The Moon’s shape, a familiar circle in the night sky, is the result of a fundamental physical law. It is spherical because of the sheer power of its own gravity, a force that acts uniformly across its entire mass. This process shapes all large celestial bodies, including planets and stars. The Moon’s diameter of roughly 3,474 kilometers provides a gravitational field strong enough to overcome the internal strength of its rocky material, forcing it into the most geometrically efficient shape possible.
The Dominance of Gravity
Gravity is an attractive force that pulls every particle of matter toward every other particle. For a massive object like the Moon, this pull is directed toward a central point. As the Moon formed through the accretion of material, this radial force smoothed out irregularities.
Any bump, mountain, or irregular protrusion on the surface represents material that is slightly farther from the center of mass than the surrounding matter. Gravity continuously pulls higher material downward, effectively leveling the surface over vast periods of time. This force seeks to minimize the object’s gravitational potential energy, which is achieved when mass is distributed as closely and evenly as possible around the center. A perfect sphere is the only shape where every point on the surface is equidistant from the center, satisfying this gravitational imperative.
Achieving Hydrostatic Equilibrium
The mechanism by which gravity creates a sphere is known as hydrostatic equilibrium. This term describes a state of balance where the inward force of gravity is precisely counteracted by the outward pressure exerted by the body’s material. This balance was important during the Moon’s formation, when its interior was hot and soft, allowing material to flow and adjust.
The Moon’s self-gravity compresses its material, generating internal pressure that pushes back against the collapse. When the object’s mass is sufficient, the material behaves like a fluid over long time scales, even if it is solid rock. This behavior allows the mass to redistribute until the inward gravitational pull is equal in all directions.
Why Size Matters
Gravity’s dominance is not absolute, which explains why smaller space objects, like asteroids and comets, are often irregularly shaped. For a body to achieve hydrostatic equilibrium, its self-gravity must be strong enough to overcome the material strength, or rigidity, of its composition. Rocky bodies require a diameter of roughly 600 kilometers or more for gravity to force them into a round shape.
Smaller objects lack the necessary mass to generate sufficient gravity to crush their internal structures. Their material strength is greater than the gravitational pull, allowing them to maintain jagged edges and other non-spherical forms. The Moon, with a diameter well over 3,000 kilometers, easily surpasses this threshold, ensuring its spherical form is maintained.
Is the Moon Perfectly Round?
While the Moon is spherical, it is not a mathematically perfect sphere. Its shape is technically described as a scalene ellipsoid, meaning its three principal axes are slightly different lengths. This subtle deviation results from external and internal forces acting upon the Moon.
One minor deviation is a slight flattening at the poles, known as oblateness, caused by its rotation. This effect is minimal for the Moon compared to larger, faster-spinning planets. A more significant factor is the tidal stretching caused by Earth’s gravity, which pulls the Moon into a subtle egg shape with bulges on the near side and the far side. The measured bulges are larger than what Earth’s current pull should create, suggesting the Moon’s shape solidified billions of years ago when it orbited much closer to Earth.