Meridian passage (LAN)

Meridian passage is the instant a body transits the observer’s meridian, reaching its greatest altitude bearing due north or south. For the Sun this is Local Apparent Noon, when apparent solar time reads 1200 and the Sun’s GHA equals the observer’s longitude. The navigator estimates the clock time of meridian passage from the almanac, prepares to observe a few minutes early, and tracks the Sun until its altitude stops rising. The meridian altitude then gives latitude without any hour-angle computation. The time of LAN, compared with GMT, also yields a longitude line, the basis of the older noon-position routine.