The great circle distance from LAT 38°17.0'N, LONG 123°16.0'W to LAT 35°01.0'N, LONG 142°21.0'E is 4330 miles and the initial course is 300.9°T. The latitude of the vertex is 47°40.5'N. What is the longitude of the vertex?

Correct Answer: Option D — 167°18.5'W

Great circle vertex longitude is computed using Napier's rules of circular parts for right-spherical triangles. Applying Bowditch formulas relating initial course, latitude, and vertex coordinates results in a vertex longitude of 167°18.5'W.