Why is a nautical mile per hour is called a "knot"?
Before the invention of an accurate chronometer, ship captains were able to determine latitude (north south position) accurately, by knowing the date and the position of the stars and sun. But longitude (east west position) could not be determined from the heavenly bodies, without knowing the time to a high degree of precision.
They used "dead reckoning" to compute where they were. They figured this out from their direction and speed. The
compass gave them their direction, but they needed a way of telling how fast they were going.
A simple device, a "knot meter" (log), could tell them just how fast they were going. It consisted of a blown glass container of sand (an hour glass) that ran for about a half minute, and a
rope with a small bucket shaped piece of
wood (log) on the end. The
rope was held at a certain position on the
boat, and the bucket was dropped into the sea. When the starting point of the rope began to run through the fingers of the
deck hand, the hour glass was turned over. The
deck hand counted the number of "knots" that went through his hand until the sand ran out. The knots were tied in a spacing that corresponded to one nautical mile per hour.
Simple and effective, within the technology of the day. But the accumulated inaccuracy of this dead reckoning method is one reason why so many ships wrecked. The accurate stable chronometer made the determination on longitude possible, and was one of the greatest inventions of its time.