What best describes a rhumb line on a map?

• A. Line cutting all meridians at the same angle; line of constant direction
• B. Shortest path between two points on a sphere
• C. Line that follows every parallel
• D. Line that stays on the equator or a meridian

Answer: (A)

A rhumb line is a path that crosses every meridian at the same angle, meaning it maintains a constant bearing or direction relative to true north. This property is why such lines plot as straight lines on a Mercator map, making it convenient to steer a constant course. The shortest route between two points on a sphere is a great-circle path, which generally changes bearing along the way, so it isn’t a rhumb line. A line that follows every parallel would be a line of latitude, which does not cross meridians at a constant angle. Staying on the equator or a meridian are special cases of constant bearing, but the defining description is the constant angle to all meridians. So the best description is crossing all meridians at the same angle with a constant direction.