This entry contributed by Ed Post

The plane of the Earth's orbit around the sun, so named because eclipses of the sun and moon can occur only when the moon (whose orbit is closer to the plane of the Earth's equator instead of its orbital plane around the sun) crosses this plane. Because of precession, the position of the ecliptic plane changes slowly over time and so must be specified together with an epoch, usually defined as 1950.0 or 2000.0.

Constellation, Eclipse, Ecliptic, Zodiac