altazimuth

Altazimuth, or alt-azimuth, is a method for locating objects in the sky that uses two perpendicular axes: altitude (height above the horizon) and azimuth (direction along the horizon). It is used both as a celestial coordinate system for describing positions and as the basis for telescope mounts that track objects as the sky appears to rotate.

In the alt-azimuth coordinate system, altitude is the angle above the local horizon, ranging from 0° at the horizon to 90° at the zenith. Azimuth measures the compass direction along the horizon, commonly 0° = north, 90° = east, 180° = south, and 270° = west, though conventions vary by country.

In practice, an alt-azimuth telescope mount moves on two perpendicular axes: an elevation or altitude axis and an azimuth axis. The design is compact and mechanically simple, making it common in portable telescopes, consumer mounts, and many modern observatories when paired with computerized control.

Tracking objects requires continuous adjustment of both axes to compensate for Earth's rotation. Modern systems use motors and software to perform this tracking, and often derotate the field of view for long exposures in astrophotography to counteract field rotation inherent to the alt-azimuth geometry.

Historically, equatorial mounts were preferred for long-exposure astronomy because they align with Earth's axis and avoid field rotation. The rise of computer-controlled alt-azimuth mounts in the 20th and 21st centuries has made the arrangement common in large telescopes and robotic observatories, offering improved ease of pointing and stability while delegating the rotational compensation to software.