In books about sea voyages or adventures, in films about desperate sailors, in articles on a geographical topic and in a conversation between sailors, the term "nautical mile" often slips. The time has come to figure out what this measure of length in shipping is equal to, and why sailors do not use the kilometers we are used to.
What is 1 nautical mile?
Initially, this value corresponded to a length of 1/60 degree of the arc of a circle on the surface of the Earth with a center coinciding with the center of the planet. In other words, if we consider any meridian, then the nautical mile will be
approximately equal to the length of one minute of latitude. Since the
shape of the Earth is somewhat different from the shape of an ideal ball, the length of 1 minute of the degree of the meridian under consideration may differ slightly depending on latitude. Most of all, this distance at the poles is 1861.6 m, and the least at the equator is 1842.9 m. To avoid confusion, it was proposed to unify the length of the nautical mile. The basis was the length of 1 minute degree at 45º latitude (1852.2 m). This definition made the nautical mile convenient for calculating navigation tasks. For example, if you need to measure a distance of 20 miles on a map, then it will be enough to measure with a compass 20 arc minutes on any meridian on the map.
Since 1954, the United States began to use the international nautical mile (1852 m). In practice, it is often rounded up to 1800 meters. The official designation of this unit has not been accepted. Sometimes the abbreviation "nmi", "nm" or "NM" is found. By the way, "nm" is a common designation for nanometers. 1/10 of the international nautical mile = 1 cables = 185.2 meters. And 3 miles is equal to 1 nautical league. In the UK, its own nautical mile, often equal to 1853.184 m, was often used before. In 1929, an international conference was held in Monaco on various hydrographic issues, at which the length of the nautical mile was determined to be 1852.00 meters. Do not forget that a mile is not only marine, but also land. In this case, its length is 1.151 times less than marine.
What is the relationship between nautical mile and knot?
A sea mile (nautical mile) or, as it is sometimes called, geographical or navigational mile, has become widespread in geography, aviation and navigation. Closely connected with it is the concept of a maritime knot used in shipping as the basic unit of speed. One knot is equal to one mile traveled per hour. The name "knot" is due to the fact that in the old days, ships used a lag to measure speed. It was a log or a board in the shape of a triangle, to which the load was tied. A line (rope) was tied to such a
floating anchor , at which nodes were tied after a certain distance. The lag was thrown overboard, after which for the selected period of time (from 15 seconds to 1 minute) it was calculated how many nodes went into the water.
There are different versions regarding the distance between nodes. Some say it was 25 feet (7.62 m), and if in 15 seconds one knot left, the result was one nautical mile (100 ft / min). According to the second version, the knots were tied after 47 feet and 3 inches (14.4018 m), and the countdown went in 28 seconds. In this case, one knot showed a speed of 101.25 ft / min.
We hope that now you will not have difficulty in understanding the marine terminology, and miles with knots will become as clear as usual kilometers.