Chart Navigation & Plotting · Sailings
In parallel sailing, a vessel steaming due east or west changes its longitude. The relationship between departure (distance east-west, in NM) and difference of longitude (in minutes) is departure = DLo ×:
- Asin(latitude)
- Bcos(latitude)✓ Correct
- Ctan(latitude)
- D1 (they are always equal)
Explanation
Departure (NM) = difference of longitude (minutes) × cosine of the latitude. Only at the equator (cos 0° = 1) does one minute of longitude equal one nautical mile; at higher latitudes the parallels converge.
Authority: Bowditch, American Practical Navigator (Pub. 9), parallel sailing
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