Nowadays, we hardly think about determining how fast we are going. Cars, planes, boats, even bicycles, can have an easy to read, and instantly read, speedometer to answer that question.
For the mariners of the 1400's, speed was determined by an eyeball guess made by looking over the side to see how fast the water flowed by.. As distances traveled increased and as the need to record those distances became important to explorers (so they could accurately tell others how to return to the lands discovered), speed measurement also became more important.
At first, as used by English sailors, speed was measured by what they called a Dutchman's log, the mid-1500's. This was simply tossing something over the side of the ship at the bow (it was important that the "something" floated) and measuring how long it took to reach the stern of their ship (Bow is the front, stern is the rear). The measuring was no more precise than counting the seconds out loud. Knowing the time it took and the length of your vessel, you could do the math to get speed. Another method was to mark two placed on the side of your vessel and note the time it took the floater to pass between them.(Speed x Time = Distance, the math is beyond the scope of this article, but see a math book or navigation book for the details of how it's done)
The next step seems to have been taken by the English in the mid-1500's. They tossed a piece of wood over the side, but they did so off the stern and had a rope attached. The rope was on a spool which could easily turn. The rope spun off as the ship moved away from the wood sitting in the water. They then used a sand-glass, 60 seconds, and measured the amount of rope which went over the stern. They then pulled the rope back in and measured how much had come off the spool. They thought that a nautical mile was 5000 feet. Therefore, the amount of rope which came off in 60 seconds could be mathematically compared to how much would come off if the rope were to spin-off for a full hour---their speed in miles per hour.
You can imagine how much trouble it was measuring the rope as you brought it back. They did so by stretching their arms and guessing that the outstretched arm had six-feet of rope between the man's hands. This figure of six-feet became known as a fathom because many English words were borrowed from Scandinavian countries, and fathmr means to measure.
To make it less troublesome, the English decided to put a knot in the rope every seven fathoms, that's 42 feet. They also shortened the measuring time to 30 seconds. This was done about 1570. The time to spin off 42 feet of rope is the same as taking an hour and spinning off 5000 feet…a mile. (If you do the math, you'll find it's actually 5040 feet, close enough for a sailor). Of course, if two knots passed through your hand, you were traveling two miles per hour. Three knots, three miles per hour. To this day, speed at sea is called knots and means one nautical mile per hour. Twelve Knots at sea means 12 nautical miles per hour. It's incorrect to say , "12 knots per hour"; for you are really saying, "12 nautical miles per hour per hour".
By the 1400's, a nautical mile meant a particular distance based on the earth's diameter at the equator--one minute of arc was defined as a nautical mile. Since there are 60 minutes to the degree and 360 degrees in a circle, the earth's circumference was divided by 21, 600 minutes.
In time, the measurement of the earth became more precise and scientists determined that a nautical mile was not 5000 feet, but exactly 1852 meters, or about 6, 076.116 feet.