C, as in the other figures, is the place of the observer and Z is the zenith, but to avoid complication of details the circle of the horizon is not drawn, only the north-and-south line, N C S, being shown. Eq Eq′ is the equator. Ec Ec′ is the ecliptic. P and P′ are the celestial poles. p and p′ are the poles of the ecliptic. Na is the nadir. V is the vernal equinox, and A the autumnal equinox. The circle through s, parallel to the ecliptic, is a latitude circle. The circle p s p' is the ecliptic meridian of the star s. The circle P V P′ A is the equinoctial colure. The circle p V p′ A is the prime ecliptic meridian. The arc of the ecliptic meridian contained between the ecliptic and s measures the star's latitude. The arc of the ecliptic contained between V and the point where the ecliptic meridian p s p′ meets the ecliptic (or the angle V p s) measures the star's longitude east from V, the vernal equinox. ]