**ρ**) is the number of degrees in a radian calculated as 180 degrees divided by pi (π). So

**ρ**is about 57.3 degrees. The nm can be calculated as the radius (6357 or 6378 km) divided by

**ρ**to get degrees and divided by 60 to get minutes of arc. So at the equator the nm is very close to 6378/57.3/60 = 1.855 km. Similarly at the pole it is nearly 1.849 Km.

This of course was the original nautical mile before we standardized it. As stated in an earlier post, today we use a fixed standard for the nm which is 1.852 km. exactly by definition. This is likely to simplify its application in computerized processes.

Seafaring navigators used this nautical mile for its convenience on a chart. The edges of a chart show the degrees and minutes of latitude on the sides and longitude at the top and bottom.

The distance between longitude coordinates vary as we go north and get closer together. But for latitude it stays pretty constant as shown in the calculations above. On a road map one would use a legend to determine a distance with a ruler or a pair of dividers, well on a nautical chart the navigator could simply use the latitude minute marks on the left and right edges of the chart and not have to go turn the chart over looking for a legend. Also on some chart projections like the Mercator projection, the scale varied with latitude. For example you know how on some maps Greenland is bigger than the US. In these cases the scale would only be accurate at the same latitude where the reading was taken off the left and right edges.

The nautical mile is a pretty ingenious invention.

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