### horizontal coordinate system

**(az/el system, Alt/Az system, altazimuth system)**
(celestial coordinate system based upon horizon and north)

The **horizontal coordinate system**
(**az/el system**, **alt/az system**, **altazimuth system**) is a
spherical coordinate system used for directions in the
celestial sphere,
fixed to the horizon and north. It is directly related to observation
from Earth, but time-dependent: the rotation of Earth
produces a constant minute-by-minute change in the whole sky,
called the **diurnal motion**. Other coordinate systems
avoid this time-dependence, e.g., for specifying locations of stars,
but to relate an observation to one of them, the position must be
converted between the other system and this one. This system is
straight-forward for a telescope with an altazimuth mount,
and modern research telescopes use computer control to handle the
aiming and conversion.

Elements of the horizontal coordinate system:

**altitude** - angle from the horizon, 90° being straight up, i.e., toward the zenith.
**azimuth** - angle eastward from north.
**zenith distance** - compliment of altitude, which can be used instead.

Some terms based upon these, describing the position of
astronomical objects in the sky:

**rise time** - time at which an object rises over the horizon.
**rise azimuth** - an object's *azimuth* at its *rise time*.
**morning width** - an object's *rise azimuth* angle measured northward from east rather than eastward from north.

(*coordinate system,horizon,celestial sphere,Earth*)
**Further reading:**

http://en.wikipedia.org/wiki/Horizontal_coordinate_system

**Referenced by pages:**

celestial coordinate system

ephemeris

zenith

Index