Why 'straight' lines are never actually straight in navigation
In short: If you steer a ship or plane on a constant bearing, you aren't necessarily traveling in a straight line. Because of the Earth's curvature, maintaining a fixed heading creates a rhumb line rather than a great circle. Understanding this distinction is the difference between efficient travel and getting lost.
In navigation, a bearing (or azimuth) is the horizontal angle between an object and a reference point like north. While it seems simple, different contexts require different systems. Absolute bearings are measured clockwise from a fixed pole—such as true north, magnetic north, or grid north. Relative bearings, however, are measured from the craft's own heading. For example, in marine navigation, 'relative' can be expressed informally using a clock face (12 o'clock being forward) or a 32-point system where each point represents 11.25 degrees.
A critical nuance exists between traveling on a great circle versus keeping a constant bearing. A great circle is the shortest distance between two points on a sphere, but following it requires constant adjustments to your heading. Conversely, maintaining a fixed steering angle creates a rhumb line. Because of this geometry, the bearing from Point A to Point B is not generally the opposite of the bearing from B back to A when traveling along a great circle. For instance, if two points in the northern hemisphere share a latitude, the path toward one might be east-northeast, while the return path appears west-northwest.
These distinctions matter for safety and precision. Navigators use absolute bearings of fixed landmarks with geometrical techniques to determine position on a chart. In contrast, relative bearings are essential for collision avoidance; by observing if the bearing of another vessel is decreasing or remaining constant, a pilot can predict whether they will pass astern or collide. Even the definition of 'bearing' has shifted over time, such as the US Army's pre-1940s system which defined it as the smallest angle to either north or south, a practice now largely abandoned for standard 360-degree azimuths.
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Sources
- Summary by Intellect · source: Bearing (navigation)
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