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Why Nautical Mile and Knot Are The Units Used at Sea?

container ship at sea

Nautical fictions and other adventure stories are often filled with horrifying instances of ships being stranded at sea- due to storms and technical failures- not knowing the right direction. It’s true that, unlike on land, navigation in the open ocean is significantly difficult. The sea, with its vastness and unpredictable chaos within, always offer an uncertain journey for the captain and other sailors.  For navigators, while the land offers a number of fixed visible cues in the landscape, the sea does not leave any useful, distinguishing features. Due to this complexity, marine navigation has evolved significantly since the origin of humankind, finding various methods and measurements to save the life of mariners.

For many, the first encounter with the measurements being followed at sea raise eyebrows, wondering why they need to be different from miles and kilometres used while talking about land. Unlike measuring distance and speed on land, sailors use Nautical Mile as well as a knot for measurements during the sail. At sea, in navigational calculations, the statute mile is considered an arbitrary length of no particular significance. And, in particular, the replacement of the ordinary measurement with nautical miles and knots at sea helps the Mariners to quickly read charts that use latitude and longitude. Currently, the nautical mile is used as the unit of measurement by all countries for air and sea navigation.

What is Nautical Mile?

A nautical mile, a unit of measurement defined as 1,852 meters or 1.852 kilometres, is based on the circumference of the earth and is equal to one minute of latitude. If one is to pick a part of the earth after cutting the planet in half at the equator and consider the equator as a circle, it can be divided into 360 degrees. Then, one degree can be split into 60 minutes, of which one minute of arc on the planet Earth is 1 nautical mile. One nautical mile is slightly more than a statute mile (1 nautical mile = 1.1508 statute miles). According to the English measurement system, a nautical mile is equal to 1.1508 miles, or 6,076 feet.

What is Knot?

Then comes the knot. Of course, here we are not talking about sailing knots, such as Figure-8 Knot.The knot here, the nautical knot, is a unit of speed equal to one nautical mile per hour (1 knot = 1.15 miles per hour) or approximately 1.15078 mph. Similar to the speed and distance measurement on land, the nautical mile and knot explain the movement of a vessel at sea. For instance, a boat or ship travelling at 15 knots could go 15 nautical miles per hour.

History of Maritime Navigation

Since the beginning of ocean navigation, a number of methods have been brought in for making the voyage through long and vast seas easier. Several of traditional practices, using geometry, astronomy and even special instruments, helped sailors to navigate to their destinations for a quite long period. In the ancient times, long before the ship’s clock became common in use, sailors relied on time derived from the position of sun, moon, and stars- now known as Celestial navigation. Sometimes, in addition to their know-how tools, it was just luck that protected them when they ventured out into the uncharted, dangerous waters.In the later periods, the mariners succeeded in developing charts depicting distant shorelines and common features of the sea during voyages. According to historical records, such charts developed in the earlier period were marked with simple outlines of coastlines made to support written or oral directions.

In addition to these, compasses, astrolabes, and callipers were the tools those were in use by ocean navigators in the earlier times. The Mariner’s Compass, which was one of the earliest navigational tools and an early form of the magnetic compass, had been used widely in the earlier periods. Primarily, this compass was used to determine the direction of the wind when the sun was not visible.  Similarly, the cross-staff, astrolabe, and quadrant were in use to help sailors determine latitude in several stages of maritime navigation.

Phoenicians are the first Western civilization known to have developed the art of navigation at sea thousands of years ago. Phoenicians relied on primitive charts as well as observations of Sun and stars to navigate their vessels to destinations. In the later period, the Phoenicians and their successors, the Carthaginians, also invented a tool known as the sounding weight. Made of stone or lead, this bell-shaped tool had a very long rope attached to tallow inside.  Sailors used to lower this weight into the bottom of the sea to determine how deep the waters were, and, using this measurement, to estimate how far they were from land. In addition, the tool, with the help of the tallow inside, could pick up sediments from the seabed, which enabled expert sailors to decide the location of their vessel.

However, centuries passed before the use of a standard method to measure the distance and speed during navigation at sea. A number of new techniques and methods were experimented from time to time, making marine navigation more meaningful. Until the fifteenth century, coastal navigation was mostly in practice, since the open sea voyages were limited to regions of predictable winds and currents. On the open sea, mariners mostly depended on dead reckoning— the process of calculating one’s current position using a ship’s last position, speed, and direction.  Further ventures by the sailors were enabled by the development of scientific and mathematically-based methods and tools in the following years.The invention of the sextant, the chip log and Chronometers, etc. made the calculation of latitude and longitude possible and much easier. And, the modern era saw the replacement of ancient navigational tools with electronic and technological equivalents and also the determination of standard measures including Prime Meridian. With the help of new technologies, from Gyroscopic Compass to GPS, now marine navigation has become more systematic and easy.

History of Nautical Mile and Knot

Years after the use of several techniques to determine the position and speed of a vessel, British mathematician Edmund Gunter succeeded in enhancing navigational tools including a new quadrant to define latitude at sea. Gunter claimed that the lines of latitude can be used as the basis for a unit of measurement for distance. Eratosthenes and his successors had already assessed the circumference of the Earth, helping other mathematicians to build on.  Gunter proposed the nautical mile as one minute or one-sixtieth (1/60) of one degree of latitude (one degree is 1/360 of a circle, one minute of arc is 1/21600 of a circle). Using the circumference of the Earth assessed by Dutch scientist WillebrordSnellius aka Snell- who assessed it at 24,630 Roman miles or 24,024 statute miles -Gunter defined a nautical mile as 6,080 feet (1853 meters), i.e. the length of one minute of arc at 48 degrees latitude.

Even decades after these developments, there was no standard definition of a nautical mile and different countries had different definitions to follow until 1929. It was in 1929, at the First International Extraordinary Hydrographic Conference in Monaco, the international nautical mile was accepted as exactly as 6,076 feet (1,852 meters). Currently, this is the standard definition of a nautical mile in use and is accepted by the International Hydrographic Organization and the International Bureau of Weights and Measures.The United States measurements were based on the Clarke 1866 Ellipsoid and based on this calculation, a nautical mile was 6080.20 feet (1,853 meters).  Similarly, the United Kingdom was defining the nautical mile based on the knot- unit of speed measured from dragging pieces of knotted string. According to this, one knot was defined as one nautical mile and one nautical mile represented 6,080 feet (1853.18 meters).  However, both the US and the UK abandoned their own definitions in 1954 and 1970 respectively and accepted the international measure of a nautical mile.

On the other hand, the term knot can be traced back to the 17th century, during which period sailors used a device called common log to measure the speed of the ship. The common log was a device consisting of a wedge-shaped piece of wood and a coil of rope with uniformly spaced knots attached to the piece of wood. During the sail, the piece of wood was allowed to float for a specific time after lowering from the back of the vessel, and also the line was allowed to play out freely from the coil as the wood floats. After some time, the line was pulled in and the sailors used to count the number of knots on the rope between the ship and the wood to measure the speed of the ship. Sailors concluded the speed of the vessel according to the average of frequent measurements taken throughout the day. At present, with the help of advanced technologies, the knot measurements are determined using methods such as mechanical tow, Doppler radar, and/or GPS.

Calculation of Nautical Mile and knot:

The nautical chart turns to be the one of the important element abroad the vessel once it sets sail. The fixed relationship between distance, speed and time helps sailors calculate the distance the vessel expected to travel in a given time. The formula being used by sailors is 60 x D = S x T which is expressed as 60D = ST.

Conversion of Kilometer to Nautical Mile  – Formula

Checkout the video for understanding the forumal for conversion of kilometre to a nautical mile.

Disclaimer: The authors’ views expressed in this article do not necessarily reflect the views of Marine Insight. Data and charts, if used, in the article have been sourced from available information and have not been authenticated by any statutory authority. The author and Marine Insight do not claim it to be accurate nor accept any responsibility for the same. The views constitute only the opinions and do not constitute any guidelines or recommendation on any course of action to be followed by the reader.

The article or images cannot be reproduced, copied, shared or used in any form without the permission of the author and Marine Insight. 

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Why Nautical Mile and Knot Are The Units Used at Sea?


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