What does "constantly changing track direction" refer to in navigation?

"Constantly changing track direction" in navigation refers to a great circle. When navigating on the surface of a sphere, like the Earth, a great circle is the shortest path between two points. This path continuously alters its directional bearing because it curves as it follows the curvature of the Earth.

For instance, when flying or sailing along a great circle route, a navigator must frequently adjust their heading to maintain the intended course, as the direction changes relative to a fixed point like the compass rose. This practice is essential for efficient travel over long distances, particularly in aviation and maritime contexts.

In contrast, other options refer to different concepts in navigation. Parallels of latitude are lines that run horizontally around the globe and remain constant in direction, while rhumb lines may appear straight on a map but do not provide the shortest distance between points because they maintain a constant compass direction, leading to direction shifts in actual navigation. The equator is a specific parallel of latitude that represents the midpoint of the Earth, but it does not embody the concept of constantly changing track direction as described in the question.