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Reference.org
Horizontal coordinate system
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<p>The horizontal coordinate system is a <a href="/facts/Celestial_coordinate_system/23HhqvHj">celestial coordinate system</a> that uses the observer's local <a href="/facts/Horizon/LxwxWfC2">horizon</a> as the <a href="/facts/Fundamental_plane_(spherical_coordinates)/n5cWNMq2">fundamental plane</a> to define two angles of a <a href="/facts/Spherical_coordinate_system/5cVnNof3">spherical coordinate system</a>: altitude and <i><a href="/facts/Azimuth/kEyt9nJ3">azimuth</a></i>. Therefore, the horizontal coordinate system is sometimes called the az/el system, the alt/az system, or the alt-azimuth system, among others. In an <a href="/facts/Altazimuth_mount/AhXsnY0f">altazimuth mount</a> of a <a href="/facts/Telescope/BDwYahhR">telescope</a>, the instrument's two axes follow altitude and azimuth. </p>