Editing Horizontal coordinate system (section)

{{Short description|Type of celestial coordinate system}}
{{distinguish|Horizontal position representation}}
[[File:Azimuth-Altitude schematic.svg|right|350px|thumb|Horizontal coordinates use a [[celestial sphere]] centered on the observer. [[Azimuth]] is measured [[east]]ward from the [[true north|north]] point (sometimes from the [[south]] point) of the [[horizon]]; altitude is the angle above the horizon.]]

The '''horizontal coordinate system''' is a [[celestial coordinate system]] that uses the observer's local [[horizon]] as the [[fundamental plane (spherical coordinates)|fundamental plane]] to define two angles of a [[spherical coordinate system]]: '''altitude''' and ''[[azimuth]]''.
Therefore, the horizontal coordinate system is sometimes called the '''az/el system''',<ref name=Keck-Az-El/> the '''alt/az system''', or the '''alt-azimuth system''', among others. In an [[altazimuth mount]] of a [[telescope]], the instrument's two axes follow altitude and azimuth.<ref name=Britannica-horizon-system/>