Question

explain briefly about equator.

Answer 1

An equator is an imaginary line around the middle of a planet or other celestial body. It is halfway between the North Pole and the South Pole, at 0 degrees latitude. An equator divides the planet into a Northern Hemisphere and a Southern Hemisphere.

The Earth is widest at its Equator. The distance around the Earth at the Equator, its circumference, is 40,075 kilometers (24,901 miles). 

Answer 2

Here your answer...
Intersection of a spheres surface with the plane perpendicular to the spheres axis of rotation and Midway between the poles.
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