How do you measure a nautical mile?

Answer 1

The Nautical mile is defined to be one angular minute (1/60 of a degree) of longitude at the equator at sea level.

The word "knot" is a measure of velocity in nautical speak... and it means "nautical mile per hour". 1 knot = 1 nautical mile / hour

The unit used for nautical mile is often times "nmi". As in, 5 nmi.

The conversion ratio between nautical miles and other more familiar units are:
1 nmi = 1.15078 statue miles (land miles) (mi)
1 mi = 0.86898 nmi
1 nmi = 6076.1155 feet (compared to the 5280 feet of the standard mile)
1 nmi = 1852 m = 1.852 km

How does one actually measure distance traveled at sea? That would be a good question. A separate question, though. And Im sure there are various answers.

In todays electronic world, we probably use GPS to measure distance traveled, by pinning down and comparing two locations: where you were and where you are.

In earlier years we undoubtedly used the sun and stars to indicate position, likely requiring a compass, maps, sextant, sundial and a mechanical clock. From this you could also approximate relative position.

I imagine boats have speedometers of a sort, too. How they mechanically work, I cannot say. But coupled with a time piece and a compass, it would be easy to compute distance traveled.