The Moon (Luna) is Earth’s only natural satellite, and its size is a frequent point of comparison when examining the hundreds of other moons within our Solar System. With a mean diameter of approximately 3,474 kilometers, Earth’s Moon sets a significant baseline for evaluating the scale of these orbiting bodies. This diameter makes it one of the largest satellites overall, but its true distinction lies in its unique proportion to its parent planet.
The Unique Scale of Earth’s Satellite
The Moon’s relationship with Earth is often considered one of the most unusual in the Solar System. The system is sometimes informally referred to as a “double-planet” system because the Moon’s diameter is about one-quarter the diameter of our planet, a ratio of approximately 1:4. No other planet in the Solar System has a moon that is so large in proportion to its host planet.
While the “double-planet” concept is not officially recognized, the location of the barycenter, or the center of mass, is key to this unusual pairing. For the Earth-Moon system, this barycenter lies within Earth itself, which prevents its formal classification as a double-planet, unlike the Pluto-Charon system. Nevertheless, the Moon is massive enough to cause a noticeable wobble in Earth’s orbit, demonstrating its significant gravitational influence.
The Moon’s density is another distinguishing feature, measuring about 3.34 grams per cubic centimeter, which is lower than Earth’s but higher than many of the large, icy moons found in the outer Solar System. This relatively high density suggests a different internal composition, primarily of rock and metal, contrasting with the ice-rock mixtures of many other large satellites. This composition and the large size ratio to Earth make the Moon a statistical outlier among the Solar System’s satellites.
Ranking Against the Solar System’s Largest Moons
When comparing the Moon to the other major satellites in the Solar System, it ranks as the fifth largest overall. The four moons exceeding Earth’s Moon in diameter all orbit the gas giant Jupiter and the ringed planet Saturn. The largest moon is Jupiter’s Ganymede, which has a diameter of about 5,268 kilometers.
The second-largest satellite is Saturn’s Titan, with a diameter of roughly 5,150 kilometers, making it only slightly smaller than Ganymede. Jupiter’s Callisto comes in third, measuring approximately 4,817 kilometers across, and its innermost Galilean moon, Io, is the fourth largest at 3,636 kilometers. Earth’s Moon, at 3,475 kilometers, is just a few hundred kilometers smaller than Io.
Rounding out the top six is Jupiter’s Europa, with a diameter of 3,122 kilometers, and Neptune’s largest moon, Triton, at 2,705 kilometers. The largest moons are concentrated around the giant planets, where their immense gravity has allowed for the formation or capture of truly colossal satellites.
Comparing Moons to Planets and Dwarf Planets
The immense scale of the largest moons is best understood by comparing them to the Solar System’s planets and dwarf planets. Both Ganymede and Titan are so large that they exceed the size of the planet Mercury, which has a diameter of about 4,879 kilometers. Ganymede is approximately 400 kilometers wider than Mercury, making it a truly planetary-scale object.
The largest moons are also significantly larger than the dwarf planet Pluto, which has a diameter of roughly 2,374 kilometers. Ganymede is over twice Pluto’s size. Furthermore, Earth’s Moon is also substantially larger than Pluto, exceeding its diameter by over 1,000 kilometers.
In fact, six of the Solar System’s moons—Ganymede, Titan, Callisto, Io, Earth’s Moon, and Europa—are all larger than Pluto. The sheer size of these satellites demonstrates that the term “moon” does not necessarily imply a small body, but rather one that orbits a planet. Many of these satellites are complex, differentiated worlds.