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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 and south poles.
In contrast, the ecliptic is in the same plane as the Earth's orbit around the Sun. The angle between the two, which is roughly 23.4° is (necessarily) the same as Earth's familiar axial tilt.