Nautical Mile vs Mile

A nautical mile was defined as one minute of arc of latitude along a meridian. A degree of latitude is 60 minutes, and the Earth has 180 degrees of latitude from pole to pole, so the unit is built into the coordinate system rather than into any object.

That gives navigators something no statute mile can. A position two degrees of latitude away is exactly 120 nautical miles away, readable straight off the chart's side scale without arithmetic. Because the Earth is not a perfect sphere the true length of a minute of latitude varies slightly with position, so in 1929 the international nautical mile was fixed at exactly 1,852 metres.

A knot is one nautical mile per hour, which makes it 1.852 km/h or 1.15078 mph. Its name is literal: speed used to be measured by paying out a knotted line behind a moving ship and counting the knots that ran out in a fixed interval timed by a sandglass.

Because the nautical mile is tied to latitude, a speed in knots converts directly into how fast your latitude is changing. That relationship is the whole reason the unit survived the metric transition in both marine and air navigation.

Aviation and maritime charts, air traffic control, weather warnings for wind at sea, and territorial waters all use nautical miles. A country's territorial sea extends 12 nautical miles from its coast, which is 13.8 statute miles.

Roads, running events and everyday distances use the statute mile. The one place they overlap and confuse people is coastal weather forecasting, where wind speeds are given in knots but distances inland are in statute miles.