01-08-2025
Why airplanes use nautical miles instead of kilometers to measure distance
We travel by road, water, or air. On roads, the odometer shows meters, and the speedometer displays kilometers per hour (km/h). But measuring distance in air or water isn't as straightforward. This is where nautical miles, a unit of length specially designed for navigation in air, sea, and space, comes in. 'The nautical mile is based on geography, history, and practicality. It equals 1.852 km (1.851853 km by international standard) and was defined in 1929 by the International Hydrographic Organization (IHO). While both units relate to Earth's size, the meter (1/10,000,000 of the Earth's meridian) suits land measurements better and lacks a clear link to latitude and longitude. The nautical mile isn't part of the International System of Units (SI),' explained Dr Harshada Babrekar, Ph.D. Physics, consultant for Academic Development, Maharashtra Knowledge Corporation Limited (MKCL).
Prof. Jayant Pande, Faculty of Physics, FLAME University, said historically, sailors used nautical miles for centuries to navigate over large distances in the sea. 'Particularly when global navigation started to be carried out widely by European explorers in the 16th century, it was a matter of great convenience for them to know that the distance traveled by ship over a sea along a longitudinal meridian held a direct relationship to the degrees of latitude that had been crossed by the ship,' said Prof. Pande.
Since the latitudes are equally spaced along the Earth's surface, such that there are 180 degrees of latitude in total from the North Pole to the South Pole, the calculation meant that one nautical mile equalled 1/(180*60)th of the distance along a meridian from the North to the South Pole. 'Again, this assumes the Earth to be a sphere, which is a close approximation to its actual shape of an oblate spheroid. This definition of a nautical mile, based on the distance between two degrees of latitude, makes it a handy unit for navigation, both for sailors and pilots, and helps them to pinpoint the latitude of a point easily and precisely,' said Prof. Pande.
Take, for instance, Dr Harshada explained that if a plane is passing from Point A with Latitude 35° N, Longitude 118° W, and Point B: Latitude 38° N, Longitude 120° W (not taking into account longitude difference just for simplicity):
Latitude Difference: 38° – 35° = 3°
Convert to Minutes: 3° * 60 minutes/degree = 180 minutes
Nautical Miles: 180 minutes * 1 nautical mile/minute = 180 nautical miles (approximately)
Kilometers: 1 nautical mile = 1.852 kilometers = 180 * 1.852 = 333 km
The International Civil Aviation Organization (ICAO) and International Maritime Organization (IMO) mandate the use of nautical miles as a global standard for ships and aircraft to reduce confusion. 'Each nautical mile equals one minute of latitude, allowing pilots to map routes precisely and navigate with ease. This standardisation helps maintain course using maps, ensures safer flights, and supports accurate GPS-based navigation,' said Dr Harshada.
Prof. Pande said that a nautical mile is more suited to navigation over a body of water, whose surface can be constantly changing, than ground-based units such as kilometers and miles. 'With that said, due to reasons of standardisation, in modern times, an international nautical mile has been globally defined to have a length of exactly 1852 meters, which means that one can, if needed, convert from the nautical mile to an SI unit such as a kilometer,' said Prof. Pande.
Using nautical miles also streamlines flight planning and promotes international coordination in aviation. 'If global adoption of the metric system grows like the gradual shift in the United States, kilometers may become common in everyday use. But in professional navigation and aviation, nautical miles will continue to dominate,' said Dr Harshada.
DISCLAIMER: This article is based on information from the public domain and/or the experts we spoke to.
