Knots and miles per hour are measures of speed. Mph is usually a measure of land speed, though, while knots measure speed in water or in air. Another difference is that miles per hour measures actual distance traveled while knots measure the speed a craft itself is making, disregarding the effects of wind or current that speed it on or hold it back.
One knot is a speed of one nautical mile per hour. However, a nautical mile is not a standard mile. The nautical mile was derived from a measurement of the earth's circumference, and was originally defined as one minute of longitude at the equator. For practical reasons, a nautical mile is now defined as a sixtieth of a degree of latitude.
This measurement was chosen because it was straightforward to determine at sea and it worked well on navigation charts. Nowadays a nautical mile is defined as 1.150779 standard miles or 1.852 kilometers.
The term knot comes from the way speed was measured on sailing ships. Ships used a method that measured distance over a short time to derive a ship's speed in knots.
It took three men, an hourglass and a chip log to measure knots. A chip log was a triangular piece of wood attached to a long rope with knots in it every 47 feet and 3 inches. The rope was wound on a reel so it would easily unroll as the ship sailed away from the chip log.
The log was thrown off the stern, the back of the ship. One man held the reel as the knotted rope unrolled. Another man timed thirty seconds with the sandglass. The third man counted the knots as they were pulled off the reel.
If the counter saw one knot, the ship was traveling at one knot; if he saw three, the ship was making three knots. Five or six knots was a fair speed for a sailing ship.
A knot is a measure of the distance something moves in an hour. The reason a thirty-second measurement gave an hour measure was multiplication. You can do something similar by counting his heartbeats at your wrist for ten seconds and multiplying by six to get your pulse, which is heartbeats per minute.
Mariners worked out a distance between the knots on their reels that spared them the multiplication. One nautical mile per hour is about 6076 feet per hour. Dividing by 60 to get feet per minute, and again by 60 to get feet per second, it turns out that a knot is equivalent to about 16.87 feet per second. Multiplying by thirty would theoretically give the distance of 47 feet and 3 inches that mariners left between the knots on their reel.
It does not exactly. Nowadays re-enactors time a chip log for 28 seconds to measure knots per hour accurately with a reel. Their measurements are slightly different and their ways of using a reel are probably slightly different, too. The chip log was extremely accurate for its day.
Modern methods of measuring knots include mechanical or electronic tows, hull-mounted logs and GPS. GPS includes the effects of current and wind to measure actual progress, rather than ship speed the way knots do.
Knots are still used in air navigation and meteorology. They measure tidal streams, river currents and wind speeds. One reason they remain in use is that they work well on charts constructed with the Mercator projection. Many people have little use for charts, but still enjoy hearing speeds expressed in knots. The unit evokes a fascinating history and a rich naval tradition.