Great Circle Route
A Great Circle Route is the shortest path between two points on the globe, lying along a great circle — a circle whose plane passes through the centre of the earth. On long east-west passages in high latitudes it can be significantly shorter than a straight line on the common Mercator chart.
The trade-off is that a great circle crosses each meridian at a different angle, so the ship must change course continually; in practice the route is sailed as a series of straight rhumb-line legs between waypoints that approximate the great circle. Voyage-planning and weather-routing systems compute the optimum track automatically.
On a long ocean passage, sailing the great circle instead of the apparent straight line on the chart can cut hundreds of miles — saving days and fuel. Choosing and adjusting the route, balanced against weather, is a direct lever on voyage cost and ETA.
What is the difference between a great circle and a rhumb line?
A great circle is the shortest distance between two points but requires constant course changes; a rhumb line is a track of constant course, longer but simpler to steer. Long passages approximate a great circle with rhumb-line legs.