Sit. Rep. #162: Knot is not a Knot, Or is it?

1A- A statute mile as we in the US know it is 5,280 feet long. How did it come to be this sort of awkward number?  Let us go all the way back to the Romans.  The Roman Empire measured the “mille passus”, or “thousand paces,” as 5,000 Roman Feet as a Roman Mile.  The Roman Foot is slightly shorter than our current foot of 12 inches.  Each Roman Pace was considered to be 5 Roman feet.  Therefore 1,000 paces at 5 feet per pace equals 5,000 feet.

1B- In come the British, several centuries later.  They measured the “furlong” as the distance that an oxen team (2 oxen) could plow in a day.  A furlong is measured as 660 feet.  In the year 1592, the British Parlement set the (current) mile to 8 furlongs.  Therefore giving us 5,280 feet.

1C- The speed of light is 983,571,056.16 feet per second, which makes a “foot” about 1.017 nanoseconds at the speed of light.

1D- The first known measure using the term “foot” was from Sumer, where a definition is given in a statue of Gudea of Lagash from around 2575 BC. (Thanks Wiki)


2- Let us merge three parts of technology together to find that a knot is not a knot – but almost – until it is.

A- Gerardus Mercator had published his “Mercator” projection maps in the late 1500s.  These maps gave nautical sailors a true direction by which they could navigate the oceans easier and quicker.  But to get somewhere you had to know the distance and your rate of speed across the fluid (water).

B- So Sailors used a chip log attached to a line with knots tied equally distant to each other.  They would throw the log into the water and measure the number of knots that went thru a sailor’s hand in 30 seconds.  The timing was done using an hourglass-like timer called a sand-glass measured for 30 seconds.  The knots were tied every 47 feet 3 inches (14.4018 m).  This gave a speed of 20.25 in/s per knot (1.1505682 mph = 1.85166 km/h).

C- Remember that a circle is 360 degrees and every degree has 60 minutes.  Therefore a circle has 21,600 minutes of arc.  So 1 minute of arc is equal to 1 nautical mile.  Because the earth is irregular and not a perfect sphere this definition does not hold water.  Now enter the French in the 1800s who define the nautical mile as 1852 meters (about 6,076.11549 feet or 1.1508 statute miles).  This definition was accepted at an International Conference in Monaco in 1929.

D- So now we have 1 nautical mile per hour is 1 knot.

3- More knots for all you aviation fans and pilots.

  • TAS is “knots true airspeed“, is the airspeed of the aircraft relative to undisturbed (fluid) air
  • KIAS is “knots indicated airspeed“, is the speed shown on an aircraft’s airspeed indicator via the pitot tube
  • CAS is “knots calibrated airspeed“, is the indicated airspeed corrected for position and/or instrument error
  • EAS is “knots equivalent airspeed“, is the calibrated airspeed corrected for the adiabatic compression flow at that particular altitude

4- Knot abbreviations:
International Organization for Standardization, ISO, and IEEE (Electrical Engineers) use kn.
International Civil Aviation Organization, ICAO, uses kt

C: Do you know what the person that invented Velcro said?
D: Why knot!

E: Which Scout won the knot tying contest?
F: Neither. They tied.

G: What do you call a tangle of rope in space?
H: An Astro-knot.

Shalom to all,

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