Definition
A moon is a natural satellite, meaning a celestial body that orbits a planet, a dwarf planet, or another non-massive stellar object. Unlike planets, moons do not orbit a star directly but revolve around a body that itself orbits a star. They may exist singly or in large numbers around a planet.
Structure
The internal structure of a moon depends on its mass and composition. Some moons are differentiated with a metallic core, rocky mantle, and outer crust, while others are made of ice, porous rock, or a mixture. Some show geological or cryovolcanic activity, others are geologically inactive.
Origin
Moons may form through accretion in a circumplanetary disk, by gravitational capture of a wandering body, or by fragmentation after a collision. Their origin influences their composition, orbital tilt, and interaction with the central object.
Dynamic Functions
Moons strongly affect the dynamic state of their host planet. They can stabilize axial tilt, cause tides in fluid envelopes (oceans, atmospheres), and slow the planet’s rotation through tidal braking. They may also interact with rings or other satellites in complex systems.
Variation
The number, size, and properties of moons vary greatly between planets. Some moons are larger than planets, others are irregular fragments. Their distribution reflects the dynamic history of the planetary system they belong to.