In sailing, what does a knot measure?
And the answer: speed.
On the ocean, a knot measures speed, with one knot measuring one nautical mile per hour. Nautical miles are based on longitude and latitude coordinates, with one nautical mile equaling one minute of latitude.
Measuring speed and distance was a whole lot harder before Google Maps. And that's not to mention measuring speed and distance on the ocean – though it may seem strange, using knots is an effective and accurate way to estimate the speed of a ship on the sea. Interestingly, 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 (you guessed it) knots at regular intervals attached to a piece of wood. Mariners would lower the wood piece into the water and allow it to float freely behind the ship for an amount of time that was measured with an hourglass, and when the time was up, they would count the knots between the ship and the piece of wood. Voila! That number of knots estimated the speed of the ship.
Nautical miles, on the other hand, offer a means of measuring distance on the ocean. This system works with the curvature of the Earth on long distance journeys, which proves effective to use in air and space travel as well. Although nautical miles weren't standardized until the mid-20th century, their use by sailors was consistent beginning in the late 16th century. Navigation at sea was actually done by eye until around 1500, when navigational instruments were developed and cartographers began using a coordinate system with parallels of latitude and meridians of longitude.
Learn more about knots and nautical travel here.