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.

References in periodicals archive ?
Since his scope moves in altitude and azimuth rather than equatorially, the setting circles are calibrated in altitude and azimuth, too.
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.
His primary finding was a precise determinationof the altitude and azimuth of a knoll on the southeast horizon.
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.
I use a program called SatSpy, which prepares tables listing a satellite's altitude and azimuth at 1-minute intervals," John explains.
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.
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.
From the length and direction of the shadow, we could determine the altitude and azimuth of the Sun.
The key to the flex rocker is that the altitude and azimuth bearing points lie on top of each other so that the telescope's weight is transmitted through these points directly to the ground via the base ring.
Using trigonometry to make small corrections that adjusted our observations of altitude and azimuth, we found that van Gogh would have seen the outcropping near azimuth 126[degrees], that is, about 36[degrees] south of due east, and extending between 4 1/2[degrees] and 4 3/4[degrees] in altitude.
You can move the stylus across the screen (the hand-held equivalent of clicking and dragging the mouse in a desktop program), enter coordinates of right ascension and declination or altitude and azimuth, tap controls in the lower-right corner of the screen to move incrementally in altitude or azimuth, display a pop-up menu with a choice of altitude or azimuth settings, or even enter shorthand characters in Palm's Graffiti alphabet.