Because it moves around a planet,earth 

The word "satellite" was in use long before being used to mean "scientific machines orbiting the Earth." It was used for the natural satellites (moons) of planets, for countries in eastern Europe that were dominated by the USSR, and for such things as smaller airport terminals separate from the main one (as in Seattle). Most people now use "satellite" to mean manmade satellites, and use the adjective "natural" when they are talking about moons, or "the Moon." The astronomical definition of a moon or satellite is a substantially smaller body that orbits a larger one. All satellites of planets also orbit the Sun at the same time, but they are considered separately from the asteroids and minor objects that do not orbit a planet but rather circle the Sun on their own.