Earth’s Moon Is Exceptionally Large Compared With Its Planet

Earth’s Moon is the Solar System’s largest and heaviest moon relative to its planet, measuring about 27% of Earth’s width and 1.2% of its mass. [oai_citation:0‡NASA Space Science Data Archive](https://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html?utm_source=chatgpt.com)

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Earth’s Moon is unusual because it is enormous compared with the planet it orbits. The Moon’s average radius is about 27% of Earth’s, while its mass is about 1.23% of Earth’s mass. That ratio is much larger than the usual gap between a planet and its moons. For comparison, Ganymede is physically larger than the Moon, but Jupiter is so much more massive that Ganymede represents a far smaller fraction of its parent planet. The same general pattern holds for other major planetary moons. The distinction matters because the claim is about the Moon’s size and mass relative to a planet, not about being the biggest moon by absolute size or mass. Charon, Pluto’s largest moon, is an important comparison because Charon is roughly half Pluto’s diameter and is the largest known satellite relative to its parent body when dwarf planets are included. Because Pluto is classified as a dwarf planet, the Earth Moon system remains exceptional when the comparison is restricted to moons of the eight planets. The unusually large Moon also makes the Earth Moon system dynamically interesting. Their shared center of mass lies inside Earth rather than at Earth’s exact center, unlike the Pluto Charon pair, whose shared center of mass lies outside Pluto.

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