ce·les·tial e·qua·tor
/səˈlɛs.tʃəl ɪˈkweɪ.tər/
The imaginary Great Circle projected from Earth into space that is the same plane as Earth's terrestrial equator.
Understanding the celestial equator is crucial for grasping the astronomical framework upon which the zodiac is built. It should not be confused with the ecliptic, which is the apparent path of the Sun around the Earth and the plane used for measuring planetary longitude. The two great circles are tilted 23.5 degrees relative to each other, and their points of intersection define the vernal and autumnal equinoxes. The vernal equinox, where the Sun crosses the celestial equator moving north, marks the beginning of the tropical zodiac at 0° Aries. Furthermore, the celestial equator serves as a fundamental reference plane for several house division systems and for measuring declination, the celestial equivalent of terrestrial latitude, which is used to identify planetary parallels and contra-parallels.