Earth’s Moon ranks as the fifth-largest moon in the solar system, with a diameter of about 3,474 km. That alone would make it a respectable world, but what truly sets it apart is its size relative to the planet it orbits. The Moon’s radius is roughly 27% of Earth’s, a ratio unmatched by any other major planet-moon pair. That proportional heft has shaped everything from our planet’s climate stability to the way scientists think about moon formation.
Where the Moon Ranks Among the Solar System’s Giant Moons
Seven moons in our solar system are larger than about 2,500 km across, and they dominate any size comparison. Ganymede, orbiting Jupiter, takes the top spot at roughly 5,268 km in diameter, making it larger than the planet Mercury. Saturn’s Titan comes in second at about 5,150 km, followed by Jupiter’s Callisto at around 4,820 km and Io at approximately 3,643 km. Earth’s Moon, at 3,474 km, sits just below Io. Europa, the smallest of Jupiter’s four large Galilean moons, comes in sixth at about 3,122 km. Neptune’s Triton, widely believed to be a captured object from the outer solar system, rounds out the major moons at around 2,707 km.
So the Moon is not the biggest, but it is firmly in the top tier. It is larger than Europa, significantly larger than Triton, and roughly two-thirds the diameter of Ganymede. For a moon orbiting a relatively small rocky planet rather than a gas giant, that is remarkable.
The Size Ratio That Makes the Moon Special
Raw diameter only tells part of the story. What makes the Earth-Moon system genuinely unusual is how large the Moon is compared to its host planet. The Moon-to-Earth radius ratio is 0.272, meaning the Moon is just over a quarter the width of Earth.1The Astrophysical Journal Letters. Worlds without Moons: Exomoon Constraints for Compact Planetary Systems No other major planet in the solar system comes close to that proportion. Ganymede is enormous in absolute terms, but Jupiter is so much bigger that Ganymede’s radius is less than 4% of its planet’s. Titan reaches only about 4.4% of Saturn’s radius. Even Triton, orbiting the much smaller Neptune, manages just 5.5%.
The only pairing in the solar system with a more extreme ratio is Pluto and its moon Charon. Charon’s diameter is roughly half of Pluto’s, which is part of why the two are sometimes described as a binary system rather than a planet and moon. But Pluto is a dwarf planet, so among the eight major planets, nothing matches the Earth-Moon relationship.
This ratio matters because it means the Moon exerts a gravitational influence on Earth that is disproportionately strong for a satellite. The tidal forces between the two bodies are powerful enough to slow Earth’s rotation, drive ocean tides, and even keep Earth’s axial tilt from wandering chaotically over millions of years.
The Tiny End of the Spectrum
At the opposite extreme from Ganymede and Titan, most moons in the solar system are small, irregularly shaped chunks of rock or ice. Mars has two of the best-known examples: Phobos, with a mean radius of about 11 km, and Deimos, with a mean radius of about 6 km. Deimos is roughly 56% the size of Phobos, and both are so small that their gravity is too weak to pull them into a spherical shape.2Monthly Notices of the Royal Astronomical Society. Phobos and Deimos surface composition: search for spectroscopic analogues Phobos is dominated by a crater called Stickney that is 9 km across, nearly as wide as the entire moon itself.
Jupiter alone has more than 90 known moons, the vast majority of which are small irregular bodies just a few kilometers across. Saturn’s count is similar. Many of these were likely captured asteroids or fragments from ancient collisions. They are worlds in name only, more like flying rubble piles than the rounded, geologically active bodies we picture when we hear the word “moon.”
Earth’s Moon, at 3,474 km in diameter, is roughly 300 times wider than Phobos. If you placed Phobos on a map of the Moon, it would fit comfortably inside a single large lunar crater. That contrast underscores how unusual it is for a rocky inner planet to have a moon this big. Mars ended up with pebbles; Earth ended up with a world.
Why the Moon Is So Unusually Large
The leading explanation for the Moon’s size is the giant impact hypothesis. According to the standard version of this model, an object roughly the size of Mars struck the early Earth, blasting an enormous amount of debris into orbit. That debris disk eventually coalesced into the Moon.3Space: Science & Technology. Research Advances in the Giant Impact Hypothesis of Moon Formation No other inner planet experienced a collision of quite this scale and geometry, which helps explain why Venus, Mars, and Mercury lack anything comparable.
One long-standing puzzle with this theory was compositional. Early simulations suggested the debris disk should consist mostly of material from the impactor, which would give the Moon a chemical fingerprint distinct from Earth’s. But actual measurements show the Moon and Earth are strikingly similar in composition. More recent simulations involving larger impactors have resolved this discrepancy by showing that a bigger collision can mix the materials more thoroughly, producing a disk with essentially the same makeup as Earth’s mantle.4PubMed Central. Forming a Moon with an Earth-like composition via a giant impact
The moons of the outer planets formed differently. Jupiter’s Galilean satellites likely accreted within a disk of gas and dust surrounding the young Jupiter, in a process that echoed planet formation around the Sun in miniature. Triton, by contrast, is thought to have formed elsewhere in the Kuiper Belt and was later captured by Neptune’s gravity.5Icarus. Reassessing the origin of Triton These different formation pathways produced moons with very different compositions, densities, and internal structures.
What Is Inside the Big Moons
Size comparisons get more interesting when you look beneath the surface. The Moon is dense and rocky, with a relatively small metallic core. Estimates place the iron core at about 310 to 320 km in radius, or roughly 430 to 440 km if the core contains a significant iron-sulfide component. That gives the Moon the lowest total iron-to-silicon ratio of any terrestrial planet, chondrite meteorite, or outer solar system satellite that has been studied.6Icarus. Core Sizes and Internal Structure of Earth’s and Jupiter’s Satellites In other words, the Moon is iron-poor compared to almost everything else in the solar system, which is consistent with it forming mostly from Earth’s rocky mantle material rather than from deep iron-rich layers.
The Galilean moons tell a very different internal story. Io, despite being slightly larger than the Moon, has a much bigger iron core, estimated at 590 to 630 km in radius. Europa’s iron core spans roughly 420 to 510 km, and Ganymede’s reaches 580 to 650 km.6Icarus. Core Sizes and Internal Structure of Earth’s and Jupiter’s Satellites These large metallic cores may be capable of generating magnetic fields, and Ganymede is in fact the only moon known to have its own intrinsic magnetosphere.
Then there are the icy moons, where the internal picture is radically different. Titan, Enceladus, Dione, and Ganymede (which has ice layers atop its rocky interior) contain substantial water-ice envelopes. Their bulk compositions include significant fractions of volatile materials, with rock-and-organic cores overlain by water or ice shells that can be hundreds of kilometers thick.7Earth and Planetary Science Letters. Carbon-rich icy moons and dwarf planets Earth’s Moon, by contrast, is bone-dry by comparison. It has no ice shell, no subsurface ocean, and only trace amounts of water locked in minerals. Two moons can be similar in diameter yet be fundamentally different kinds of worlds.
How the Moon’s Size Shapes Life on Earth
A moon’s size is not just a matter of astronomical trivia. The Moon’s gravitational pull has concrete consequences for Earth’s habitability. One of the most significant is its stabilizing effect on Earth’s axial tilt, or obliquity. Earth’s tilt currently sits at about 23.4 degrees and varies only modestly over tens of thousands of years, which keeps seasonal temperature swings within a range that complex life can handle.
Without the Moon, the picture would be dramatically different. Modeling has shown that without a large satellite, the gravitational tugs of Jupiter and other planets would push Earth’s obliquity into a chaotic zone ranging from nearly 0 degrees up to about 85 degrees.8Nature. Stabilization of the Earth’s obliquity by the Moon A planet tipped at 85 degrees would essentially roll along its orbit like a ball, with each pole alternately baked by direct sunlight and plunged into months of darkness. The climate swings would be extreme. In this sense, the Moon acts as a climate regulator, and it can do so precisely because it is massive enough for its gravitational pull to matter.
Mars provides a cautionary comparison. Its two moons are far too small to stabilize anything. Mars’s obliquity has varied wildly over geological time, swinging by tens of degrees, which likely contributed to the loss of surface habitability. If Earth had ended up with Phobos-sized companions instead of a world nearly 3,500 km across, our climate history could look very different.
Moons We Have Not Seen Yet
The search for moons around planets beyond our solar system, known as exomoons, is still in its early stages. No exomoon has been conclusively confirmed, though a handful of candidates have been proposed. The difficulty is partly one of detection: a moon produces subtle shifts in the timing and duration of its planet’s transit across the host star, and these signals are small. The Earth’s Moon, for example, has a radius ratio of 0.272 relative to Earth, which means it would block only about 7.4% as much starlight as the planet itself during a transit.1The Astrophysical Journal Letters. Worlds without Moons: Exomoon Constraints for Compact Planetary Systems That is detectable with precise instruments, but moons much smaller than ours would be extraordinarily hard to spot.
Theoretical work suggests that moons in the range of 0.1 to 0.5 Earth masses could form naturally within circumplanetary debris disks or through capture from binary systems, and that such moons could be habitable and detectable with current technology.9PubMed Central. Formation, habitability, and detection of extrasolar moons For reference, Earth’s Moon is only about 0.012 Earth masses, well below that range. So if moons around gas giants in other solar systems formed in conditions anything like Jupiter’s or Saturn’s, some of them could dwarf our Moon several times over and potentially host conditions friendly to life.
The fact that researchers are already thinking about habitable exomoons is a direct consequence of what we have learned from our own solar system. Enceladus, with its subsurface ocean and hydrothermal vents, is only about 500 km in diameter. Europa, at roughly 3,100 km, is considered one of the best candidates for extraterrestrial life in our neighborhood. Size clearly matters for a moon’s geological activity and ability to hold onto an atmosphere or internal ocean, but even modestly sized moons have turned out to be far more geologically alive than anyone expected a few decades ago.
The Moons That Defy Easy Categories
Not every moon fits neatly into the “giant world” or “tiny rock” bins. Triton is an instructive example. At about 2,707 km in diameter, it is smaller than the Moon, yet it has a thin nitrogen atmosphere, active geysers, and a surface that appears geologically young. Its status as a likely captured Kuiper Belt object means it has more in common, compositionally and historically, with Pluto than with the Galilean moons or our own Moon.5Icarus. Reassessing the origin of Triton Triton is smaller than the Moon in diameter but arguably more geologically interesting right now, a reminder that size alone does not determine how active or complex a world can be.
Saturn’s moon Titan offers another case where size comparisons only scratch the surface. Titan is larger than the Moon and even larger than Mercury, with a diameter of about 5,150 km. But its thick nitrogen atmosphere, methane lakes, and organic-rich surface chemistry make it an entirely alien kind of world. You could put the Moon and Titan side by side, note that Titan is roughly 48% wider, and still not capture the most important differences between them. Titan has weather. It has river valleys carved by liquid methane. The Moon is an airless, nearly pristine record of the inner solar system’s bombardment history. Both are fascinating, but for completely different reasons.
Even among Jupiter’s moons, the contrasts are stark. Io is only slightly larger than Earth’s Moon, yet it is the most volcanically active body in the solar system, its surface constantly reshaped by sulfur-rich eruptions driven by tidal heating from Jupiter. Europa, slightly smaller than the Moon, hides a global saltwater ocean under kilometers of ice. Ganymede, the largest moon anywhere, has its own magnetic field and a layered interior of metal, rock, and ice. The Moon, sitting in the middle of this size range, is geologically quiet by comparison. Its volcanism shut down billions of years ago, and it lacks the tidal energy sources that keep the outer solar system’s moons active. What it does have, uniquely, is a gravitational relationship with its planet that has shaped the development of life on a neighboring world in ways no other moon in the solar system can claim.