In sailing, what does a knot measure?

In sailing, a knot measures speed, with one knot equalling one nautical mile per hour (1.852 km/h or 1.151 mph). Nautical miles are based on longitude (East to West) and latitude (North to South) coordinates, with one nautical mile equaling one minute of latitude.

The term "knot" dates back to the 17th century, when sailors measured the speed of their ship using a device called a "common log." The common log was a rope with knots at regular intervals attached to a piece of wood. Sailors lowered the wood into the water and allowed it to float behind the ship for an amount of time, which they measured with an hourglass. When the time was up, the sailors counted the knots between the ship and the piece of wood. That number of knots estimated the speed of the ship.

Nautical miles, on the other hand, measure distances on the ocean. The system uses the curvature of the Earth to determine distances, which is also effective to use in air travel. Until 1500, navigation at sea was done by eye until navigational instruments were developed. Nautical miles were not standardized until the mid-20th century, although most sailors agreed on distance in the 16th century.

Although knots do not conform to the usual metric and imperial units of speed used on land, sailors continue to use knots when at sea. This is because most distances continue to be measured in terms of longitude and latitude, rather than miles or kilometres.