Equator (Lat. oequare, to make equal), an imaginary circle drawn around any symmetrical solid of revolution (for example, a sphere or an ellipsoid) so that all its points are equally distant from the extremities of the axis of revolution. These extremities are called poles. Thus the earth is an ellipsoid revolving about an axis the extremities of which are called the north and south poles; a circle conceived to be drawn around the earth so that all its points are equally distant from the poles is called a terrestrial equator. - The celestial equator, or as it is often called by astronomers the equinoctial, is a great circle of the celestial sphere, situated in the indefinite extension of the plane of the terrestrial equator. All its points are equally distant from the poles of the celestial sphere. The name takes its origin from the fact that when the sun in his annual course passes through the plane of the celestial equator the days and nights are equal in all parts of the world. This occurs about the 21st of March and the 23d of September in each year, which periods are hence called respectively the vernal and the autumnal equinox; and the points where the plane in which the sun moves intersects the celestial equator are called the equinoctial points.

The celestial meridian which passes through these points is called the equinoctial colure.