Altitude (astronomy)

Altitude is the angle between a celestial object and an observer’s horizon, measured upward from 0° at the horizon to 90° at the zenith.

Horizon coordinate system: A local astronomical coordinate system that locates objects by altitude and azimuth relative to an observer’s horizon. Altitude is its vertical coordinate, paired with azimuth.
Diurnal motion: The apparent daily movement of celestial objects across the sky, caused mainly by Earth's rotation. It makes most objects' altitude rise and fall during the night.
Airmass: A measure of the amount of atmosphere light traverses on its way to an observer. Low altitude generally means greater airmass and stronger atmospheric effects.
Declination: The angular distance of a celestial object north or south of the celestial equator. Unlike altitude, declination does not depend on the observer's local horizon.
Azimuth: The horizontal direction of an object, measured around the horizon from a reference direction. Together with altitude, it specifies an object’s direction in the local sky.
Meridian (astronomy): The great circle through the celestial poles and an observer’s zenith. An object reaches its daily maximum altitude when it crosses the local meridian.
Altitude-azimuth mount: A telescope mount that points by rotating around vertical and horizontal axes. Its horizontal axis changes telescope altitude directly.
Elevation: The vertical height of a place or object above a specified reference level, often mean sea level. Astronomical altitude is an angle, not a physical height.
Zenith: The point in the sky directly above an observer. It marks the 90° endpoint of altitude measurements.
Circumpolar star: A star that never sets below the horizon for a particular observer. Its altitude varies through the night but remains above zero.

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