Altitude

Altitude, in Astronomy, is the height of a heavenly body above the horizon. It is measured, not by linear distance, but by the angle which a line drawn from the eye to the heavenly body makes with the plane of the horizon, or by the arc of a vertical circle intercepted between the body and the horizon. Altitudes are taken in observatories by means of a telescope attached to a graduated circle (see CIRCLE), which is fixed vertically. The telescope being directed towards the body to be observed, the angle which it makes with the horizon is read off the graduated circle. The altitude thus observed must receive various corrections—the chief being for Parallax (q.v.) and Refraction (q.v.)—in order to get the true altitude. At sea, the altitude is taken by means of a Sextant (q.v.), and then it has further to be corrected for the dip of the visible horizon below the true horizon (see HORIZON). The correct determination of altitudes is of great importance in most of the problems of astronomy and navigation (see LATITUDE and LONGITUDE).—An ALTITUDE AND AZIMUTH INSTRUMENT, called by contraction ALTAZIMUTH (q.v.), consists essentially of a vertical circle with its telescope so arranged as to be capable of being turned round horizontally to any point of the compass. See AZIMUTH.