What defines a great circle on the Earth's surface?

• A. The intersection of a plane that passes through the Earth's center with the Earth's surface.
• B. A circle of latitude at any fixed distance from the equator.
• C. A circle formed by intersecting a plane tangent to the Earth's surface.
• D. The equator only.

Answer: (A)

A great circle is formed when a plane that passes through the Earth's center cuts the sphere. The intersection is a circle whose center is the Earth's center and whose radius equals the Earth’s radius, making it the largest possible circle on the surface. This is why it represents the shortest path between two points on the globe, so navigation often uses great-circle routes for long distances. The equator is a familiar example, but any plane through the center creates one that extends from pole to pole along a continuous circle. In contrast, a circle of latitude away from the equator lies on a plane that does not include the center, so it’s a smaller circle (not a great circle). A plane tangent to the surface would touch at just one point, not generate a true circle on the surface.