Why is speed at sea measured in knots?

When ships first ventured out to sea, they had no sure way of knowing their location. Eventually, this was done by finding the latitude and longitude of the place. Latitude is distance north or south of the equator. Longitude tells how far east or west a place is. It was decided that zero degrees longitude would be the longitude line that goes through Greenwich, England.

To get an idea of their longitude, early ships first calculated how far they had traveled in a certain period of time. They used a "log" to find this out. It was a log of wood, weighted at one end, with the other end fixed to a long piece of rope. The log, thrown over the stern of the ship, floated, and the rope was let out as the ship sailed on. The speed of the ship could be calculated by seeing how much rope had been let out in a given time.

In later years, knots were tied at equal distances along the rope. A sailor counted how many knots passed through his hands in a certain time. This gave the speed of the ship. Sailors came to use the word "knots" to mean the speed of a ship.

Today, a knot has come to mean one nautical, or sea, mile per hour. A nautical mile equals 6076.1 feet (1852 meters), a little more than a land mile. Suppose a ship is sailing at a speed of 15 knots. This means that it is sailing at a speed of 15 nautical miles an hour, or 28 kilometers an hour.

Logs are still used to show how fast a ship is traveling. But today the logs are special metal rods with flat blades around them. As the ship sails through the water, the metal rod rotates and twists the rope round and round. The spinning rope works a device back on the ship that shows the actual speed.