Nautical mile
Unit of length for air, marine, and space navigation.
The nautical mile is a unit of length used in air, marine, and space navigation, and for the definition of territorial waters. Historically defined as the meridian arc length corresponding to one minute of latitude at the equator, it remains in common use globally due to its correspondence with geographic coordinates, though it is not part of the International System of Units (SI).
- type
- Unit of length
- derived_unit
- Knot (one nautical mile per hour)
- historical_basis
- One minute (1/60 of a degree) of latitude at the equator
- symbols
- NM (ICAO), nmi (IEEE, US GPO), M (IHO), nm (non-standard)
Lore & Background
The word mile derives from the Latin phrase for a thousand paces: mille passus. An earlier manuscript by Germanus stated 62½ miles to a degree, but the reason for the change is not explained. By the late 16th century, English geographers and navigators knew that the ratio of distances at sea to degrees was constant along any great circle, assuming a spherical Earth.
Reader's Guide
The nautical mile's significance lies in its direct link to Earth's geography, enabling navigators to translate angular measurements (degrees and minutes of latitude) into linear distances. This correspondence made it indispensable for celestial navigation and charting long before modern electronic systems. Despite not being an SI unit, it remains globally prevalent in air, marine, and space contexts, with no single internationally agreed symbol.
Did You Know?
- A Martian nautical mile, using 1/60 of a degree of latitude on Mars, equals 983 metres.
- The symbol 'nm' is a non-standard abbreviation for nautical mile that conflicts with the SI symbol for nanometre.
Frequently Asked Questions
Who is Nautical mile?
Nautical mile is a unit of length that serves as the standard measuring stick for air, marine, and space navigation. It also underpins the legal definition of territorial waters in international maritime law.
What are Nautical mile's powers/role?
Its core ability is tying distance directly to geographic coordinates—one nautical mile corresponds to one minute of latitude along a meridian. This makes it the natural companion to the knot (one nautical mile per hour) and keeps pilots, sailors, and spacecraft operators aligned with the Earth's coordinate grid.
How does Nautical mile's story end?
Nautical mile never fully retired; it simply stepped outside the SI family and settled into a permanent non-SI role. It remains legally and operationally essential in navigation and maritime law despite not being an International System of Units member.
More in Units & Measurement 1-24
Elsewhere in the Units & Measurement universe
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record