[teamonkey]

The knot isn't quite as arbitrary as most imperial measures.

1 knot = 1 nautical mile/hour

1 nautical mile ~= the distance travelled by someone at the equator in one minute due to the Earth's rotation

Since navigation at sea was strongly dependent on the position of the sun and stars, it makes sense to use a unit of speed that makes your calculations easier.

Even for modern navigation it makes sense to use a unit of measurement based on the circumference of the great circle surrounding the Earth.

[ScottBurson]

> 1 nautical mile ~= the distance travelled by someone at the equator in one minute due to the Earth's rotation

No. It's approximately one minute of arc, that is, 1/60 of one degree.

In fact the speed of the earth's rotation at the equator is more like 15 nautical miles per minute.

[teamonkey]

Yes, you're right of course. The Earth rotates through 15 arcminutes in 1 minute.