Editing Horizontal coordinate system (section)

==General features==
The horizontal coordinate system is fixed to a location on Earth, not the stars. Therefore, the altitude and azimuth of an object in the sky changes with time, as the object appears to drift across the sky with [[Earth's rotation]]. In addition, since the horizontal system is defined by the observer's local horizon,{{efn|name=geometric_horizon_footnote}} the same object viewed from different locations on Earth at the same time will have different values of altitude and azimuth.

The [[Cardinal direction|cardinal points]] on the horizon have specific values of azimuth that are helpful references.
{| class="wikitable" style="text-align:center"
|+Azimuth values for the cardinal directions
!scope="col"| Cardinal point
!scope="col"| Azimuth
|-
!scope="row"| North
| 0°
|-
!scope="row"| East
| 90°
|-
!scope="row"| South
| 180°
|-
!scope="row"| West
| 270°
|}

Horizontal coordinates are very useful for determining the rise and set times of an object in the sky. When an object's altitude is 0°, it is on the horizon.{{efn|name=geometric_horizon_footnote}} If at that moment its altitude is increasing, it is rising, but if its altitude is decreasing, it is setting. However, all objects on the [[celestial sphere]] are subject to [[diurnal motion]], which always appears to be westward.

A northern observer can determine whether altitude is increasing or decreasing by instead considering the azimuth of the celestial object:
* If the azimuth is between 0° and 180° (north–east–south), the object is rising.
* If the azimuth is between 180° and 360° (south–west–north), the object is setting.

There are the following special cases:{{efn|name=geometric_horizon_footnote|Note that the special conditions described are strictly true only regarding the ''geometric'' horizon. That is, the horizon as it ''would'' appear for an observer on a perfectly smooth Earth without an atmosphere, for an observer at [[sea-level]]. In practice, the ''apparent'' horizon has a slight negative altitude due to the curvature of Earth, the value of which gets more negative as the observer ascends higher above [[sea level]]. In addition, [[atmospheric refraction]] causes celestial objects very close to the horizon to appear about half a degree [[looming and similar refraction phenomena|higher]] than they would if there were no atmosphere.}}

* All directions are south when viewed from the [[North Pole]], and all directions are north when viewed from the [[South Pole]], so the azimuth is undefined in both locations. When viewed from either pole, a star (or any object with fixed [[equatorial coordinates]]) has constant altitude and thus [[circumpolar star|never rises or sets]]. The [[Sun]], [[Moon]], and [[planets]] can rise or set over the span of a year when viewed from the poles because their [[declination]]s are constantly changing.
* When viewed from the [[equator]], objects on the celestial poles stay at fixed points, perched on the horizon.