Nautical Mile

The nautical mile (NM or nmi) is the fundamental unit of distance in maritime and aeronautical navigation. Historically defined as the arc length corresponding to one minute (1/60th of a degree) of latitude along a meridian, it directly ties geographic coordinates to physical distance on sea charts. Today, by international standard adopted in 1929, one international nautical mile is defined as exactly 1,852 meters (approximately 6,076.1 feet or 1.151 statute miles).

Because latitude scales on Mercator projection paper charts represent arc minutes of latitude, sailors can directly measure distances in nautical miles using a pair of dividers against the latitude scale on the side of the chart. One minute of latitude equals one nautical mile.

The derived unit for speed at sea is the knot (kn), defined as one nautical mile per hour. For instance, a yacht sailing at 6 knots covers 6 nautical miles in one hour. Distance calculations in passage planning, fuel consumption, and collision avoidance rely entirely on nautical miles and knots rather than kilometers or land miles. In classical sailor lore, milestones were celebrated through traditions such as swallow tattoos, where each swallow traditionally represented 5,000 nautical miles sailed. In modern navigation, electronic systems like GPS and AIS plot coordinates and calculate distances using nautical miles.