Nautical time communicating with land, the land time zone must be employed. The nautical time zone system is an ideal form of the terrestrial time zone system for use on high seas. Under the system time changes are required for changes of longitude in one-hour steps. The one-hour step corresponds to a time zone width of 15° longitude. The 15° gore that is offset from GMT or UT1 (not UTC) by twelve hours is bisected by the nautical date line into two 7°30′ gores that differ from GMT by ±12 hours. A nautical date line is implied but not explicitly drawn on
Longitude then the longitude at a point would be equal to the angle between a vertical north–south plane through that point and the plane of the Greenwich meridian. Everywhere on Earth the vertical north–south plane would contain the Earth's axis. But the Earth is not radially homogeneous and has rugged terrain, which affect gravity and so can shift the vertical plane away from the Earth's axis. The vertical north–south plane still intersects the plane of the Greenwich meridian at some angle; that angle is the astronomical longitude, calculated from star observations. The longitude shown on maps and GPS devices is the