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
Mercator 1569 world map 12 by using compass roses on the equator and it is only in Legend 10 that he introduces the Organum Directorium and also addresses the inverse problems: given the initial point and the direction and distance of the second find the latitude and longitude of the second. Mercator's construction is simply an evaluation of the rhumb line distance in terms of the latitude difference and the azimuth as If the latitude difference is expressed in arc minutes then the distance is in nautical miles. In later life Mercator commented that the principles of his map had not been understood by