Which circle on the celestial sphere is the projection of the Earth's equator onto the celestial sphere?

Which circle on the celestial sphere is the projection of the Earth's equator onto the celestial sphere?

• A. Celestial poles
• B. Zenith
• C. Celestial equator
• D. Horizon

Answer: (C)

The projection of Earth's equator onto the celestial sphere forms a great circle known as the celestial equator. This circle lies in the same plane as Earth's equator, cutting the celestial sphere into northern and southern celestial hemispheres and crossing the sky at the vernal and autumnal equinox points. It has declination zero and serves as the baseline for celestial coordinates, with right ascension measured along it from the vernal equinox. The celestial poles are points where Earth's rotation axis meets the celestial sphere, the zenith is the point directly overhead, and the horizon is the observer’s local boundary circle—none of these are the projection of Earth's equator.

The projection of Earth's equator onto the celestial sphere forms a great circle known as the celestial equator. This circle lies in the same plane as Earth's equator, cutting the celestial sphere into northern and southern celestial hemispheres and crossing the sky at the vernal and autumnal equinox points. It has declination zero and serves as the baseline for celestial coordinates, with right ascension measured along it from the vernal equinox. The celestial poles are points where Earth's rotation axis meets the celestial sphere, the zenith is the point directly overhead, and the horizon is the observer’s local boundary circle—none of these are the projection of Earth's equator.