# Definition:Nautical Mile

## Definition

The nautical mile is a unit of length which is used for air and sea navigation.

It was originally defined as the length measured along Earth's surface of one minute of latitude along a meridian.

It is now defined as exactly $1852$ metres.

 $\displaystyle$  $\displaystyle 1$ nautical mile $\displaystyle$ $=$ $\displaystyle 1852$ metres (by definition) $\displaystyle$ $=$ $\displaystyle 10$ cables $\displaystyle$ $\approx$ $\displaystyle 6076$ feet $\displaystyle$ $\approx$ $\displaystyle 1 \cdotp 151$ (international) mile $\displaystyle$ $\approx$ $\displaystyle 0 \cdotp 999 \, 363$ admiralty (UK nautical) mile

## Also defined as

Before $1929$, the nautical mile was defined as either:

$6080$ feet (what is now known as the British admiralty mile)
$6080 \cdotp 2$ feet (the American nautical mile)

Older textbooks may still refer to either of these now obsolete units as the nautical mile.

## Also known as

Some sources refer to this unit as the international nautical mile.

## Historical Note

The concept of defining base units of measurement based on the dimensions of Earth appears to have been introduced by Edmund Gunter.

In the early $1600$s he proposed the nautical mile as one minute of arc, or $\dfrac 1 {60}$ of a degree of latitude on Earth's surface.

However, as Earth is not perfectly spherical, but an oblate spheroid, one minute of latitude is not constant.

At the equator it is about $1843$ metres, but at the poles it is about $1861$ metres.

The French standard evolved such that a nautical mile is one minute of arc of a meridian at a latitude of $45 \degrees$, which was a measure of convenience, as it happened to match a more mundane measurement made a century earlier.

By the middle of the $19$th century, France had defined the nautical mile in terms of the metre that had been defined in $1791$.

This worked out at $1851 \cdotp 85 \mathrm m$, which was rounded to $1852 \mathrm m$.

This then became the French legal standard in $1906$, and was then sanctioned by many other countries using the metric system in $1929$, at the first International Hydrographic Conference in Monaco.

The USA and UK both used their own measurement, based on the average minute of arc of latitude defined on a sphere with the authalic radius of Earth.

This minute of arc of latitude measures $1853 \cdotp 2480$ metres or $6080 \cdotp 210$ feet.

The USA took the first $5$ significant figures, giving their nautical mile a measurement of $6080 \cdotp 2$ feet.

The UK took the first $4$ significant figures, giving their nautical mile a measurement of $6080$ feet, which is now referred to as the admiralty mile.

The USA finally adopted the international nautical mile of $1682 \, \mathrm m$, while the UK did not adopt it until $1970$.

Legacy measurements in admiralty miles are for convenience considered as being $1683 \, \mathrm m$.