celestial equator

The great circle in which the plane of the Earth's equator cuts the celestial sphere. This is the primary circle for the co-ordinates' right ascension and declination.

The celestial equator is a great circle on the imaginary celestial sphere, which is actually the plane of the terrestrial equator extended out into the universe (i.e., it could be constructed by extrapolating the Earth's equator until it touches the celestial sphere).

If an observer stands on earth equator s/he sees the celestial equator passing through the zenith. Because celestial equator is theoretically infinitely far (on the [[celestial sphere), irrespective of observer position, s/he always sees this equator as a semicircle ending in the horizon, exactly in the local east/west directions.

Celestial objects near the celestial equator are visible worldwide, but they culminate the highest in the sky in the tropics.