Unlike the standard mile that Americans are used to using, the nautical mile is based on standard dimensions of the Earth.
The international mile, used in the United States as a measurement of length, is derived from an ancient Roman measurement of one thousand paces. Much like the foot, based on the length of a man’s foot, and the inch, based on the measure of the distance from the last knuckle of the thumb to the tip, the mile is fairly arbitrary and, in its original definition, varies from person to person. While these units of measure have been standardized, they still are quite subjective.
The nautical mile, however, was based on a basic measurement of the size of the Earth.
The nautical mile was originally defined as one minute of arc along a meridian on the Earth. Early cartographers (map makers) defined the circumference of the Earth in degrees, much like we define circles. Since the Earth is roughly a sphere there are 360 degrees around the Earth from east to west and 360 degrees around the Earth from north to south. And, also like a circle, the degrees of measurement around the Earth are broken down into minutes and seconds for greater precision. 60 seconds makes a minute and 60 minutes makes a degree. (I know that sounds odd, like minutes, seconds and hours on our clock. Why 60? It’s because of the counting system of the ancient Sumerians in Mesopotamia.)
Degrees of longitude are mapped on the Earth like slices of an orange. They are thin at the top and bottom and wide in the middle, at the Equator. Degrees of longitude, therefore, differ in measurement depending on the latitude.
Latitude, however, is different.
Latitude is measured as angles from the center of the Earth to the Equator. Each degree of latitude would be the exact same distance from the next if the Earth were a perfect sphere. After the nautical mile was originally defined it was discovered that the Earth isn’t a perfect sphere, however. The Earth is flatter at the top and bottom than it is in the middle. This is because of the Earth’s rotation. The problem with this is that one minute of latitude at the Equator isn’t quite the same as one degree of latitude at the poles. So the official definition of the nautical mile is the mean of the length of all of the minutes of arc between the Equator and the poles.
As a unit of measurement, like most every other unit of measurement, a nautical mile is a nautical mile. However, for reference, it is common to compare a unit of measurement to other units of reference. For that purpose we can say that a nautical mile is 1,852 meters (1.852 km) or 6,076 feet (1.151 miles).
