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°. ] Then another set of circles is drawn perpendicular to the horizon, and all intersecting at the zenith and the nadir. These are called vertical circles, from the fact that they are upright to the horizon. That one of the vertical circles which cuts the horizon at the north-and-south points coincides with the meridian, which we have already described. The vertical circle at right angles to the meridian is called the prime vertical. It cuts the horizon at the east and west points, dividing the visible sky into a northern and a southern half. Like the altitude circles, vertical circles may be drawn anywhere we please so as to pass through a star in any quarter of the sky—but the meridian and the prime vertical are fixed. With the two sets of circles that have just been described, it is possible to indicate accurately the location of any heavenly body, at any particular moment. Its altitude is ascertained by measuring, along the vertical circle passing through it, its distance from the horizon. (Sometimes it is convenient to measure, instead of the altitude of a star, its zenith distance, which is also reckoned on the vertical circle.) 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.