What is a moon? Funny thing is, scientists haven't decided yet! You might recall the IAU or International Astronomical Union, changed the definition of what a planet is recently recently and some scientists think the definition for Moon is next. For now a moon is simply defined as an object (not manufactured), in orbit around another object.

Where do Moons come from? Some moons are caused by collisions with their planet that break off a piece of the crust. Other moons may have been created while their planet was forming or could have been passing objects, captured by their planet's gravity. (Image Right: Earth's moon, Calisto, Rhea, Mimas, Io (Original images courtesy of NASA. Edits by A.Rickhoff, Adler Planetarium))