The Moon’s shape is often called “lemon-like,” and the comparison is not as silly as it sounds. Rather than being a perfect sphere or even a simple oblate spheroid like Earth, the Moon is a lumpy triaxial body with a slight elongation pointing toward and away from our planet, a modest equatorial bulge, and a flattened profile at the poles. These deformations are tiny relative to the Moon’s overall size, amounting to just a couple of kilometers across a body roughly 1,737 kilometers in radius, but they are real and measurable. The story behind that shape pulls in frozen tidal forces, ancient giant impacts, lopsided volcanism, and a crustal asymmetry so pronounced that the Moon’s center of mass does not even line up with its geometric center.
What “Lemon-Shaped” Actually Means
When planetary scientists describe the Moon as lemon-shaped, they are talking about two features working together. First, the Moon is slightly elongated along the axis that connects it to Earth. If you could stand at the Moon’s north pole and look down, the equatorial cross-section would be a very slight oval, not a circle, with the long axis aimed at Earth. Second, there are raised regions near the equator on both the near side and far side. Combine the elongation with those equatorial bumps and the polar flattening, and you get something that, if exaggerated enormously, would look a bit like a lemon with its pointed ends aimed at and away from Earth.
The deformation is subtle. The difference between the Moon’s longest and shortest radii is on the order of a few kilometers. On a basketball-sized model, you would not feel it with your fingers. But for spacecraft engineers, geophysicists, and anyone trying to understand the Moon’s internal structure, those few kilometers matter enormously because they record billions of years of gravitational, thermal, and impact history.
The Fossil Bulge From a Closer Moon
The elongation toward Earth is the most distinctive feature of the Moon’s non-spherical shape, and it has puzzled scientists since Laplace first noted it over two centuries ago. The Moon’s present tidal-rotational bulges are significantly larger than what you would expect if the Moon were in perfect gravitational equilibrium with Earth at its current distance. In other words, the Moon bulges more than it should for where it sits right now.
The leading explanation is that these bulges are fossils. Early in its history, the Moon orbited much closer to Earth, and tidal forces stretching it toward our planet were far stronger. As the Moon’s outer shell cooled and thickened into a rigid lithosphere, it locked in those bulges. The Moon then gradually spiraled outward to its current distance, but its crust was too stiff to relax back to the shape that current tidal forces would produce. The bulges represent an equilibrium condition preserved from a time when the Moon was roughly 25 Earth-radii away, compared to about 60 Earth-radii today.1Physics of the Earth and Planetary Interiors. The lunar fossil bulge hypothesis revisited The stresses in the thickening lithosphere were what allowed the bulges to survive against hydrostatic adjustment as the Moon receded.2Geophysical Research Letters. Formation of the Lunar Fossil Bulges and Its Implication for the Early Earth and Moon
This is a genuinely elegant piece of planetary detective work. The shape of the Moon today tells you something concrete about where it was billions of years ago, because it literally could not shed the old shape fast enough.
How We Mapped the Shape So Precisely
Understanding the Moon’s shape at the level of a few kilometers of deviation required extraordinary measurement tools. Two missions in particular transformed our knowledge.
The Lunar Orbiter Laser Altimeter (LOLA), riding aboard the Lunar Reconnaissance Orbiter, has fired billions of laser pulses at the Moon’s surface and timed the reflections. By mid-2010, it had collected over two billion elevation measurements, producing the highest-resolution global topographic model of the Moon ever made, with radial accuracy of about 10 meters and spatial accuracy of roughly 100 meters relative to the Moon’s center of mass.3Geophysical Research Letters. Initial observations from the Lunar Orbiter Laser Altimeter (LOLA) That kind of precision is what lets scientists distinguish genuine shape deformations from random surface roughness.
The Gravity Recovery and Interior Laboratory (GRAIL) mission complemented LOLA by mapping what is beneath the surface. Twin spacecraft flew in formation around the Moon, and tiny changes in the distance between them revealed variations in the Moon’s gravitational pull. GRAIL constructed a gravitational field model out to spherical harmonic degree and order 420, detailed enough that over 98 percent of the gravitational signature from moderate to fine scales could be linked directly to surface topography, reflecting the preservation of crater shapes in the Moon’s highly fractured crust.4PubMed. Gravity field of the Moon from the Gravity Recovery and Interior Laboratory (GRAIL) mission Later refinements pushed this to degree and order 1200, enabling detailed maps of lateral and vertical density variations in the lunar crust.5Journal of Geophysical Research: Planets. High‐Resolution Gravity Field Models from GRAIL Data and Implications for Models of the Density Structure of the Moon’s Crust
Together, LOLA gave us the Moon’s outer shape and GRAIL gave us its internal mass distribution. Without both, the “lemon” description would be a curiosity. With both, it becomes a window into the Moon’s deep history.
The Center of Mass Is Not Where You Would Expect
One of the most telling asymmetries in the Moon’s shape is that its center of mass does not coincide with its geometric center. The offset is about 1.935 kilometers, with the center of mass shifted toward the near side.6Journal of Geophysical Research: Planets. Can the Moon’s Center of Mass–Center of Figure Offset Be Explained With a Uniform Primordial Crust? That might not sound like much, but it means the Moon is heavier on the side facing Earth. The cause is a combination of thinner, denser crust on the near side and thicker, lighter crust on the far side.
This offset is separate from the fossil bulge, though both contribute to the Moon’s overall departure from a simple sphere. The fossil bulge is an elongation along the Earth-Moon axis. The center-of-mass offset is a density imbalance between the two hemispheres. Together, they make the Moon a geometrically and gravitationally lopsided body in ways that a lemon analogy only begins to capture.
Why the Near Side and Far Side Look So Different
If you compare images of the Moon’s near side with its far side, the difference is dramatic. The near side is dominated by dark, flat maria, the ancient lava plains visible to the naked eye. The far side is almost entirely bright, cratered highlands. This is not just a cosmetic difference. The Moon shows striking geological asymmetries in elevation, crustal thickness, and composition between its two hemispheres.7Journal of Geophysical Research: Planets. Are the Moon’s Nearside‐Farside Asymmetries the Result of a Giant Impact?
The far side’s crust is substantially thicker than the near side’s, in some places by tens of kilometers. This thickness difference is one reason the far side sits higher on average, contributing to the overall non-spherical shape. It also explains why lava had a much harder time reaching the far side’s surface: to erupt, magma had to push through a much thicker barrier of solid rock.
What caused this hemispheric divide is still debated, but a compelling hypothesis involves a giant impact on the early Moon. Numerical simulations show that a large impactor striking the near side at low velocity could have formed a mega-basin and thinned the crust on that hemisphere, while the far side retained its thick primordial crust.7Journal of Geophysical Research: Planets. Are the Moon’s Nearside‐Farside Asymmetries the Result of a Giant Impact? An alternative or complementary idea proposes that a companion moon, roughly a third the size of the main Moon, collided at subsonic velocity and piled onto the far side rather than carving a crater, building up a hemispheric layer consistent with the dimensions of the farside highlands.8Nature. Forming the lunar farside highlands by accretion of a companion moon Both scenarios are physically plausible, and neither has been definitively ruled out. The truth could involve elements of both.
Giant Impacts and the Moon’s Topographic Extremes
The Moon’s shape is not just about gentle bulges and hemisphere-wide thickness differences. Individual impact basins carved enormous depressions that still dominate the topographic map. The South Pole-Aitken (SPA) basin on the far side is one of the largest and oldest impact structures in the solar system, stretching roughly 2,500 kilometers across and plunging kilometers below the surrounding terrain.
Recent simulations suggest SPA was formed by a differentiated impactor about 260 kilometers in diameter striking on a north-to-south trajectory. That angle of impact explains the basin’s distinctive tapered elliptical shape, narrowing toward the south pole. The collision dispersed lunar mantle material in the cross-range and downrange directions, with much of the mantle ejecta collapsing back into the basin interior.9PubMed Central. A southward differentiated impactor forms the tapered shape of the South Pole-Aitken impact basin on the Moon SPA alone introduces a major departure from any smooth geometric shape, and it is just the most dramatic example. The Moon is peppered with basins that locally distort the surface by kilometers.
These impact features interact with the larger-scale shape. The SPA depression sits on the far side, which is already elevated on average due to its thicker crust. The near side’s great basins, like Imbrium and Procellarum, were later flooded with lava, partially filling them in and creating the smoother terrain visible from Earth. The end result is a surface that, at every scale from global to regional, resists being described by any simple geometric shape.
Lopsided Volcanism and the Heat Beneath
The near side’s dark maria did not form randomly. Nearly all significant volcanic activity on the Moon was concentrated within a single crustal province called the Procellarum KREEP Terrane, named for its enrichment in potassium (K), rare-earth elements (REE), and phosphorus (P). This region contains extraordinarily high concentrations of radioactive uranium and thorium, roughly 300 times the levels found in primitive solar system material. That radioactive enrichment meant the mantle beneath this terrane generated far more internal heat, making partial melting and volcanic eruptions an inevitable outcome over billions of years.10Journal of Geophysical Research: Planets. The “Procellarum KREEP Terrane”: Implications for mare volcanism and lunar evolution
This concentration of heat-producing elements in one hemisphere shaped the Moon’s surface in ways that feed back into its overall form. The lava floods that filled near-side basins added dense basaltic material to the near-side crust, contributing to the mass imbalance that shifts the center of mass toward Earth. Without the KREEP Terrane’s radioactive hotspot, the near side might look much more like the far side, and the Moon’s shape would be measurably different.
Why all these heat-producing elements ended up in one place is itself a puzzle. One theory ties it to the aftermath of the magma ocean that covered the early Moon. As that ocean crystallized, the last dregs of liquid, enriched in incompatible elements like uranium and thorium, were concentrated in a single region. Whether an impact event helped funnel them there or whether it was a quirk of crystallization dynamics remains an open question.
The Moon Is Still Changing Shape
The Moon’s shape is not entirely frozen. The Moon has been slowly shrinking as its interior cools, and that global contraction produces compressional stress in the crust. The result is lobate scarps, low cliff-like features found across the surface where sections of crust have been thrust over neighboring sections along fault lines. These scarps are geologically young, some likely less than a hundred million years old, and seismic data from the Apollo era suggests some faults may still be active.
The orientations of these thrust faults are not random. When orbital recession stresses from the Moon’s continued outward spiral are combined with global contraction and daily tidal stresses from Earth, the resulting stress patterns match the observed scarp orientations. Tidal stresses at apogee, when the Moon is farthest from Earth in its monthly orbit, produce peak stresses that may help trigger slip events on active faults.11Geology. Global thrust faulting on the Moon and the influence of tidal stresses In other words, Earth’s gravity is still subtly sculpting the Moon, even if the big shape-defining events happened billions of years ago.
The shrinkage is modest in human terms, probably on the order of a hundred meters of radius reduction over the past billion years. But it means the Moon’s shape, as precisely as we can measure it today, is a snapshot of a body still in very slow motion.
Why It Matters for Getting There
The Moon’s lumpy shape has practical consequences for anyone trying to orbit or land on it. Spacecraft in lunar orbit experience gravitational tugs that vary from place to place because of the uneven mass distribution below. The near side’s mass concentrations, called mascons, associated with dense lava-filled impact basins, can perturb a low orbit enough to crash an unattended spacecraft into the surface within months. Early lunar missions discovered this the hard way, when tracking data showed unexpected orbital changes.
GRAIL’s high-resolution gravity maps were partly motivated by the need to improve spacecraft navigation. Knowing the gravitational field to fine detail allows mission planners to choose stable orbits and calculate fuel budgets for station-keeping maneuvers. For future surface missions, especially those targeting the south polar region near the SPA basin, the interplay of topography and gravity becomes even more critical. Landing sites that look flat in photographs may sit in gravitational environments that complicate descent trajectories.
The center-of-mass offset matters too. Because the Moon’s mass center is shifted toward the near side by nearly two kilometers, the gravitational “downhill” for a spacecraft is not always where the geometric surface suggests it should be. Navigation models that treat the Moon as a simple sphere, or even a simple ellipsoid, accumulate errors that grow with mission duration.
How the Moon’s Shape Compares to Other Worlds
Earth is also not a perfect sphere. It bulges at the equator due to its spin, with an equatorial radius about 21 kilometers longer than its polar radius. But Earth’s deformation follows a clean pattern driven almost entirely by its rotation rate and fluid interior. The Moon’s deformation, by contrast, is driven by a cocktail of frozen tidal forces, impact history, and internal composition that makes its shape harder to summarize with a single geometric description.
Most large moons and planets in the solar system are close to hydrostatic equilibrium, meaning their shapes are roughly what you would predict from their spin and tidal environment. The Moon is an outlier because its fossil bulge records conditions that no longer exist, and its hemispheric asymmetry reflects a violent early history that left permanent structural scars. Among well-studied solar system bodies, few carry such a visible record of their past embedded in their present shape.
So is the Moon lemon-shaped? As a first approximation, yes, the analogy captures something real about the elongation and the equatorial bumps. But a lemon is smooth and symmetric, and the Moon is neither. Its shape is the accumulated result of tidal stretching when it was young, catastrophic impacts that rearranged its crust, radioactive heating concentrated in one hemisphere, and slow contraction that continues today. The lemon comparison gets you in the door. The full story of the Moon’s shape is far stranger and more interesting than any piece of fruit.