Why all the Conversions?

If you've ever had to convert feet to nautical miles or been asked how fast you fly, you probably start to wonder why we have all these different units of measurement. The 23 June 2008 Democrat-Gazette has an article that sheds a little light on NM, miles, and KM.


Don’t know why a mile is 5,280 feet? You’re not alone
BY MARSHALL BRAIN MCCLATCHY NEWSPAPERS

If you live in the United States, you know all about miles. We measure any long distance in miles, and we also have things like miles per hour and miles per gallon. A mile is 5,280 feet. But where did this unit of measurement come from? Why is it so bizarre? And what is the difference between a statute mile, a nautical mile and a kilometer? Let’s explore how these different measurements work. It is pretty obvious where the “foot” came from. It started with the length of a person’s foot. Anyone wearing a size-10 shoe has a sole that is almost exactly a foot long. Having a unit of measurement attached to your body is obviously quite convenient, so this unit stuck. But putting 5,280 feet into a mile is more obscure. To understand the number, you have to understand the furlong, which the English have used for measuring parcels of land for centuries. A furlong is 660 feet. A mile is 8 furlongs. Eight times 660 is 5,280 feet. In other words, the length of a mile is totally arbitrary, but at least you now understand where the obscure number came from. You might ask, “Why did the English want a unit of measurement that was about 5,000 feet long?” That’s because the Romans, who once ruled the English, had a unit called a mille passuum, which measured 1,000 paces. A pace was five feet. So a Roman mile was 5,000 feet. Since the furlong was an important unit of measurement in England, it appears that the British chose a furlong-based system when defining their own mile. And 5,280 feet was pretty close to 5,000 feet. What about a nautical mile? Instead of being based on human anatomy, a nautical mile is based on the circumference of the Earth. If you were to cut the Earth in half at the equator, you could pick up one of the halves and look at the equator as a circle. You could divide that circle into 360 degrees. You could then divide a degree into 60 minutes. A minute of arc on the Earth is 1 nautical mile. This unit of measurement is used by all nations for air and sea travel. A knot is a unit of measure for speed. If you are traveling at a speed of 1 nautical mile per hour, you are said to be traveling at a speed of 1 knot. (Sailors used to measure knots using a length of rope weighted with a log. The rope was knotted at regular intervals. The sailors would toss the log overboard and count how many knots played out through their hands as the log drifted away behind the boat. They used sandfilled hourglasses to measure the time.) And then there is the kilometer. A kilometer is also defined using Earth as a standard of distance. If you were to take the Earth and cut it in half along a line passing from the North Pole through Paris, and then measure the distance of the curve running from the North Pole to the equator on that circle, and then divide that distance by 10,000, you would have the traditional unit for the kilometer as defined in 1791 by the French Academy of Sciences. A kilometer is 1,000 meters. Today the scientific community uses the metric system. The meter has been defined as the distance that light will travel in a vacuum in 1/299,792,458th of a second. So a mile is now defined as 1,609.344 meters. To travel around the Earth at the equator, you would have to travel (360 x 60) 21,600 nautical miles, or 24,857 miles, or 40,003 kilometers.