The longitude of the upper vertex of a great circle track is 169°E. What is the longitude of the lower vertex?

Correct Answer: Option C — 011°W

According to Bowditch, the upper and lower vertices of a great circle track are separated by 180 degrees of longitude. Given an upper vertex longitude of 169°E, subtracting this from 180° yields 11°, and reversing the hemisphere gives a lower vertex longitude of 011°W.