The horizontal coordinate system is
a celestial coordinate system that uses the observer's local horizon as
the fundamental plane to define two angles: altitude and azimuth.
Therefore, the horizontal coordinate system is sometimes called as 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.
This celestial
coordinate system divides the sky into two 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. 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. In practice, the horizon can be defined as
the plane tangent to a quiet, liquid surface, such as a pool of mercury.
The pole of the upper hemisphere is called the zenith. The pole of the
lower hemisphere is called the nadir.
The
following are two independent horizontal angular coordinates:
·
Altitude
(alt.), sometimes referred to as elevation (el.) or 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°.
·
Azimuth (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.
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,[a] 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.
|
Azimuth values for the cardinal directions |
|
|
Cardinal point |
Azimuth |
|
North |
0° |
|
East |
90° |
|
South |
180° |
|
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. 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, and planets can rise or set
over the span of a year when viewed from the poles because their declinations are
constantly changing.
·
When
viewed from the equator, objects on the celestial poles stay at fixed
points, perched on the horizon.
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