The plane of the horizon, with the north, south, east, and west points, and the zenith, are represented as in Fig. 2. P and P′ are the poles of the celestial sphere, the dotted line connecting them representing the direction of the axis, both of celestial sphere and the earth. The circle Eq Eq′ is the equator. V is the vernal equinox, or the point on the equator whence right ascension is reckoned round toward the east. The circle passing through s, parallel to the equator, is a declination circle. The circle P s P′ is the hour circle of the star s. The arc of this hour circle contained between s and the point where it meets the equator is the star's declination. Its right ascension is measured by the arc of the equator contained between V and the point where its hour circle meets the equator, or by the angle V P s. The hour circle P V P′, passing through the vernal equinox, is the equinoctial colure. When this has moved up to coincidence with the meridian, N Z S, it will be astronomical noon. ]