Horizontal coordinate system

A local sky-coordinate system that locates objects by altitude above the horizon and azimuth around it. Its coordinates depend on the observer’s location and time.

Altitude (astronomy): The angular height of a celestial object above the horizon, measured from 0° to 90°. It supplies the vertical coordinate in this system.
Diurnal motion: The apparent daily movement of celestial objects across the sky, caused chiefly by Earth’s rotation. It makes an object’s altitude and azimuth vary over the course of a night.
Altazimuth mount: A telescope mount that moves in altitude and azimuth axes. Its mechanical axes directly follow the system’s two coordinates.
Equatorial coordinate system: A celestial coordinate system that locates objects by right ascension and declination relative to the celestial equator. Its coordinates are tied to the celestial sphere rather than the observer’s horizon.
Azimuth: The horizontal angle of a direction, measured from a reference direction around the horizon. It supplies the directional coordinate around the horizon.
Meridian (astronomy): The great circle passing through the celestial poles and an observer’s zenith. An object crossing this local arc reaches its greatest altitude for that day.
Theodolite: An instrument that measures horizontal and vertical angles for surveying and alignment. Its angle readings can establish a celestial object’s azimuth and altitude.
Ecliptic coordinate system: A celestial coordinate system based on the ecliptic plane and its pole. It suits planetary and solar-system positions where horizon coordinates change continuously.
Horizon: The apparent boundary between the sky and the ground or sea, whose position depends on the observer. It defines altitude zero and the plane in which azimuth is measured.
Transit (astronomy): The passage of a celestial object across an observer’s meridian. At upper transit, an object’s altitude is at its daily maximum.

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