A Knot
As a follow-up to my Jamestown post, one of the things we learned was where the sailing term “Knot” came from. I have always known that a knot is 15% smaller than the same speed in miles/hour. But where did that term come from?
Below is what the sailing term “a Knot” is based off of. The sailor used a spool of rope like the one shown below. They put the end of the rope with the wooden “paddle” in the water and allowed it to un-spool with the speed of the ship. They timed the process with a sand timer (about 30 seconds) and then counted the number of actual knots that had been spooled into the water (the knots are on the rope and you can see one of them on the right side of the spool). They used this in the 1700’s to get from England to the “New World”.
Today I use a wrist watch which receives microwave signals from a constellation of 30+ satellites which can tell me within 15 feet where I am anywhere on earth.
Oh, and this marvel of technology can still tell me how fast I am going in Knots!!!
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But you leave out one part: you could build a knot-and-spool device to measure speed on any scale you want - it’s just a matter of how far apart you space the knots.
Nautical miles, which sailors use, are different than statute miles because they are based on the circumference of the Earth. One nautical mile is equal to one minute (e.g., one sixtieth of a degree) of latitude at the equator. Sailors care a lot about such things, land-lubbers not so much.
So while the rope-and-spool thing is the reason that sailors call miles-per-hour knots, the reason that those miles-per-hour are different than MPH on land relates to the reasoning behind nautical miles.