The equator is:

• A. A line of longitude.
• B. A circle that lies entirely within the Earth.
• C. A line of longitude that passes through the poles.
• D. A great circle that is also a parallel of latitude.

Answer: (D)

The equator is a great circle that is also a parallel of latitude. A great circle is a circle on the Earth’s surface whose plane passes through the planet’s center, giving the largest possible circumference. The equator fits this perfectly, circling the globe midway between the poles and lying in a plane that cuts through the center. It’s also a zero-degree parallel of latitude, meaning it is the reference line from which north and south positions are measured. It isn’t a line of longitude, which would run from pole to pole and meet at the poles, nor is it a circle that lies inside the Earth—the equator lies on the surface and represents the intersection of the Earth with the plane perpendicular to its axis.