Just what is a nautical mile, why is it called a knot, and why can’t sailors just use a regular mile?
All very good questions. Let’s start with what a nautical mile is and why it is referred as knot instead of miles per hour.
In the days of wooden ships and iron men they did not have a speedometer near the ship’s wheel to tell how fast the ship was going. They used a log. A log is a quarter round piece of wood with a line attached to it. The line has knots tied in it at regular intervals. When the log in thrown in the water behind the ship the line is allowed to go out for exactly one minute. At the end of the minute the number of knots that went out is counted and that is how many knots, or nautical miles an hour, the ship is travelling.
Now a land mile, or statute mile, is 5,280 feet. A nautical mile is 6,080 feet. A nautical mile is 6,080 feet because 1/60th of a degree in latitude is – yup you guessed it – 6,080 feet. One degree of latitude is divided into 60 minutes (60 nautical miles) and one minute of latitude is divided into 60 seconds.
So in navigational terms one minute is a distance, one mile, but it is also time as well. A ship at sea will “shoot the sun” at noon. That means they try to figure out when the sun reaches its apex where they are. The ship keeps two clocks, one set on local time and one set to Greenwich Mean Time (GMT this is the time at zero degrees longitude). When they shoot the sun they will adjust the local time clock, but the GMT clock is never reset. They compare the local time to the GMT time. For an example let’s say on our ship GMT is 2 PM and local time is 12 noon. So that means GMT is two hours ahead of local time or 120 minutes, or two degrees of longitude, which is also 120 nautical miles. So our ship would be 2 degrees west of 0 degrees longitude and two hours behind GMT.
So as I said a nautical mile is not just a distance, but it is time too. Some people understand this the first time through. The rest of us have to reread it a few times before it makes sense. It you are one of the many just reread it, you will get it. And when you do get it will make perfect sense.