Meridian

A meridian is half of a great circle running pole to pole, defining a single longitude; the prime meridian at Greenwich is 0 degrees longitude and the reference for time zones and Greenwich Mean Time. A vessel’s local meridian passage of the sun (local apparent noon) is the moment the sun bears due north or south and reaches its greatest altitude, the basis of the noon latitude sight. Meridians converge at the poles, which is why a minute of longitude shrinks with latitude.