Written by Todd DePastino

Sand timer and moving sailing vessel for nautical mile and knot used for maritime speed referrences

I’m convinced that Navy jargon exists mainly to befuddle the rest of us.

But our main seafaring branch of service can’t be blamed for creating its strange unit of speed measurement, the “Knot.” That word, it turns out, came from previous generations of mariners.

To be honest, I was never sure whether the word was “knot,” as in a tied piece of rope, or “naut,” short for nautical. Only recently did I discover it’s the former. But, this being the Navy, there is also something called a “nautical mile” that differs from a land mile.

First, let’s look at the “Knot” as a term for speed at sea. This unit of measurement evolved over centuries as a way to measure how fast a ship was moving in a salt water environment bereft of fixed landmarks.

Two sailors at the stern of the ship would uncoil a long rope (“line,” in Navy parlance) into the ocean as the ship moved forward. The rope had a triangular piece of wood attached to the end and was knotted at intervals of 47 feet and 3 inches.

At the order, one sailor would toss the end of the “chip log” or “log line” into the water below and begin counting the knots slipping overboard as the rope unspooled.

The other sailor used a sand-filled hourglass to keep time. The sailors together tracked the number of knots passing into the water over a certain amount of time.

Speed equaled the number of knots divided by the amount of time. The more knots passing overboard, the faster you were going. Hence, marine jargon for speed became the word “knot.” One knot equals one mile per hour. Any relation to the word “nautical” is just an accident of phonetics, nothing more.

The Air Force adopted this term in the 20th century. So, when your next flight’s captain says over the loudspeaker that your plane is traveling “at 500 knots,” you’ll know why.

Of course, that’s not the end of the story. The mile a knot measures is not the 5,280-foot distance defined by the Elizabethan Parliament in the 1593 Weights and Measures Act and known to us all.

Rather, the knot’s mile is a special 6,076.40-foot distance officially defined at the First International Extraordinary Hydrographic Conference held in Monaco in 1929 and adopted by the United States in 1954. The term we give to this super-sized mile is “nautical mile.”

The Elizabethan mile–the one was use everyday–was passed down from Ancient Rome, which defined the unit as mille passus, the distance traveled by 1,000 normal human paces.

A nautical mile is a whole different animal. It is based on the Earth’s circumference.

Specifically, a nautical mile represents one “minute” of latitude. Latitude and Longitude are divided into degrees, minutes, and seconds. And there are 90 degrees latitude from the equator to the North Pole. Each degree is made up of 60 minutes. Each minute is made up of 60 seconds.

Right now, the chair in which I’m sitting as I type this post, is located at latitude 40° (degrees) 22′ (minutes) 48” (seconds) N (north of the equator). If I head one nautical mile north–that is, one minute of latitude, I’ll be at a great Thai restaurant where I’ll probably order Tom Yum soup.

Cutaway of globe with latitude and longitude from the equator to show nautical mile

This system makes the nautical mile especially useful for navigation because it directly correlates with the geographical coordinates used in mapping and charting courses at sea.

And, of course, it has the added benefit of confusing landlubbers like me.