Planets are not perfect spheres. Their true geometric shape is an oblate spheroid, meaning the object is slightly flattened along its axis of rotation and bulges around its middle. This deviation from perfect roundness results from two competing forces acting on a massive, rotating body.
The Ideal Shape Driven by Gravity
Gravity pulls all of a planet’s massive material toward a central point. This immense, inward-pulling force minimizes the total gravitational potential energy of the body, and the sphere is the single shape that achieves this minimum energy state.
When a body is massive enough, gravity overcomes the structural strength of its material, causing it to behave like a fluid over long periods. This state is hydrostatic equilibrium: the balance between the inward force of gravity and the outward pressure of the compressed material. Gravity pulls down any significant irregularity until the body settles into a generally spherical form. This is why small asteroids are often potato-shaped, as their weak gravity cannot overcome the rock’s rigid strength, while all planets are round.
The Effect of Planetary Rotation
While gravity strives for a perfect sphere, rotation prevents this ideal. As a planet spins, it introduces the centrifugal force, which acts perpendicular to the axis of rotation and pushes material outward. Since rotational speed is greatest at the equator, the outward push is strongest there.
This centrifugal force partially counteracts the inward pull of gravity near the equator, causing material to accumulate and form a bulge. The faster a planet spins, the greater the centrifugal force and the more pronounced this equatorial widening becomes. This deforms the planet’s structure, making its diameter across the equator distinctly larger than its diameter measured from pole to pole.
Measuring the Equatorial Bulge
The resulting shape is defined by oblateness or flattening, a measurable quantity that compares the polar radius to the equatorial radius.
Earth
Earth rotates relatively slowly and has a modest equatorial bulge. Its equatorial diameter is approximately 43 kilometers greater than its polar diameter, a difference of about 0.3%. This means a person standing at sea level on the equator is 21 kilometers farther from Earth’s center than a person standing at the North Pole.
Other Planets
The magnitude of the bulge varies significantly across the solar system, depending on the rotation rate. Venus rotates extremely slowly, taking 243 Earth days to complete one turn, and is nearly a perfect sphere showing almost no measurable flattening. In contrast, the gas giant Jupiter spins very rapidly, completing a rotation in less than 10 hours. This fast spin results in a severe equatorial bulge, making its diameter across the equator approximately 10% larger than its diameter across the poles.