Meridian altitude

Meridian altitude is the altitude of a body at the instant it crosses the observer’s meridian, when it reaches its highest point and bears due north or south. It gives latitude by a one-line reduction with no hour angle needed: the zenith distance (90 degrees minus the corrected altitude) combined with the body’s declination yields latitude directly, adding or subtracting by name (same or contrary). The noon Sun sight is the classic case, and the Sun’s slow change of altitude near meridian passage gives the navigator several minutes to catch the maximum. Ex-meridian tables handle a sight taken slightly off the meridian.