English: A rhumb line (blue) compared to a great-circle arc (red) between Lisbon, Portugal and Havana, Cuba. Top: orthographic projection. Bottom: Mercator projection. The rhumb line (constant compass bearing; straight line on the Mercator projection) is slightly longer than the great-circle arc (shortest distance on the sphere), but is a close enough approximation to be used in marine navigation, especially when a chart, a magnetic compass, and the stars were the only available tools for determining position and orientation of a ship.