Chart Navigation & Plotting · Sailings — Departure from dLong
At latitude 60°N you change longitude by 50' of arc (dLong = 50'). What is the departure (east-west distance) in nautical miles?
- A25 NM✓ Correct
- B50 NM
- C100 NM
- D43 NM
Explanation
Departure = dLong (in minutes) × cos(latitude) = 50' × cos 60° = 50 × 0.5 = 25 NM. One minute of longitude equals one nautical mile only at the equator; it shrinks with the cosine of the latitude toward the poles.
Authority: Bowditch (Pub. No. 9), Mid-Latitude Sailing
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