What Are Nautical Miles and How Are They Measured?

A nautical mile is a unit of measurement that sailors or navigators use in aviation and shipping on water. A nautical mile is the average length of one minute of one degree along a great circle of the Earth. One nautical mile equals one minute of latitude, which means that degrees of latitude is about 60 nautical miles apart. On the other hand, the distance of nautical miles between degrees of longitude is not that constant, given the fact that the lines of longitude become closer together as they join at the poles. 

Nautical miles are usually abbreviated with the symbols NM, nm, or nmi. Aside from being used in aviation and navigation, nautical miles are also used in polar explorations and international laws and treaties regarding territorial water limits. 

History of Nautical Miles

Before 1929, there was not an internationally agreed-upon definition or distance for the nautical mile. During that year, the First International Extraordinary Hydrographic Conference was held, and it was when they determined that the international nautical mile should be at precisely 6,076 feet or 1,852 meters. Today, this is the only description that is used widely. It is the only one accepted by the International Bureau of Weights and Measures and the International Hydrographic Organization. 

Before 1929, different countries had their own interpretations of the nautical mile. The United States measurements follow the Clarke 1866 Ellipsoid wherein one minute of arc along a great circle. If you follow these calculations, one nautical mile would be 6,080.20 feet or 1,853 meters. In 1954, the United States of America decided to disregard this nautical mile definition and adapt the international measure of a nautical mile. 

While in the United Kingdom, they base their nautical mile on the knot, which is a unit of speed that is derived from dragging pieces of knotted strings from sailing ships. Knots are measured by the number of knots that are falling into the water over a given period of time, and this is how they determine the knots per hour. The United Kingdom determined that one knot is equal to one nautical mile, which is equivalent to 6,080 feet or 1,853.12 meters. In 1970, the United Kingdom decided to abandon this computation and started to use exactly 1,853 as the nautical mile’s definition.

How to Use Nautical Miles

As we mentioned, the agreed-upon measure of one nautical mile is 1,852 meters or 6,076 feet. When it comes to an understanding of the nautical mile is to remember its essential relation to latitude. This is because nautical miles are based on the Earth’s circumference, which is why some say that the concept of a nautical mile is easier to understand if you imagine the Earth cut in half. The circle of the half will be divided into equal 360° portions. These degrees are then divided into 60 minutes. One of these minutes along the great circle on Earth is equated to one nautical mile. 

A nautical mile equals 1.15 miles because one degree of latitude is about 69 statute miles in length when it comes to the statute or land miles. 1/60th of that said measure would be 1.15 statute miles. 

Aside from its navigational purposes, nautical miles are also significant speed markers because the term “knot” is used today to mean one nautical mile per hour. This is why a ship that is moving at 10 knots means that it is moving at ten nautical miles per hour. As we mentioned, the term knot is derived from the United Kingdom’s previous practice of using a log, a knotted rope tied to a ship to gauge the speed of a boat. In order to measure knots, the people would throw the log into the water and trail behind the ship. The number of knots that passed off of the boat and into the water during a particular time would be counted, and the result will determine the ship’s speed in knots. Today, knots are measured using technologically advanced gadgets such as a Doppler radar, mechanical tow, or GPS.