A nautical mile is a distance on the earth's surface of 6,080 feet, which is equal to one minute of latitude at the earth's equator. Since there are 360 degrees around the earth, and each degree equals 60 minutes, the distance around the earth, at the equator or any other great circle, is 21,600 nautical miles. (A great circle is like a circumference.)

The origin of the nautical mile started with the realization that the earth was spherical and not flat. It was Pythagoras who first put forward the theory in 580 b.c.

A major advance that made early navigation much more accurate was the invention of the chip log (c.1500-1600). Essentially a crude speedometer, a light line was knotted at regular intervals and weighted to drag in the water. It was tossed overboard over the stern as the pilot counted the knots that were let out during a specific period of time.

The knots were spaced at a distance apart of 47 feet 3 inches and the number of these knots which ran out while a 28-second sand glass emptied itself gave the speed of the ship in nautical miles per hour. The proportion of 47 feet 3 inches to 6,080 feet is the same as 28 seconds to one hour.

Interestingly, the chip log has long been replaced by equipment that is more advanced but we still refer to miles per hour on the water as knots.

For more information, check out The History of Navigation.
Have fun with our calculator that converts statute miles to nautical miles, and vice versa.