Horizontal coordinate system - a simple definition<BR>
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Its reference plane is the tangent plane of the Earth passing through the 
observer; this horizontal plane intersects celestial sphere along the 
horizon.<BR>
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One of the coordinates is azimuth, A; it is the angular distance of the 
vertical of the object from some fixed direction. Although the practice 
varies, in this program azimuth is measured counterclockwise from north 
(i.e. west = 90 deg). The values are between 0 and 360 degrees, given 
in format (degrees minutes seconds).<BR>
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The second coordinate is altitude, a, which is measured from the horizon 
along the vertical passing through the object. Its values are between -90 and 
+90 degrees, given in format (degrees minutes seconds). Values are positive 
for objects above the horizon and negative for objects below the horizon.<BR>
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