Celestial equator

By extension, it is also a plane of reference in the equatorial coordinate system.

[1] Due to Earth's axial tilt, the celestial equator is currently inclined by about 23.44° with respect to the ecliptic (the plane of Earth's orbit), but has varied from about 22.0° to 24.5° over the past 5 million years[2] due to perturbation from other planets.

As the observer moves north (or south), the celestial equator tilts towards the opposite horizon.

The celestial equator is defined to be infinitely distant (since it is on the celestial sphere); thus, the ends of the semicircle always intersect the horizon due east and due west, regardless of the observer's position on Earth.

At the poles, the celestial equator coincides with the astronomical horizon.