Have you ever noticed that the Moon appears larger when it’s near the horizon? Although the phenomenon is an optical illusion—known as the Moon illusion—it can make the lunar disk seem almost as big as the Earth. In reality, the Moon is just under one‑third the diameter of Earth and only about 1.2 % of its mass.
By definition, a moon is a smaller body that orbits a planet. If a satellite were more massive than the planet it orbits, the planet would instead orbit the satellite. That is why the Moon can never exceed Earth’s mass. Even so, Earth’s moon is unusually large compared to its parent, and its gravitational pull is strong enough to shape the tides that sustain life on our planet.
The only other inner‑planet’s moon is Mars, which has two tiny natural satellites. In contrast, Earth’s Moon is the largest satellite relative to its planet in the Solar System.
While a moon’s mass can’t surpass its planet’s, the ratio can be remarkably high. The most extreme example is Charon, Pluto’s largest moon. Charon is roughly half the diameter of Pluto and has a mass that, together with Pluto, creates a tidally locked pair that always shows the same face to each other.
The overall largest moon in the Solar System is Ganymede, a satellite of Jupiter. Ganymede exceeds the size of the planet Mercury and is the only known moon that possesses its own intrinsic magnetic field.
Because of their massive size, moons like Ganymede raise the possibility of “sub‑moons” or “moonmoons.” A sub‑moon would orbit a moon and remain gravitationally bound to it while still staying far enough from the planet to avoid being stripped away. While no sub‑moons have yet been confirmed, theoretical models suggest the conditions for such bodies could exist.
In short, a moon can never outsize its planet, but it can still be impressively large, as demonstrated by Charon, Ganymede, and Earth’s own Moon.
Jose A. Bernat Bacete/Getty Images
Claudio Caridi/Shutterstock
