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Origins of Knot and Nautical Mile

by Jeremy Cox 1. June 2006 00:00
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On June 26th it will be the sixtieth anniversary of the day that the United States Army Air Forces and the United States Navy officially adopted the ‘Knot' and ‘Nautical Mile' as the official measurement for Speed and Distance in the air. With this auspicious historical event in mind, have you, like me, ever wondered what the origins are of these perplexing measurement units?


According to a free online Encyclopedia, a Knot is a unit of speed… it is not a Standard International (SI) unit, and although discouraged, its use is accepted for use around the world for Marine and Aviation purposes… …Although knots do not fit within the SI system their retention for nautical and aviation use is important for navigational reasons because the length of a nautical mile is almost identical to a minute of latitude. I.e. if you slice Earth into two equal halves right through its centre along the equator for example, then divide the perimeter (its circumference) into three hundred and sixty degrees, then each degree into sixty arc minutes, the resultant length is approximately one nautical mile. Therefore one nautical mile is the arc distance of about one minute of a degree (or one sixtieth of a degree) of the Earth. As a result distance in nautical miles on a navigational chart can easily be measured by using dividers and the latitude indicators on the side of the chart.

It goes on to provide the following definition:… 1 knot = 1 nautical mile/hour = 1.852 km/h exactly. This is based on the internationally agreed length of the nautical mile… …Knot is sometimes mistakenly used to refer to the nautical mile itself, but this is incorrect…

Okay, so how did a ‘Knot' come into being?

This is where it really gets interesting.

Shortly after the invention of the sailing vessel, early sailors wanted to determine how fast their vessels were travelling as they sailed out of sight of the land. Ingeniously they achieved this by throwing a wooden log into the water and then observing how fast the vessel moved away from it. This method was named" 'Heaving the Log' and was not improved upon until the Sixteenth Century when the 'Chip Log' method came into being. This improved method of speed determination employed the use of a weighted wood panel tied to a reel of knotted rope. This rope had knots tied in it at approximately every fifty feet. When the Chip Log went over the side, the number of Knots that slipped through the hands of the sailor holding the rope, would be counted off during the time that it took for the grains of sand in an inverted ‘Hour-Glass', Sand-Timer to completely leave the time bulb that was calibrated to time the elapse of thirty seconds. This simple method provided a fairly accurate measurement of the number of nautical miles per hour that the vessel was traveling. For example, if ten knots went overboard in thirty seconds, then the ship was moving forward at the speed of ten knots or ten nautical miles per hour.

Another free encyclopedia site tackles the issue of how the approximation of the ‘Chip Log' Knot was tightened up, as follows: …We say approximate because if you choose to slice Earth along the line that goes through the North and South poles you would get a slightly different result due to the fact that Earth is not a perfect sphere - it is slightly flattened at the poles. The difference between the polar and equatorial diameter being about twenty three point four nautical miles out of six thousand, eight hundred and eighty nautical miles. The exact value for the nautical mile is taken to be the average of the two (i.e. polar and equatorial) which equals:

1 nautical mile = 1.15 miles = 1,852 meters = 6,067 feet

Since the majority of the world now uses the Metric System, interestingly enough, in the seventeenth century the Meter came into being and was defined as one part in ten million of the distance from the North Pole to the Equator along the meridian (an imaginary circle perpendicular to the horizon) that passes through the city of Paris.

Staying with the Knot, or Nautical Mile, later on in naval history, the hour-glass, sand timer was recalibrated to measure twenty eight seconds, and the knots were spaced out at exactly forty eight feet (which is equivalent to eight fathoms.) Under this arrangement a vessels speed was pretty much ‘spot-on'.

How about that!?!

As an aside, did you know that the term ‘Cockpit' comes from the Royal Navy of the Seventeenth Century? The cockpit was the area where the junior officers were stationed. Eventually this lead to the word being used to refer to the area towards the stern of a small decked vessel that houses the rudder controls.

Equally of interest, did you know that the name: ‘Pilot' comes from the Dutch word "pyl-lood". According to the encyclopedia at Wikipedia.org …The word pilot is supposed to have been derived from the word "pyl" and "lood". The word "pyl" stands in Dutch for everything vertical or straight as well as the height of the water. The word "lood" stands for "lead"; the dense metal used when making soundings. The combination of the two words describes "the person who is using the lead in a vertical way to obtain the water depths". In very ancient Dutch also the word "lodes-man" is sometimes used, having more or less the same implication.

As the great actor Michael Caine sometimes says: "Not a lot of people know that!"

So what other nautical terms do you know that have made it into the vocabulary of our aviation industry? Any input that you care to make will be of great interest to all of the readers here at Globalair.com. So please don't be bashful and go ahead and write your comments and suggestions here. Please don't forget that whatever you write here, can be seen publicly by everyone that visits this page, so please be funny, be inspired, but most importantly of all, please be nice. ‘Au revoir' until next Month.

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Jeremy Cox


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