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The celestial equator is a great circle around the celestial sphere in the same plane as the Earth's equator. Declination is measured from the celestial equator. Similarly, the celestial poles (celestial north pole and celestial south pole) are the points on the celestial sphere directly above the Earth's North Pole and South Pole.
In contrast, the ecliptic is in the plane in which the Earth's orbit around the Sun resides. The angle between the two planes, which is roughly 23.4°, is (necessarily) the same as Earth's familiar axial tilt.