Nautical Miles
Abbreviation/Symbol:
nmi
Unit of:
Worldwide use:
A nautical mile is one minute of latitude, which the international definition fixes at exactly 1,852m. Ships and aircraft both measure distance in them.
Nautical miles are the unit on sea charts and aviation instruments worldwide, and speed in both fields is given in knots, ie nautical miles per hour. A ship at 20 knots covers 20 nautical miles an hour, so 'knots per hour' is a mistake.
Definition:
A nautical mile is a unit of measurement used primarily in navigation and maritime contexts. It is defined as one minute of latitude along any meridian of the Earth. This means that a nautical mile is equal to one-sixtieth of a degree of latitude. The reason for using nautical miles instead of regular miles or kilometers in maritime navigation is that it provides a more accurate representation of distances on the Earth's surface.
Origin:
A nautical mile is one minute of latitude, one sixtieth of a degree along a meridian, which is why it works so neatly on charts. With a circumference of 40,000 kilometers that comes to 1.852 kilometers (the meter was originally defined from the Earth's size, so the circumference is almost exactly 40,000). 1,852 meters is now the fixed international value – the US adopted it in 1954 and the UK in 1970.
Common references:
Usage context:
A nautical mile is exactly 1,852 meters, about 1.15 statute miles, and was set internationally in 1929. It is close to one minute of latitude, which is how it began.
Navigators measure distance against the latitude scale at the side of a chart, where 1 minute is about 1 nautical mile and 1 degree about 60. That is why ships and aircraft work in nautical miles rather than kilometers or statute miles.
A knot is one nautical mile per hour, which is why ship and aircraft speeds are quoted in knots ("knots per hour" is a common mistake). Weather reports and marine charts also give distances to ports and hazards in nautical miles.