In the horizon coordinate system, which two quantities define the position of a celestial object?

• A. Altitude, Azimuth
• B. Right Ascension, Declination
• C. Local Hour Angle, Azimuth
• D. Local Hour Angle, Declination

Answer: (A)

Position in the horizon coordinate system is described by how high the object is above the horizon and which direction along the horizon it lies. Altitude (or elevation) tells how far up from the horizon the object appears, from 0° on the horizon to 90° at the zenith. Azimuth gives the compass direction around the horizon, typically measured from a reference like north toward the east. Together, these two values pin down the object’s exact location in the observer’s sky at that moment.

Right Ascension and Declination locate objects on the celestial sphere relative to the equator and the vernal equinox, not with respect to the local horizon, so they aren’t the horizon-based pair.

Local Hour Angle relates to time and the observer’s longitude and, by itself, doesn’t specify altitude; pairing it with azimuth isn’t the standard way to define horizon position. Similarly, combining Local Hour Angle with Declination still doesn’t give the horizon coordinates directly.

So the two quantities that define position in the horizon coordinate system are Altitude and Azimuth.