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Monday, 25 June 2007

How long is a mile? What kind of mile? Distance - arbitrary or absolute!


I was just thinking about distances - aviation, maritime, land. Here are some thoughts and definitions.

Britannica Concise Encyclopedia Defines a mile as follows:
Any of various units of distance, including the statute mile of 5,280 ft (1.61 km). It originated from the Roman mille passus, or "thousand paces," which measured 5,000 Roman ft (4,840 English ft [1.475 km]). A nautical mile is the length on the Earth's surface of one minute of arc or, by international definition, 1,852 m (6,076.12 ft [1.1508 statute mi]); it remains in universal use in both marine and air transportation. A knot is one nautical mile per hour.
I wish to correct the above.

Nautical Mile: The distance on the surface of the earth subtended by one minute of arc for the center of curvature for that point on the earth.

This means the length of a nautical mile (Nm) is variable. A close approximation is 6080 - 30.1 * COS(latitude). This assumes that the earth is a regular spheroid. Early satellite orbital data proved that there’s actually more mass in the southern hemisphere, therefore south of the equator a nautical mile is slightly longer than for the same latitude north of the equator.

So what? Well, the NM is probably the only unit of distance measurement that makes any mathematical sense. Let’s face it, the standard or land mile is arbitrary – selected from historical estimates of how long it takes a horse to go a distance.

See what happens, for example, when you measure a distance on a map (any projection) and when you calculate it using your handy GPS! Such differences may appear trivial, but now that we rely on GPS more and more, the accuracy suffers if we pay no attention to these details.

Standard Nautical Mile: From above this has been arbitrarily defined as 6076.12 ft. Actually, here’s a little British history – I believe the so called Standard Nautical Mile was taken from the length af a NM in the English Channel using the formula historically in vogue at the time this was defined. Somewhat like the way the British gave the world the Greenwich Meridian!

Even more crazy is the Kilometer. Who ever said that 1/10000th of the distance between the poles is a good idea. Later, through French influence, the length of a meter was redefined as so many wavelengths of a particular laser light. Why?

Metric Units

Marketing Dictionary

Well, according to Baron’s, the metric system is a system of measuring size, weight, and volume, based upon decimal units. The basic metric system units are grams, meters, and liters. One gram = 0.035 ounce. One meter = 39.37 inches. One liter = 61.025 cubic inches (cubic capacity), 0.908 quart (dry measure), or 1.057 quarts (liquid measure).

All nations of the world with the exception of the United States and two very small countries use the metric system. (my emphasis!) The U.S. Reluctance to conform to these worldwide standards makes it difficult to market packaged goods globally. For example, a consumer abroad accustomed to buying in liter or gram quantities will not understand a package label that uses quarts or ounces. Also, food package recipes cannot be easily translated from teaspoons and cups to grams and liters as required for non-U.S. Cooking utensils. However, for international marketing, the expense of printing metric versions of labels and packages is a costly necessity.

Is there a lesson to be learned? Possibly, but for myself, I still love those old statue miles, feet and inches I learned as a child. Of course, all this was brought home to me when my son asked how long a foot was! He wasn’t even taught those units at school in Great Britain fifteen years ago.

Plus ça change.

The full phrase in French is "Plus c'est la meme chose, plus ça change" which translates as "The more things change, the more things stay the same"

Captain Kirk

1 comment:

Jason said...

Now you know this subject is a Tell Tail sign that we are Geeks, Nerds, whatever you want to call it. Good stuff. Very interesting!