Chart Navigation & Plotting · Sailings — Departure from dLong
At latitude 48°N you change longitude by 30' of arc. The departure (E-W distance) is about:
- A15.0 NM
- B30.0 NM
- C44.8 NM
- D20.1 NM✓ Correct
Explanation
Departure = dLong × cos(latitude) = 30' × cos 48° = 30 × 0.6691 = 20.1 NM. A minute of longitude shrinks with the cosine of latitude.
Authority: Bowditch (Pub. No. 9), Mid-Latitude Sailing
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