Which property distinguishes great circles from other circles on Earth?

• A. They result from planes that pass through Earth's center.
• B. They are always located at the equator.
• C. They have smaller radii than Earth.
• D. They are always perpendicular to the equator.

Answer: (A)

A great circle comes from intersecting the Earth with a plane that passes through the planet’s center. This geometry makes the circle’s radius equal to Earth's radius and places its center at Earth’s center, so it’s the largest circle you can draw on the sphere. That property—the plane through the center—is what sets great circles apart from all other circles on Earth, which come from planes that don’t cross the center and have smaller radii. The equator is one example of a great circle, but many others exist (such as meridians). The statements that a great circle must lie at the equator, have a smaller radius, or be always perpendicular to the equator aren’t correct for all great circles.