Collier's New Encyclopedia (1921)/Azimuth
AZIMUTH, the angular distance of a celestial object from the N. or S. point of the horizon (according as it is the N. or S. pole which is elevated), when the object is referred to the horizon by a vertical circle. Or the angle comprised between two vertical planes, one passing through the elevated pole, the other through the object. It is generally reckoned eastward or westward, from the N. or S. point for 180° either way; but Herschel prefers always reckoning it from the points of the horizon most remote from the elevated pole westward, so as to agree in its general direction with the apparent diurnal motion of the stars. Of course, he therefore counts from 0° to 360°. Azimuths, called also vertical circles, are great circles intersecting each other in the zenith and nadir, and cutting the horizon at right angles in all the points thereof. On these are reckoned the altitude of the stars, and of the sun when he is not in the meridian. A magnetical azimuth is an arch of the horizon, contained between the sun's azimuth circle and the magnetical meridian; or it is the apparent distance of the sun from the N. or S. point of the compass.