Equatorial mount

A telescope mount with one axis parallel to Earth’s rotational axis. Rotating around that axis lets a telescope track the apparent motion of stars across the sky.

Right ascension: The celestial coordinate analogous to terrestrial longitude, measured eastward along the celestial equator. The mount’s polar axis rotates in right ascension to compensate for Earth’s rotation.
Celestial sphere: An imaginary sphere centered on an observer, used to map the apparent positions of celestial objects. The mount’s axes are arranged to move a telescope across this coordinate framework.
Altazimuth mount: A telescope mount that moves in altitude and azimuth, the two directions of the local horizon system. Unlike an equatorial mount, it needs two coordinated motions to track a star.
Astrophotography: The practice of recording astronomical objects using cameras and telescopes. Long exposures benefit from equatorial tracking that keeps stars fixed in the camera frame.
Declination: The celestial coordinate measuring angular distance north or south of the celestial equator. The mount’s perpendicular axis sets a telescope’s declination before tracking begins.
Celestial equator: The projection of Earth’s equator onto the celestial sphere. Right ascension is measured along this circle, perpendicular to the mount’s polar axis.
Dobsonian telescope: A Newtonian reflector on a simple, low-cost altazimuth mount, commonly used for visual astronomy. Its accessible design favors visual observing over the equatorial mount’s tracking geometry.
Telescope tracking: The motion of a telescope mount that follows a celestial object across the apparent sky. Equatorial mounts achieve tracking by rotating primarily around their polar axis.
Sidereal rate: The angular rate at which Earth rotates relative to distant stars, about 15 arcseconds per second. Matching this rate in right ascension keeps a star centered.
Celestial pole: Either of the two points where Earth’s rotational axis meets the celestial sphere. The mount’s aligned axis points toward a celestial pole.

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