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.Ī horizontal coordinate system should not be confused with a topocentric coordinate system. Azimuth (az.) is the angle of the object around the horizon, usually measured from true north and increasing eastward.For visible objects, it is an angle between 0° and 90°. Altitude (alt.), sometimes referred to as elevation (el.) or apparent height, is the angle between the object and the observer's local horizon.The following are two independent horizontal angular coordinates: The pole of the lower hemisphere is called the nadir.
The pole of the upper hemisphere is called the zenith. In practice, the horizon can be defined as the plane tangent to a quiet, liquid surface, such as a pool of mercury. 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. 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. In an altazimuth mount of a telescope, the instrument's two axes follow altitude and azimuth. Therefore, the horizontal coordinate system is sometimes called the az/el system, the alt/az system, or the alt-azimuth system, among others. 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. Azimuth is measured eastward from the north point (sometimes from the south point) of the horizon altitude is the angle above the horizon. Horizontal coordinates use a celestial sphere centered on the observer.
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