When viewed from a distance, the earth looks like a sphere, but it is actually an oblate spheroid — a sphere compressed along the polar axis and bulging slightly along the equator.
A cross-section through the poles gives an ellipse rather than a circle. The equator, positioned midway between the poles, is the largest possible circumference of the spheroid. According to the International Ellipsoid of Reference, the polar diameter is 7899.98 miles and the equatorial diameter is 7926.68 miles.
Earth's oblateness, as expressed by flattening at the poles, is therefore 26.70 miles or 26.70/7927=.0033682, a fraction slightly greater than 1/297.