Definition
The distance arc d between two positions satisfies cos d equals sin(lat1) sin(lat2) plus cos(lat1) cos(lat2) cos(difference of longitude). The result in degrees is multiplied by 60 to give nautical miles, assuming a spherical Earth. For example, positions 50 degrees North 005 degrees West and 40 degrees North 070 degrees West give an arc near 50 degrees, about 3,000 nautical miles. Accuracy degrades for very short distances because the cosine changes slowly.
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