The horizonal coordinate system depends on the location of the observer and the time of the observation. The altitude measures the angle of the object above or below the observer’s horizon.

Along with the azimuth, the ‘altitude’ of an object is used to define its position on the celestial sphere in the horizontal coordinate system. Specifically, it is a measure of the angular distance of an object above or below the observer’s horizon.

It ranges from 0o for objects located on the horizon, to 90o at the zenith. Objects below the horizon have negative altitudes, and an object located at the nadir (anti-zenith) has an altitude of -90o.

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