Editing Horizontal coordinate system (section)

==Definition==
This [[celestial coordinate system]] divides the [[sky]] into two [[Sphere#Hemisphere|hemispheres]]: The upper hemisphere, where objects are above the [[horizon]] and are visible, and the lower hemisphere, where objects are below the horizon and cannot be seen, since the Earth obstructs views of them.{{efn|name=geometric_horizon_footnote}} The [[great circle]] separating the hemispheres is called the '''''celestial horizon''''', which is defined as the great circle on the [[celestial sphere]] whose plane is normal to the local [[gravity vector]] (the [[vertical direction]]).<ref name=Clarke-Roy-2003/>{{efn|name=geometric_horizon_footnote}} In practice, the horizon can be defined as the plane [[tangent]] to a quiet, liquid surface, such as a pool of [[mercury (element)|mercury]], or by using a [[bull's eye level]].<ref name=Young-Kattawar-Parviainen-1997/> The pole of the upper hemisphere is called the ''[[zenith]]'' and the pole of the lower hemisphere is called the ''[[nadir]]''.<ref name=Schombert-Earth-co-ords/>

The following are two independent horizontal [[angular coordinate]]s:
* Altitude (alt. or altitude angle{{efn|Not to be confused with linear [[altitude]], in units of length.}}), sometimes referred to as ''{{visible anchor|elevation}}'' (el. or ''elevation angle''{{efn|Not to be confused with linear [[elevation]], in units of length.}}) or {{visible anchor|apparent height}}, is the angle between the object and the observer's local horizon. For visible objects, it is an angle between 0° and 90°.{{efn|Alternatively, [[zenith angle]] may be used instead of altitude, as they are [[complementary angles]] (the sum of the altitude angle and the zenith angle is 90°).}}
* [[Azimuth]] (az.) is the angle of the object around the horizon, usually measured from [[true north]] and increasing eastward. Exceptions are, for example, [[European Southern Observatory|ESO]]'s [[FITS]] convention where it is measured from the south and increasing westward, or the [[FITS]] convention of the [[Sloan Digital Sky Survey]] where it is measured from the south and increasing eastward.

A horizontal coordinate system should not be confused with a ''[[topocentric coordinate system]]''. Horizontal coordinates define the observer's orientation, but not location of the origin, while topocentric coordinates define the origin location, on the Earth's surface, in contrast to a [[Geocentric Celestial Reference System|geocentric celestial system]].