Loxodrome

A loxodrome, or rhumb line, is the track that crosses every meridian at the same angle, the line of constant true course a ship steers by holding one heading. On a Mercator chart it plots as a straight line, which is why Mercator sailing works directly with it, but on the globe it spirals toward the pole and is longer than the great circle between the same two points except along the equator or a meridian. Mercator sailing and plane sailing solve the loxodrome for course and distance. The great-circle alternative is shorter but needs continuous course changes, so navigators often combine the two.