vertex

i. The last fix adapted on the arrival speed segment. Normally, it will be the outer marker of the runway in use. However, it may be the actual threshold or another suitable common point on the approach path for the particular runway configuration.

ii. In astronavigation, the vertex of a great circle is the point at which the great circle is nearest the geographic pole—that is, the point at which it reaches the maximum latitude. Every great circle has two vertices (one in each hemisphere). In the solution of great-circle sailing problems, only the vertex nearest the arc forming the great-circle track is considered.

Every great circle intersects two points, 180° apart. These points are 90° away from the longitude of the vertices. The angle at which the great circle intersects the equator is equal to the co-latitude of the vertices. The purpose of determining position of the vertex is to facilitate calculation of intermediate position along the great circle track between which short rhumb-line courses can be steered. Based on the fact that the great circle track intersects the meridian of the vertex at 90°, Napier’s rule can be used to solve the successive right angles to obtain intermediate positions.