Aviation numbers
The nautical mile, and the shortest way round.
A quote from us is priced against the great-circle distance in nautical miles. Neither of those is the number a road map would give you, and both have a reason.
- 1,852 mOne nautical mile — fixed by definition in 1929, not measured.
- 1′ of latitudeWhat the unit was built to be: one minute of arc on a chart.
- A floor, not a planThe shortest track that exists. The one actually flown is longer.
Why a curve is shorter than a straight line
On a flat map the shortest route between two cities looks like the line you would draw with a ruler. On a sphere it is not. Slice a globe through both points and through its centre and the cut traces a circle — a great circle, the largest circle the sphere allows. The shorter of the two arcs between your points is the shortest path across the surface, and that arc is the great-circle distance.
Flattening a globe onto paper has to stretch something, so the true shortest path arrives on most maps looking bent. It is the map that is distorted, not the route. This is why a flight from Türkiye to North America appears to wander north instead of heading straight west.
Where 1,852 metres comes from
The nautical mile was not invented to be tidy in metres. It was built to be tidy on a chart: one nautical mile is one minute of arc along a meridian. Read a latitude off the side of a chart and you have read a distance — sixty minutes to the degree, sixty nautical miles.
Because the Earth is not a perfect sphere, that minute of arc is not the same length everywhere, and nations used slightly different values for decades. In 1929 the International Extraordinary Hydrographic Conference in Monaco settled on a single figure for everyone:
1 nautical mile = 1,852 metres exactly — a definition, not a measurement.
1 NM = 1.852 km · 100 NM = 185.2 km · 1 km = 0.53996 NM
The knot follows directly from it: one knot is one nautical mile per hour, so 450 knots is 450 nautical miles covered in an hour, or 833.4 km/h.
Aviation's standard is metric — the exceptions just outlived it
It is commonly said that aviation runs on imperial units. The formal position is the opposite. ICAO's standard for units of measurement, published as Annex 5 to the Chicago Convention, is the International System of Units. The nautical mile, the foot and the knot are carried as a temporary dispensation alongside it.
They were meant to be retired. No termination date has ever been fixed, because withdrawing the units that every chart, altimeter and clearance in the world is written in would introduce more risk than it removed. The temporary exception is now permanent in practice, and that is the honest answer to why a modern quotation still measures in NM.
Where else the figure is used
Great-circle distance is not only a navigation number. Airline performance reporting is built on it: available seat kilometres and revenue passenger kilometres both multiply traffic by great-circle distance, and freight tonne kilometres do the same for cargo. It is the neutral yardstick that lets two carriers on different routings be compared at all.
What the distance does not tell you
The great-circle figure is a floor, not a plan. Between it and the aircraft's actual track sit several things:
- Airways and airspace. Aircraft follow published routes and avoid closed or restricted airspace, which is rarely the geometric shortcut.
- Wind. The air itself moves. A headwind subtracts from ground speed and a tailwind adds to it, which is why the same leg is often quicker one way than the other.
- Climb, descent and taxi. Block time runs from the moment the aircraft moves off the stand to the moment it stops at the other end — always longer than time in the air.
So a distance divided by a cruise speed is not a flight time, and we do not quote it as one. Where we publish a duration it is block time, estimated with routing and taxi included.
Distances are computed with the haversine formula on a mean Earth radius of 6,371.0088 km. Airport coordinates come from aviowiki open data (CC BY 4.0). Unit definitions: International Extraordinary Hydrographic Conference, Monaco 1929; ICAO Annex 5, Units of Measurement to be Used in Air and Ground Operations. A great-circle figure ignores terrain, airways and the Earth's flattening, so it will differ slightly from a flight-planned distance.