C is the place of the observer. N C S, a north-and-south line drawn in the plane of the horizon. E C W, an east-and-west line in the plane of the horizon. N E S W, the circle of the horizon. Z, the observer's zenith. N Z S, vertically above N C S, the meridian. E Z W, the prime vertical. Z s s′, part of a vertical circle drawn through the star s. The circle through s parallel to the horizon is an altitude circle. The angle s C s′, or the arc s′ s, represents the star's altitude. The angle s C Z, or the arc Z s, is the star's zenith distance. To find the azimuth, the angular distance round the horizon from S (0°), through W, N, E, to the point where the star's vertical circle meets the horizon, is measured. In this case it is 315°. But if we measured it eastward from the south point it would be—45°. ]