Today there is an international standard for the nautical mile: 1.1508 miles, or 1.852 kilometers. Strictly speaking, these figures are not derived from surface distances, but rather from the curvature of the earth at the Equator.
Since the earth is basically a sphere, the Equator is basically a circle. Neither the sphere nor the circle is perfect in mathematical terms, but each is close enough to permit navigation by degrees of arc for centuries. What is important is that sea charts have long been measured by degrees of longitude and latitude, which break up the earth into a series of circles on the north-south and east-west axes. Any given point can be measured as the intersection of a longitude and a latitude line.
Sections of a circle are measured by degrees, so the Equator (as a full circle) is comprised of 360 degrees. Each degree can further be subdivided into 60 minutes; one minute is a nautical mile. Each degree is therefore 60 nautical miles, and the Equator itself spans 21,600 nautical miles.
As earlier stated, the earth is not a perfect sphere. The curvature of the earth varies slightly at different latitudes, and consequently, the actual distance of a single minute of a degree of arc will also vary. When the local figure is used instead of the international standard, it is known as a sea mile.
A knot applies the nautical mile to a unit of time in order to measure speed. One knot is one nautical mile per hour. The term “knot” comes from the age of sailing ships, when the ship’s speed would be measured by a log chip and line. The chip was a wedge-shaped piece of wood attached to a line with a knot tied every 47 feet, 3 inches. This was the number of feet that a ship would cover in 30 seconds if the ship were sailing at one knot. When the sailors needed to measure their speed, they would throw the chip from the stern and count the number of knots that left the ship while a 30-second hourglass played out. When the hourglass had emptied itself, the number of knots that had been counted would be accepted as the ship’s approximate speed. Naturally, speed can be measured far more precisely today, but seafarers have kept the term out of tradition.
It is also worth noting that nautical miles and knots are no longer used solely by sailors; they are also used in the aviation industry. Measuring distances by degrees of arc instead of surface distances traveled makes perfect sense when one is measuring the distance between two terrestrial points, such as JFK airport, New York and Heathrow, London, crossed by a vehicle that covers most of that distance at an altitude of 25,000 feet. The further one goes from the center of the earth, the longer is the distance covered by one minute of one degree; since the important figure is the distance between two terrestrial airports, however, the use of nautical miles to measure that distance is more logical than is the use of standard miles or kilometers.
