Definition
Altitude h is the angular distance of a celestial object above the horizon, measured along the vertical circle through the object. The zenith has h = 90°; the horizon h = 0°. Air mass and refraction increase near the horizon.
Altitude
Angular height of a celestial object above the horizon.
3D illustration
Altitude is the angle above the local horizon — 90° at the zenith.
Use the menu and time controls to explore. Based on Jean Meeus' Astronomical Algorithms.