Alt-Azimuth Instrument. One that measures altitude and azimuth simultaneously—such as a theodolite.
Alternating Current. Electric current with rapidly alternating
positive and negative polarities.
Alternating Light. Navigational beacon light that changes colour in each period of its visibility.
Altimeter. Aneroid barometer graduated to show height instead of pressure.
Altitude. Angular distance of a heavenly body above the horizon. Linear distance above sea level or other datum.
Altitude a Double.* Obsolete term for a pair of altitudes taken to determine latitude.
Altitude Azimuth. Usually shortened to 'Alt-azimuth'. Applied to problems, methods, tables and instruments in which these two co-ordinates are inter-dependent.
Altitude Circle. Great circle of celestial sphere that passes through zenith and so cuts horizon at right angles. Also called 'Vertical Circle' or 'Circle of Altitude'.
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