A great and interesting question, I should have put this in a blog sooner! Thank you to the person who asked me, why my flightlog booklet was in "miles". Keep the questions coming, I am always looking for new content and ideas.
I haven't gotten too technical here but living in Greenwich, UK where time literally begins, with history of time and space all around me, I am always fascinated by this topic. See the Fun Facts section for more about the Prime Meridian, when countries adopted the international nautical mile measurement, etc.
Flights use a measurement called Nautical Miles
Before air and space travel, nautical miles, were, and still are used to measure the distance traveled through the water. A nautical mile is slightly longer than a mile on land, equaling 1.1508 land-measured (or statute) miles. The nautical mile is based on the Earth’s longitude and latitude coordinates, with one nautical mile equaling one minute of latitude.
Nautical miles are used because they are equal to a specific distance measured around the Earth. Since the Earth is circular, the nautical mile allows for the curvature of the Earth and the distance that can be traveled in one minute.
Why use a different measurement for Sea Navigation?
This is because using latitude and longitude coordinates is more practical for long-distance travel, where the curvature of the Earth becomes a factor in accurate measurement. Nautical charts use latitude and longitude, so it’s far easier for mariners to measure distance with nautical miles.
Air and Space Travel
This is earth curvature is the reason that air and space travel also use latitude and longitude for navigation and nautical miles to measure distance.
Why not use the metric system?
The word “mile” might leave you wondering if there’s a “nautical kilometer,” too. There’s not. The international nautical mile is used throughout the world. The measurement was officially set at exactly 1.852 kilometers in 1929 by what is now known as the International Hydrographic Organization.
1. International Nautical Mile - The U.S. and the United Kingdom both use to use slightly different measurements after that time, but the U.S. adopted the international nautical mile in 1954 and the U.K. in 1970.
2. Prime Meridian - "The prime meridian is zero degrees longitude and divides Earth into the Western and Eastern hemispheres. It is also known as the Greenwich Meridian because it passes through Greenwich, a borough of London, and terminates at the North and South poles. The meridian is also a key element in timekeeping, as in the 12-hour clock."
Greenwich Meridian at the Greenwich Observatory - photo credit: ©CRiney2023 & AMestrov2023
3. Knots - "Ancient mariners used to gauge how fast their ship was moving by throwing a piece of wood or other float-able object over the vessel’s bow then counting the amount of time that elapsed before its stern passed the object. This method was known as a Dutchman’s log." Boats & Planes calculate speed in knots which is equal to one nautical mile.
4. Nautical miles & knots - A nautical mile measures distance and a knot measures speed. Nautical miles are used for charting and navigating. A knot is one nautical mile per hour (1 knot = 1.15 miles per hour ).