Everything about Nautical Miles totally explained
A
nautical mile or
sea mile is a unit of
length. It corresponds approximately to one
minute of
latitude along any
meridian.
It is a non-
SI unit used especially by
navigators in the
shipping and
aviation industries. It is commonly used in
international law and
treaties, especially regarding the limits of
territorial waters. It developed from the
geographical mile.
Definition
The
international standard definition is: 1 nautical mile = exactly. while
M is used by the
BIPM and Canada. For aviation use, the preferred abbreviation of the
ICAO is
NM. The abbreviation
nm, though conflicting with the SI symbol for the
nanometre, is also in widespread use.
Conversions to other units
One nautical mile converts to:
History
The nautical mile was historically defined as a
minute of arc along a
meridian of the Earth, making a meridian exactly 180×60 = historical nautical miles. It can therefore be used for approximate measures on a meridian as change of
latitude on a
nautical chart. The originally intended definition of the metre as 10
-7 of a half-meridian makes the mean historical nautical mile exactly (2)/ = historical metres. Based on the current
IUGG meridian of (standard) metres the mean historical nautical mile is .
The historical definition differs from the length-based standard in that a minute of arc, and hence a nautical mile, isn't a constant length at the surface of the Earth but gradually lengthens with increasing distance from the equator, as a corollary of the Earth's
oblateness, whence the need for "mean" in the preceding sentence. This length equals about at the poles and at the Equator, a variation of one percent.
Other nations had different definitions of the nautical mile. This variety in combination with the complexity of angular measure described above along with the intrinsic uncertainty of geodetically derived units mitigated against the extant definitions in favor of a simple unit of pure length. International agreement was achieved in 1929 when the
International Extraordinary Hydrographic Conference held in
Monaco adopted a definition of one (1) international nautical mile as being equal to
1,852 metres exactly, in excellent agreement (for an integer) with both the above-mentioned values of historical metres and standard metres.
Since the 1929 agreement, all nations have now adopted the international definition. The
United States, formerly using a value of , didn't however adopt this definition until July 1, 1954.
British nautical mile (admiralty)
The British definition of the nautical mile originally related to the length on the surface of the Earth just south of
Great Britain. It wasn't specified according to a calibrated measurement of the Earth, but chosen as exactly 800 feet longer than a
statute mile, namely . For disambiguation, this is sometimes called the "admiralty mile" after the
British Admiralty. The precise definition of the
foot varied slightly around the world until the international
yard, always equal to exactly three feet, was standardized at exactly 0.9144 m in 1959, making the admiralty mile exactly . The Royal
Hydrographic Office of the
United Kingdom converted to the international definition in 1970.
US navy nautical mile (rocketry)
As a simpler approximation, designers of radar systems for ballistic and
cruise missiles for use by the
United States Navy in the 1950s would take as their equivalent of a nautical mile. In the past, some ship-borne
computer systems developed for the
Royal Navy also used the "data mile" of, and the more unusual "foot*", equivalent to about nine inches, defined as (223 mm).
Associated units
The derived unit of
speed is the
knot, defined as one nautical mile per
hour. The term "log" is used to measure the distance a vessel has moved through the water, it can also be used to measure the speed through the water (see
chip log) as the speed and distance are directly related.
The term knot and log originally are derived from the practice of using a "log" tied to a knotted rope as a method of gauging speed of a ship. The log would be thrown into the water and the rope trailed behind the ship. The number of knots that passed off the ship and into the water in a given time would determine the speed in "knots". The present day measurement of knots and log are determined using a mechanical tow, electronic tow, retractable hull-mounted unit,
doppler or
ultrasonics, or
GPS.
Further Information
Get more info on 'Nautical Miles'.
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