Refer to figure.
EQUATOR
The Equator is the intersection of the Earth's surface with the plane perpendicular to the Earth's axis of rotation. It is an imaginary line on the Earth's surface equidistant from the North Pole and South Pole that divides the Earth into a Northern Hemisphere and Southern Hemisphere.
A Great Circle is any circle that circumnavigates the Earth and passes through the center of the Earth. A Great Circle always divides the Earth in half, thus the Equator is a Great Circle (the only line of latitude which is also a Great Circle).
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