Celestial Coordinates - Horizon Coordinates

These refer to an observer on the Earth's surface at the center of an imagined celestial sphere.

Altitude h, an object's arc distance along a vertical circle from the horizon, positive for objects above the horizon [between it and the zenith (h=+90°)] and negative for objects below it [between it and the nadir (h=+90°)].

Azimuth A, the arc distance from the north point of the horizon (h=0°, A=0°) eastward along it to where it meets the object's vertical circle, going from 0° to 360° from the north point to full circle back to the North Point. Azimuth is undefined at Earth's north and south poles, where the north and south celestial poles (NCP and SCP) coincide with the zenith and nadir, and the celestial meridian become undefined.