Nautical mile

The nautical mile or nautical mile is a unit of length used in sea and air navigation. Today, the international definition, adopted in 1929, is the conventional value of 1,852 m, which is roughly the length of a 1' arc. (one minute of arc, one sixtieth of a sexagesimal degree) of terrestrial latitude. It has been adopted, with very slight variations, by all Western countries. This unit of length does not belong to the International System of Units (SI).

A nautical mile is approximately the length of a 1' meridian arch of the Earth's globe.

The nautical mile is derived from the length on the earth's surface of one arc minute of latitude. Sixty nautical miles of latitude is then equivalent to a latitude difference of one degree. From this derives the use of the nautical mile in navigation. For distances less than one nautical mile, the usual in the nautical world is to use tenths of a nautical mile.

The measure of speed used in the nautical field, the knot, is also derived from the nautical mile. A knot is a speed equal to one nautical mile per hour.

There is no single universally accepted symbol. SI gives preference to M, but mn, nmi, NM and nm (from English: nautical mile). Not to be confused with the statute, statutory or English mile, which is still used in some Anglo-Saxon countries and is equivalent to 1609,344 m.

Use

The nautical mile and the knot are practically the only measures of distance and speed used in maritime and air navigation, as they simplify the observer's position calculations. This position is measured by means of the geographic coordinates of latitude (North or South) and longitude (East or West) from the equator and a reference meridian, using sexagesimal degrees. The navigator's problem is to know the position in degrees and minutes of latitude and longitude after having traveled a certain distance, or vice versa (knowing the current coordinates and the destination point, calculate the distance at which it is located).

By using spherical trigonometry it is easy to determine the spherical distance between two points given their latitude and longitude coordinates. This spherical distance is obtained in sexagesimal degrees so that reducing the angle to sexagesimal minutes gives directly the distance in nautical miles.

Point A: Latitude = 44° 36' 0" No; Longitude = 4° 55' 10" W.
Point B: Latitude = 40° 10' 20" No; Longitude = 12° 10' 0" E.
Spherical distance between A and B = 12° 46' 05" = 766' = 766 NM (nautical miles).

The direct obtaining of the spherical distance expressed in sexagesimal minutes explains the success of this distance measure since its initial use in maritime navigation, normally long distances and on the surface of the terrestrial globe. Obviously the simplification of considering the spherical terrestrial globe is used (it really is an ellipsoid).

Charts and directions —also many terrestrial maps in Mercator projection— allow you to know the coordinates of lighthouses, capes, islands, reference points (landmarks), etc., and even measure spherical distances with a compass (about the meridians). Before the current era, which has seen the popularization of global positioning systems by satellite (GPS, GLONASS, Galileo), in the open sea the sextant was used to measure the observed height of the stars with respect to the horizon, which allowed determining the latitude of the place. The measurement of longitude required the use of chronometers and the observation of the sun. Accuracy in determining latitude and longitude has been significantly increased by using global positioning satellite systems.

Historical aspects

In 1929, the First Extraordinary International Hydrographic Conference, meeting in Monaco, set the value of the nautical mile at exactly 1,852 meters. (a rounded value very close to the average length of a one-minute meridian arc, which is 1,851.85 meters). Countries were adopting this convention, for example, the United States in 1954 and the United Kingdom in 1970.

When the International System of Units was created in 1960, the International Bureau of Weights and Measures discouraged the use of the nautical mile, but in 1982 the United Nations Convention on the Law of the Sea, meeting in Montego Bay (Jamaica)), adopted the nautical mile as a unit of distance to define certain types of maritime space (for example, territorial waters, contiguous zone, exclusive economic zone, and continental shelf). This Montego Bay Convention is now a reference in defining the law of the sea. As a consequence, it has contributed to perpetuate the use of the nautical mile.

In 2019, the International Bureau of Weights and Measures stopped recognizing the use of the nautical mile. However, it is still legal in some countries.

Notes