A moon is considered a natural satellite because it orbits around a planet. A natural satellites is any celestial object that orbits around another larger object.
Mercury and Venus are the only planets in the solar system that do not have natural satellites. The Earth's only natural satellite is the Moon. The Moon orbits around the Earth once every 27.3 days, which is the sidereal period, but because the Earth is moving around the Sun, the synodic period is about 29.5 days. The moon is held in orbit by gravity. Objects such as planets, asteroids, and comets orbit around stars and are also considered natural satellites.