Nautical Mile

Hello Fishies!

This may be more information than you ever wanted to know about a common term:

1 nautical mile = 1853 meters = 2000 yards = 6080 feet

Now for those of you with a math or science background:

From a Forum By Trevor Kenchington

Contrary to some earlier replies, a nautical mile is (or was) the length of a minute of latitude at the latitude in question, not at the equator. (Since the Earth isn't a perfect sphere, the length on the surface that is subtended by a degree or a minute of latitude decreases slightly towards the poles and the length of a nautical mile decreases with it.) The confusion may have arisen because a "geographic mile", a rarely used unit, is the length of a minute of _longitude_ on the equator. As someone has already noted, a nautical mile is approximately 6080 English feet and that is often useful as a working measurement. I noted above that the definition of "nautical mile" might no longer be the same because, around 25 years ago, it was admitted as a metric unit under the System Internationale (SI). Since the original designers of the metric system, back in about 1780, got their calculations wrong, distances in kilometres cannot readily be related to the spherical geometry used in navigation. [Maybe the idea was that one-kilometer should have been the distance subtended by one centigrade or 1/100 or 1/100 of a right angle, meaning that 10,000 km would have equaled 3,600 nautical miles, though that implies an unbelievably large error.] So, sometime in the 1970s, the committees, which control SI, were persuaded to accept the nautical mile as a valid unit. Unfortunately, they gave it the singularly silly abbreviation "M", which, in their unlimited wisdom, they knew nobody could confuse with "m" for metre. I have only seen that used once, by a Russian scientist, and it sure confused me!

While there are 3600 seconds in a degree, there are (of course!) 5400 minutes in a right angle. Thus, 10,000 km should be equal to 5400 nautical miles, if the former was defined correctly and the world was a perfect sphere. That makes 1.85185... (the three figures go on recurring into infinity), which suggests that the people who marked the metal bar to define the metre had their dimension very close indeed -- and did not make the substantial error that I suggested in my last.

The nautical mile was originally defined as one minute of angle of the Earth's meridian. Since the meter originally was defined as a 10.000.000 part of the distance from Equator to the pole, it follows that a nautical mile is 10000000/5400 = 1851,851851... meters.

Stay tuned!

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