What Is the Celestial Equator?

Celestial equator: refers to the great circle obtained by intersecting the equator plane with the celestial sphere. The celestial equator divides the celestial sphere into the northern hemisphere and the southern hemisphere. ^[1]

The celestial equator is a large circle on the celestial sphere. The base circle (or main circle) of the celestial equatorial coordinate system, which is formed by the infinite expansion of the Earth's equatorial plane and the celestial sphere. The celestial equator and the ecliptic intersect at the bisector, and its intersection angle is 23º26 '(2000 AD), which is called the "yellow-red intersection". ^[2]

An imaginary large circle perpendicular to the Earth's rotation axis on the celestial sphere is located on the same plane as the Earth's equator. It can also be said to be a large circle that divides the celestial sphere into two parts, the north and the south, perpendicular to the earth's axis, and theoretically has an infinitely long radius. Relative to the ecliptic plane, the celestial equator is inclined 23º26 '. This angle is called the "yellow equator angle" and is the result of the tilt of the earth's axis.

When the sun is on the equator, day and night are equal everywhere, so the equator is also known as the midnight or day equinox; at that time, the northern and southern hemispheres were at the vernal equinox or autumn equinox. During the year, the sun has two chances to be on the equator. As long as we continue to expand the earth's equator outwards to infinity, this infinite circle is the equator of the sky. Simply put, it is a large circle that intersects the celestial sphere after the equatorial plane of the earth extends indefinitely, which is called the celestial equator, or the celestial equator.

Relative to the ecliptic plane, the celestial equator tilts 23.5º, which is the result of the tilt of the earth's axis. When the sun is on the equator, day and night are equal everywhere, so the equator is also known as the midnight or day equinox; at that time, the northern and southern hemispheres were at the vernal equinox or autumn equinox. ^[3]