horizontal coordinate system

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horizontal (or horizon) coordinate systemclick for a larger image
horizontal (or horizon) coordinate system

horizontal (or horizon) coordinate system

A coordinate system in which the fundamental reference circle is the observer's astronomical horizon and the zero point is the north point (see cardinal points). The coordinates are altitude and azimuth (see illustration).

The altitude (h) of a celestial body is its angular distance north (counted positive) or south (counted negative) of the horizon; it is measured along the vertical circle through the body and ranges from 0°, when the object rises or sets, to 90°, when the object is directly overhead at the zenith. The zenith distance (ζ) is the complement of the altitude (90° – h) and is frequently used instead of altitude in the horizontal system. The azimuth (A) of a body is its angular distance measured eastward along the horizon from the north point (or sometimes the south point) to the intersection of the object's vertical circle.

The horizontal system is simple but is strictly a local system. At any given moment a celestial body will have a unique altitude and azimuth for a particular observation point, the coordinates changing with observer's position. The coordinates of a star, etc., also change with time as the Earth rotates and the observer's zenith moves eastward among the stars.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006
References in periodicals archive ?
The user defines the rectangular coordinates (pixel locations) at the end points of three selected stars and enters them into a spreadsheet that calculates what, if any, altitude and azimuth adjustments to the mount's polar axis are necessary to align it with the celestial pole.
However, azimuth instruments (in the sense of the instruments specifically used in the simultaneous measurements of altitude and azimuth) were apparently not so novel.
Observations and images, including the date, time, rough altitude and azimuth limits of the clouds, should be sent to the Aurora Section--further details and report forms are available on the Section webpage, http://www.britastro.org/aurora/
His primary finding was a precise determinationof the altitude and azimuth of a knoll on the southeast horizon.
Additional options for the Polarie include an unilluminated polar-alignment scope, ball head, time-lapse adapter, and a Polar Fine Adjustment Unit that's very useful for exact altitude and azimuth adjustments.
Since his scope moves in altitude and azimuth rather than equatorially, the setting circles are calibrated in altitude and azimuth, too.
The mount features 120-mm and 100-mm bronze worm gears in its altitude and azimuth axes for smooth tracking, driven by DC servo drives with optical encoders for precise slewing up to 3.5[degrees] per second.
Using a software package that provides real-time altitude and azimuth coordinates, John also locates difficult deep-sky objects by dialing in their positions.
Precision slow-motion controls on both altitude and azimuth axes let you track objects smoothly even at high magnifications.
Once you get the shimming set correctly, you'll find your scope is much easier to control and has a good friction match between altitude and azimuth. Furniture glides aren't as good as virgin Teflon, but they do work surprisingly well.
The altitude and azimuth axes have friction clutches that can be locked down, and each axis is equipped with a 360[degrees]-rotation, worm-driven slow-motion control.
The large locking knobs on the altitude and azimuth axes are, however, easy to engage and disengage even when you are wearing the thickest of gloves--a nice touch.