Azimuth and elevation (or altitude):

Reference plan: Perpendicular plan in O to the astronomical vertical of the place. The azimuth is the angle A = (OS, OE') measured from 0° to 360° or from -180° to 180° in retrograde sense. The elevation (or altitude) is angle (OE, OE '), positive if the star is above the horizon, or negative otherwise.

Note: Azimuth is also computed as angle (ON, OE').

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