Nautical Miles and Knots: Why Ships and Aircraft Use Their Own Distance Unit
A nautical mile is not an arbitrary imperial survival. It is one minute of latitude, which makes navigation arithmetic work on a sphere.
Aviation and marine navigation use nautical miles and knots while the rest of the world uses kilometres or statute miles. This is not stubbornness. The nautical mile is tied directly to the geometry of the Earth, and that makes chart work dramatically simpler.
One minute of latitude
A circle has 360 degrees, each divided into 60 minutes, giving 21,600 minutes. Take a great circle around the Earth and divide its circumference by 21,600, and you get the nautical mile.
It is defined as exactly 1,852 metres, fixed by international agreement in 1929. That equals about 1.15078 statute miles or 6,076 feet.
This is why the unit survives. On a paper chart in a moving vessel, that property is worth more than metric tidiness.
Why longitude does not work the same way
Meridians of longitude converge at the poles. One minute of longitude equals one nautical mile only at the equator, shrinking to nothing at the poles — it scales with the cosine of latitude.
At 60 ° north, one minute of longitude is about half a nautical mile. This is why navigators are trained to measure distance only from the latitude scale on the side of the chart, never from the longitude scale along the top.
The knot
One knot is one nautical mile per hour, about 1.852 km/h or 0.514 m/s.
The name comes from the practice of measuring speed with a chip log: a weighted board on a line knotted at regular intervals, thrown astern and allowed to run out for a timed interval measured by sandglass. Counting the knots that passed gave the speed directly. The unit is named after the physical knots in the rope.
"Knots per hour" is wrong — the hour is already inside the unit.
Why aviation kept it
Air navigation inherited marine practice and had the same reason to keep it: aircraft navigate on a spherical Earth using latitude and longitude.
ICAO standardised knots for speed and feet for altitude internationally, with nautical miles for distance. There have been metrication attempts; Russia and China used metric altimetry for decades, though both have largely moved to the ICAO convention for international traffic.
The safety argument is strong: mixed units in aviation have killed people. Uniform convention matters more than which convention.
Related units
- Cable — one tenth of a nautical mile, about 185 m. Used for short distances in pilotage.
- Fathom — 6 feet, 1.8288 m. Traditional depth measure, originally the span of a man's outstretched arms.
- League — three nautical miles. Largely historical.
- Mach number — speed relative to the local speed of sound, which varies with air temperature. Above roughly 36,000 feet, aircraft switch from indicated airspeed to Mach for exactly this reason.
Speed conventions differ by domain
Marine practice distinguishes speed through water from speed over ground. A vessel making 10 knots through water against a 2-knot current makes 8 knots over the ground. Currents change passage planning substantially.
Aviation has a richer set: indicated airspeed, true airspeed, and groundspeed. Indicated airspeed is what the instrument shows, and it matters for handling because it reflects dynamic pressure. True airspeed corrects for air density and rises with altitude. Groundspeed adds wind, and is what determines arrival time.
A jet indicating 280 knots at 35,000 feet may have a true airspeed near 480 knots, and with a 100-knot tailwind a groundspeed of 580 knots. All three are correct measurements of different things.
Our speed converter includes knots, Mach and the usual metric and imperial units.