I had heard of the term ‘nautical mile’ and also of the term ‘knots’ but had no idea where they came from, only that they were terms used by aircraft pilots and seafarers. I knew that a nautical mile was a little longer than a standard mile (one nautical mile is now defined as exactly 1,852 meters or 1.151 miles) while a knot is just one nautical mile per hour. Why we still used two units of distance and speed that were so close to each other was unknown to me. I assumed that nautical miles and knots were retained for sentimental reasons.
But I have learned that those units have an interesting meaning and practical use in that one nautical mile originally was defined as one minute of latitude. If one takes the circumference of the Earth and divide it by 360 degrees (the number of degrees in a full circle) and then again by 60 (the number of minutes in a degree), the number that you get is one nautical mile. So by knowing the difference of two latitudes in degrees, one could immediately calculate the distance between them along a great circle (i.e., following a line of longitude) in nautical miles by multiplying it by 60.
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