In the horizon coordinate system, which pair of values defines the position of a celestial object?

• A. Latitude, Longitude
• B. Right Ascension, Declination
• C. Altitude, Azimuth
• D. Altitude, Declination

Answer: (C)

In the horizon coordinate system, where you’re measuring position relative to your local sky, the object’s location is given by how high it is above the horizon (altitude) and where it lies around the horizon (azimuth). Altitude is the angle from the horizon up to the object, ranging from 0° at the horizon to 90° at the zenith, while azimuth is the compass-direction around the horizon, typically measured from a reference like north. These two angles move with your observing position and time, since the sky shifts as you move and as Earth rotates. The other pairings don’t describe horizon coordinates: geographic latitude and longitude locate positions on Earth, right ascension and declination define positions on the celestial sphere relative to the celestial equator, and combining altitude with declination mixes horizon and celestial coordinates in an inconsistent way.