noun Astronomy, Navigation.
- the great circle of the celestial sphere, lying in the same plane as the earth’s equator.
- the great circle lying on the celestial sphere the plane of which is perpendicular to the line joining the north and south celestial polesAlso called: equinoctial, equinoctial circle
- A great circle separating the northern and southern hemispheres of the celestial sphere and lying in the same plane as the Earth’s equator. The celestial equator is a projection of the Earth’s equator outward onto the celestial sphere and is used as the reference point in determining a celestial body’s declination.