Which coordinate system is used for mapping positions on the celestial sphere?

• A. Terrestrial Coordinate System
• B. Horizon Coordinate System
• C. Celestial Coordinate System
• D. Geodetic Coordinate System

Answer: (C)

Mapping positions on the sky uses the Celestial Coordinate System. It sets up a universal frame on the celestial sphere with reference to the celestial equator and the vernal equinox, giving objects coordinates of right ascension and declination. Right ascension acts like longitude projected onto the sky and is measured along the celestial equator, while declination acts like latitude, measured north or south of the celestial equator. This frame remains consistent for celestial objects, even though their observed positions can drift slightly due to precession over long periods.

The other systems describe locations on or relative to Earth or the local sky. A Terrestrial Coordinate System locates points on Earth with latitude and longitude (and height). The Geodetic Coordinate System is tied to Earth's ellipsoid model. The Horizon Coordinate System uses altitude and azimuth relative to the observer’s local horizon and changes with the observer’s position and time.