Why sailors steer a curved path to travel in a straight line

Countries & Cities · Published 3 October 2026 Swipe more like this →

In short: A rhumb line allows a navigator to maintain a constant compass bearing across the globe. While it appears as a straight line on a Mercator projection map, it is actually a complex spiral that winds toward the poles, contrasting sharply with the shortest-distance path known as a great circle.

A rhumb line, also known as a loxodrome, is a path on the Earth's surface that crosses every meridian of longitude at the same constant angle. By maintaining a fixed bearing relative to true north, a navigator can steer a ship or aircraft using only a magnetic compass, provided they account for magnetic declination. This method of travel is distinct from following a great circle, which represents the shortest distance between two points on a sphere but requires constant adjustment of the steering angle.

The mathematical nature of the rhumb line was first explored by Portuguese mathematician Pedro Nunes in 1537, with further development by Thomas Harriot in the 1590s. On a Mercator projection map, a rhumb line is represented as a straight line, which is a defining feature of that specific projection. However, in reality, these paths are curves with non-zero geodesic curvature. As a traveler approaches the poles, they must turn more sharply to maintain their constant bearing, causing the path to spiral infinitely around the pole while covering a finite total distance.

The term 'rhumb' itself has a complex history, evolving from early nautical usage to its current, more precise definition as a loxodrome. While early portolan charts used lines that were often mistaken for rhumb lines, those maps lacked a formal projection. Today, the rhumb line remains a fundamental concept in navigation, particularly useful for its simplicity in steering, even if it is not the most direct route. Special cases exist where the rhumb line coincides with a great circle, such as when traveling directly north-south or along the equator.

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