Chart Navigation & Plotting · Sailings — Mid-Latitude
You make good 30 nautical miles of departure (east) at a mean latitude of 40°N. Using dLong = departure ÷ cos(mean lat), what is the difference of longitude?
- A30.0 minutes of longitude
- B39.2 minutes of longitude✓ Correct
- C23.0 minutes of longitude
- D46.7 minutes of longitude
Explanation
dLong = departure ÷ cos(Lm) = 30 ÷ cos(40°) = 30 ÷ 0.766 = 39.2'. Because meridians converge, a given east-west distance corresponds to more minutes of longitude as latitude increases.
Authority: Bowditch (Pub. No. 9), mid-latitude sailing
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