Nautical mile Nautical mile A nautical mile (also known as an air mile) is a unit of measurement defined as exactly . Historically, it was defined as one minute of latitude, which is one sixtieth of a degree of latitude – i.e. since a minute of a degree is x = , the Earth's polar circumference is approximately 21,600 nautical miles. It is a non-SI unit which today has a continued use in both air and marine navigation, and for the definition of territorial waters. There is no single internationally agreed symbol. While using M itself, the International Bureau of Weights and
Longitude at sea, thus revolutionising and extending the possibility of safe long distance sea travel. Though the Board of Longitude rewarded John Harrison for his marine chronometer in 1773, chronometers remained very expensive and the lunar distance method continued to be used for decades. Finally, the combination of the availability of marine chronometers and wireless telegraph time signals put an end to the use of lunars in the 20th century. Unlike latitude, which has the equator as a natural starting position, there is no natural starting position for longitude. Therefore, a reference meridian had to be chosen. It was a popular