Parallel of latitude

A parallel of latitude is a small circle on Earth parallel to the equator, joining all points of equal latitude, except the equator itself which is a great circle. A ship steering due east or west runs along a parallel, the parallel-sailing case where departure equals d.long times cosine of the latitude. Parallels are the horizontal grid lines on a chart, spaced by the projection: equally on a gnomonic, increasingly toward the poles on a Mercator. The vertex of a great circle touches its highest parallel, and a composite great-circle track is bounded by a limiting parallel of latitude.