σελήνη

The Moon is the natural companion of the planet Earth.

In general, astrophysics defines a moon as a natural satellit of a planet, dwarf planet or asteroid. It is a celestial body that orbits a non-fusor. A fusor is star., i.e. a celestial body that creates its own energy by performing nuclear fusion.

In the solar system, moons are found orbiting the planets Mars, Jupiter, Saturn, Uranus and Neptun and some asteroids (like the Ida-Dactyl pair). Small bodies orbitting other small bodies sometimes cause scientific debates in term of classification because due to the laws of gravitation, the "moon" and its central body in fact orbit a common center. For instance, dwarf planet Pluto has one big companion (Charon) and two smaller ones. Thus, before 2006, it had been discussed to define a mass ration limit for moons with respect to the object that they orbit or to call the Pluto-Charon system a double planet. This could have led to the absurd situation that the Earth's moon (that is called The Moon) would have called a planet or dwarf planet because it is rather big (a third in size and a tenth in mass) with regard to its planet. Hence, there is no specific limit.