The idea of the Moon being shaped like a lemon is not accurate. While it appears perfectly round to the naked eye, the Moon’s shape is far more complex than a perfect sphere. Its deviations are subtle, only detectable through precise scientific measurement. The Moon’s figure deviates slightly due to immense gravitational forces and the inherent asymmetry of its internal structure, giving it a slightly non-spherical, or “lumpy,” appearance when measured accurately.
The Moon’s True Shape: A Near-Perfect Sphere
The Moon is generally categorized as a sphere, a classification given to any celestial body massive enough for its own gravity to pull it into a round shape. For a body of the Moon’s size, its self-gravity is the overwhelming force, attempting to make every point on the surface equidistant from its center. This is why small asteroids are irregularly shaped while the Moon is not.
While the Moon is visually spherical, detailed measurements show it is technically a triaxial ellipsoid, meaning it has three slightly different axes of length. The difference in the equatorial radius and the polar radius is minimal, only about a few kilometers across its diameter of approximately 3,474 kilometers. This small deviation means the Moon is, in fact, more spherical than Earth, which is an oblate spheroid noticeably flattened at the poles due to its faster rotation.
Scientists use advanced techniques like Lunar Laser Ranging (LLR) to measure the Moon’s exact shape with high precision. LLR involves bouncing laser pulses off retroreflectors placed on the lunar surface during the Apollo and Soviet-era Lunokhod missions, allowing for distance measurements accurate to within centimeters. Orbital data from missions like the Gravity Recovery and Interior Laboratory (GRAIL) are also used to map the Moon’s gravitational field, providing indirect information about its internal mass distribution and overall figure.
How Tidal Forces Distort Lunar Geometry
The primary external force responsible for the Moon’s slight deviation from a perfect sphere is the Earth’s gravity, which generates permanent tidal forces and causes the lunar material to bulge. The Moon is tidally locked with Earth, meaning it rotates on its axis at the same rate it orbits our planet, always showing us the same face.
The gravitational pull from Earth is strongest on the side of the Moon facing us, creating a permanent bulge of rock toward Earth. A second bulge forms on the opposite, or far, side of the Moon because the Earth’s gravity pulls the Moon’s center away from the far side material. This phenomenon stretches the Moon along the axis pointing toward Earth, giving it a slightly elongated, rugby-ball-like shape, which is a prolate spheroid.
This elongation is a remnant of a time billions of years ago when the Moon orbited much closer to Earth. The tidal forces were much stronger then, stretching the Moon’s figure into a larger bulge. As the Moon solidified and moved farther away, this ancient, over-sized bulge became “fossilized” in its structure, contributing to the current, measurable distortion.
Understanding Lunar Asymmetry and Mass Concentrations
Beyond the globally stretched shape caused by tidal forces, the Moon’s structure is complicated by internal asymmetry and localized mass anomalies. The Moon’s near side, the face we see, is fundamentally different from the far side, which was first photographed in 1959. This difference is most apparent in the thickness of the lunar crust.
The crust on the near side is significantly thinner than the crust on the far side, which is, on average, about 20 kilometers thicker. This crustal difference is thought to be a result of the Moon’s early formation and the thermal influence of the nearby, hot Earth, which kept the near side warmer and allowed for a thinner crust to form. The thinner crust on the near side enabled magma to flow more easily to the surface, creating the vast, dark volcanic plains known as maria.
Another factor contributing to the complex figure is the existence of Mass Concentrations, or Mascons. These are regions, primarily beneath large impact basins on the near side, that exhibit a positive gravitational anomaly. Mascons are areas of denser material, such as high-density lava flows or uplifted mantle material, that were not fully compensated after the initial impact.
The Mascons locally increase the gravitational pull, meaning the Moon’s center of mass is slightly offset from its geometric center. This internal structural imbalance causes low-flying spacecraft to have unstable orbits unless continuous adjustments are made.