By Tradewinds instructor Tony Johnson
As we all learn in our first navigation class, a nautical mile equals one minute of latitude. Therefore, the distance in nautical miles between the equator and the poles is, by definition, 90 degrees times 60 minutes, or 5400 nautical miles, which is very close to 10000 kilometers. This seems pretty plain, until we come across the inconvenient fact that the earth is not a perfect sphere. It’s bloated at the equator, and so is what is called an oblate spheroid. It’s a bit squashed, in other words, which means that at the poles, the radius of the earth is 6356.752 kilometers, and at the equator, the radius is 6378.137 kilometers. After various mathematical analyses in the 19th century, an Englishman named Alexander Ross Clarke came up with the spheroid, referred to by scientists as the “figure of the earth,” which became the standard way of conceiving of this shape. The investigations continue to this day and get pretty arcane. WGS 84 (World Geodetic System of 1984) is the datum for the GPS units currently in use.
Here’s a funny result of this bulge: If you are standing at the equator, your weight is reduced by centripetal force. You’re being flung into space at 1000 miles an hour, just as you would be on a playground merry-go-round. However, the centripetal force is a very small percentage of the force of gravity, so you stay on the planet, but weigh just a little less. Or you would, except for that bloatedness. Because the world is thicker at the equator, there is more mass pulling you towards the center of the earth, increasing the earth’s gravity and mostly offsetting the opposing force throwing you away from the earth. The end result is that you weigh only about .5% less at the equator than you do at the poles.
That oblateness also messes with the nautical mile. If you take this shape into account, one minute of latitude at the equator is 1842.9 meters; but at the north pole it is 1861.7 meters. In 1929, the First International Extraordinary Hydrographic Conference in Monaco decided to standardize the distance at 1852 meters or 6077 feet, which we often round to 6080, as the length of the nautical mile. This compromise between the distance at the equator and the poles works out to be the length of the nautical mile at 48 degrees of latitude.