to the celestial equator by about 23.4 degrees, a tilt known as the obliquity of the ecliptic. This tilt causes the Sun’s declination to vary through the year, producing the seasons in temperate latitudes. The ecliptic also serves as a fundamental reference plane in celestial coordinate systems. Ecliptic longitude measures position along the ecliptic from the vernal equinox, while ecliptic latitude measures a body’s angular distance north or south of the ecliptic. The zodiac comprises twelve major regions along the ecliptic, corresponding to the familiar signs.

The vernal equinox point—used as the origin of ecliptic longitude—drifts westward along the ecliptic over long timescales due to axial precession. This precession means the origin is not fixed in the background stars, an important consideration for historical and astronomical calculations.

Planets in the Solar System generally follow paths near the ecliptic because their orbits lie close to Earth’s orbital plane; their apparent motions on the sky are thus confined to a narrow band around the ecliptic. The term ecliptic is also used in astrology to refer to the same region along the zodiac.