Celestial Equator

The celestial equator is the imaginary great circle in the sky formed by projecting Earth's own equator outward onto the celestial sphere, and it serves as the fundamental reference line for the equatorial coordinate system that astronomers use to catalog the positions of stars. Just as Earth's equator divides the planet into northern and southern hemispheres, the celestial equator divides the sky into a northern celestial hemisphere and a southern celestial hemisphere, and it is defined as being exactly zero degrees declination, the celestial equivalent of latitude. Because it is directly tied to Earth's equator, the celestial equator's position in the sky depends entirely on an observer's own latitude: from the equator itself, the celestial equator passes directly overhead through the zenith, while from either pole, it runs exactly along the horizon. The celestial equator is tilted by about 23.4 degrees relative to the ecliptic, the Sun's apparent yearly path across the sky, because of Earth's axial tilt; the two points where the ecliptic crosses the celestial equator mark the spring and fall equinoxes, when day and night are of equal length worldwide.