Earth looks perfectly round in images from space, but it’s really a changing, uneven geoid. The planet’s spin makes the equatorial region bulge, so its basic shape sits closer to an ellipsoid than a sphere.
An ellipsoid describes Earth’s basic geometry, not the outline of its surface. The planet’s rotation creates centrifugal force around the equator, widening it, while vast variations in terrain further distort that basic shape. Mountain peaks rise almost 9,000 meters above sea level, and ocean trenches plunge more than 11 kilometers.
Even the open ocean surface isn’t level. Variations in Earth’s gravitational field create persistent rises and dips in the water, with height differences of more than 100 meters.
Earth’s shape changes over time, too. Daily tidal movements in the seas and solid crust make the surface rise and fall periodically. Slower processes reshape the planet over thousands of years, including tectonic plate movement and the crust’s rebound after ice sheets melt. Earthquakes, volcanic eruptions and meteorite impacts can alter the terrain suddenly and dramatically.
Geodesy helps scientists track these changes and accurately determine the positions of points on Earth’s surface. Its specialists, including researchers at the U.S. Geological Survey, continually monitor Earth’s physical framework, dimensions and gravitational field to pinpoint surface locations.
