The horizontal coordinate system is a celestial coordinate system that uses the observer's local

horizon The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whethe ...

as the fundamental plane to define two angles of a spherical coordinate system: altitude and ''

azimuth An azimuth (; from ) is the horizontal angle from a cardinal direction, most commonly north, in a local or observer-centric spherical coordinate system. Mathematically, the relative position vector from an observer ( origin) to a point ...

''. Therefore, the horizontal coordinate system is sometimes called the az/el system, 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.

Definition

This celestial coordinate system divides the

sky The sky is an unobstructed view upward from the planetary surface, surface of the Earth. It includes the atmosphere of Earth, atmosphere and outer space. It may also be considered a place between the ground and outer space, thus distinct from ...

into two hemispheres: The upper hemisphere, where objects are above the

and are visible, and the lower hemisphere, where objects are below the horizon and cannot be seen, since the Earth obstructs views of them. 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). 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. The pole of the upper hemisphere is called the '' zenith'' and the pole of the lower hemisphere is called the '' nadir''. The following are two independent horizontal angular coordinates: * Altitude (alt. or altitude angle), sometimes referred to as ' (el. or ''elevation angle'') or , is the angle between the object and the observer's local horizon. For visible objects, it is an angle between 0° and 90°. *

Azimuth An azimuth (; from ) is the horizontal angle from a cardinal direction, most commonly north, in a local or observer-centric spherical coordinate system. Mathematically, the relative position vector from an observer ( origin) to a point ...

(az.) is the angle of the object around the horizon, usually measured from true north and increasing eastward. Exceptions are, for example, 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 system.

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, the same object viewed from different locations on Earth at the same time will have different values of altitude and azimuth. The cardinal points on the horizon have specific values of azimuth that are helpful references. 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. 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: * 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 never rises or sets. The Sun,

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

, and planets can rise or set over the span of a year when viewed from the poles because their

declination In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. The declination angle is measured north (positive) or ...

s are constantly changing. * When viewed from the

equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...

Definition

General features

See also

Footnotes

References

External links