Definition
Departure is the east-west component of a run measured in nautical miles along a parallel of latitude, and it must be converted into difference of longitude because a minute of longitude shrinks with the cosine of latitude. The relationship is departure equals difference of longitude in minutes times cosine of mean latitude. At 60 degrees north, 10 nautical miles of departure corresponds to 20 minutes of longitude. Departure also names the act of taking a final fix on leaving the coast.
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