The Knot and the Mile

 

Why is a land mile different from a nautical mile?

The old land mile dates back from Roman times. The Romans called it a “mille passuum” which means a thousand paces. Not steps. It was the total distance of the left foot of a Roman legionnaire hitting the ground 1000 times. Put is simply it is 2000 steps. If your average legionnaire’s step was, say, 0.74m, a Roman mile would have been 1,480 m.

A bit shorter than the international mile that was defined at the 1959 International Yard and Pound Agreement reached by the USA, UK, Canada, Australia, NZ and South Africa. This one is about 1,609m. A legionnaire on steroids, maybe?

The nautical mile is used for air, marine and space navigation. Historically, it was defined in the 1500’s as one minute of arc on the Earth meridian. Very convenient. The Earth circumference is about 40,000km. Divide that by 360 to get one degree, then again by 60 for one minute. You get about 1,852 m.

A knot is a nautical mile per hour. The origin of “knot” in this context is interesting. In the old days, sailors measured a ship speed with a chip log: a triangular piece of wood attached to a long string (the log-line) with knots at uniform intervals. The idea was to drop the wood bit at the ship’s stern, then to count the number of knots on the log-line for a given time while the ship moved away. That gave the ship’s speed, in knots!

References:

Mile: https://en.wikipedia.org/wiki/Mile#English_mile

Chip log: https://en.wikipedia.org/wiki/Chip_log