All Meridians Are Great Circles: Here's Why

Every meridian (line of longitude) on Earth is a great circle. This fundamental geographic principle arises from how meridians are defined.

The Geometry

A great circle is any circle drawn on a sphere whose plane passes through the sphere's center. Meridians are formed by the intersection of Earth's surface with planes that:

1. Pass through Earth's center
2. Contain the rotational axis (North–South Poles)

Since each meridian's defining plane cuts through Earth's center, every meridian—from the Prime Meridian (0°) to the 180th meridian—is a great circle.

Key Properties

• Maximum circumference: Great circles are the largest possible circles on a sphere (~40,075 km for Earth)
• Shortest path: The arc of a great circle between two points is the shortest distance on the sphere (geodesic)
• Polar convergence: All meridians intersect at the North and South Poles
• Right-angle intersections: Meridians cross the equator at 90°

Contrast with Parallels

Unlike meridians, only the Equator is a great circle among lines of latitude. Other parallels (e.g., Tropic of Cancer, Arctic Circle) are small circles—their planes don't pass through Earth's center.

This geometric reality underpins navigation, cartography, and understanding Earth's coordinate system.