Horizontal coordinate system

[a] 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).

[3][a] In practice, the horizon can be defined as the plane tangent to a quiet, liquid surface, such as a pool of mercury, or by using a bull's eye level.

In addition, since the horizontal system is defined by the observer's local horizon,[a] the same object viewed from different locations on Earth at the same time will have different values of altitude and azimuth.

Horizontal coordinates are very useful for determining the rise and set times of an object in the sky.

A northern observer can determine whether altitude is increasing or decreasing by instead considering the azimuth of the celestial object: There are the following special cases:[a]

Horizontal coordinates use a celestial sphere centered on the observer. Azimuth is measured eastward from the north point (sometimes from the south point) of the horizon ; altitude is the angle above the horizon.