To ascertain the azimuth, we must first choose a point of beginning on the horizon. Any of the cardinal points, _i.e._, east, west, north, or south, may be employed for this purpose, but in astronomy it is customary to use only the south point, and to carry the measure westward all round the circle of the horizon, and so back to the point of beginning in the south. This involves circular, or degree, measure, to which a few words must now be devoted.