bruce

Actually, if we must insist on being sticklers, **By definition**, one nautical mile is equal to the distance covered at the equator by **one minute** of arc, not one degree.

When I said "Approximation" that's what I meant. Had I meant "exactly" then I would have used my computer to calculate things and specified the time of day, the day of the week, the week of the month, the month of the year and the temperature of the water around the point in question (that is if I knew what longitude the point was at.

My point was that AE made a mistake and it was a costly mistake.

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As long as we are being sticklers, one minute of arc along the equator is not a nautical mile, it is a geographic mile and is 6,087 feet long. Nor is a nautical mile one minute of latitude as that varies from 6,045 feet at the equator to 6, 108 feet at the poles. This is due to the bulge of the equator due to centrifugal force making it further around the earth along the equator than it is going north and south. The nautical mile is based on a perfect sphere having the same area as the earth. The same assumption is made for the purposes of navigation and the slight difference between the nautical mile and the length of a minute of arc on the real earth are ignored. On such a sphere there are 21,600 NM around the circumference along any great circle since there are 360 degrees around the great circle and there are 60 NM per degree (360X60=21,600.) Since there are 90 degrees from the equator to the north pole there are 5,400 NM along the same arc (90X60=5,400.)

Going back to first principles, the meter was originally defined as one-ten-millionth of the distance from the equator to the north pole. Since there are 5,400 NM covering the same distance, divide 10,000,000 by 5,400 and you find the length of a nautical mile is 1,852 meters. Since one meter is 3.2808390 feet a nautical mile is 6,076.11548556 feet.

(In case someone notices that the division doesn't work out to be exactly 1852 meters (1,851.85), that problem was dealt with by international agreement making it 1,852 meters exactly.)

gl