Equator is defined as which of the following?

• A. A Small Circle
• B. A Line of Longitude
• C. A Great Circle, Perpendicular to the Axis and Halfway Between the North and South Poles
• D. An Imaginary Circle Around the Equator at a Fixed Distance North or South

Answer: (C)

The equator is a great circle on the Earth's surface. It lies in a plane that passes through the planet’s center and is perpendicular to the rotation axis, so it sits exactly halfway between the North and South Poles. Because that plane goes through the center, the equator is the largest possible circle on the globe and it divides the Earth into Northern and Southern Hemispheres. This is why great-circle routes—the shortest paths between two points on the surface—often use the equator as a reference.

Why the other ideas don’t fit: a small circle is any circle on the sphere whose plane does not pass through the center, which isn’t the case for the equator. A line of longitude is a meridian—half a great circle from pole to pole—not a complete circle around the globe. A circle at a fixed distance north or south describes a circle of latitude, which are generally smaller circles; only the equator itself (0° latitude) is a great circle.