191. The great circle distance from LAT 25°50'N, LONG 77°00'W to LAT 35°56'N, LONG 06°15'W is 3616 miles, and the initial course is 061.7°T. Determine the longitude of the vertex, given the latitude of the vertex as 37°34.9'N.

The vertex longitude is determined using the right spherical triangle P-Z-V and the relation cos Δλv = tan φ1 / tan φv. Given latitudes of 2550'N and 3734.9'N, the longitude difference is approximately 5101', resulting in a vertex longitude of 2559.0'W.