The Shape of the Earth Oblate Spheroid

Blue Marble The Earth is not the “Blue Marble” like astronauts dubbed it. Rather the shape of the Earth is an Oblate Spheroid a sphere that is squashed at its poles and swollen at the equator.

Earth’s Distances The equatorial circumference is about 40,076 km. The polar circumference is slightly shorter at 40,008 km. The radius of Earth's center to sea level is roughly 21 kilometers (13 miles) greater at the equator than at the poles.

Why is it shaped like this? The spinning Earth has a bit of plasticity that allows the shape to deform very slightly. Our globe, however, is not even a perfect oblate spheroid, because mass is distributed unevenly within the planet. The greater a concentration of mass is, the stronger its gravitational pull, creating bumps around the globe.

The earth’s bulge Equatorial bulge.

Earth’s Changing Shape Earth's shape also changes over time. Mass shifts around inside the planet, altering those gravitational anomalies. Mountains and valleys emerge and disappear due to plate tectonics. The gravitational pull of the moon and sun causes tides as well.

Stabilizing Earth To even out Earth's imbalanced distribution of mass and stabilize its spin, the entire surface of the Earth will rotate and try to redistribute mass along the equator, a process called true polar wandering.

True Polar Wondering Cases of true polar wander have occurred several times in the course of the Earth's history. The speed of rotation is limited to about 1° per million years. http://earthsky.org/earth/earth-is-undergoing-true-polar-wander-scientists-say