Planets, moons and artificial satellites all stay in their circular orbits because of gravity. The force of gravity acts towards the centre of the orbit and pulls the orbiting object towards the body it circles.
The planets orbit the Sun, and moons orbit planets. Planets and moons are natural satellites, because they formed naturally. An artificial satellite is built by people and launched into orbit, usually around the Earth, for uses such as communications, weather forecasting and navigation. All of them are held in orbit by gravity, but they differ in what they orbit and how they came to be there.
(Higher tier) In a circular orbit the speed stays the same, but the velocity keeps changing. Velocity includes direction, and the direction of travel changes constantly as the object moves round the circle. The force of gravity causes this change in velocity.
(Higher tier) An orbit is only stable at one speed for each radius. If the speed of a satellite changes, the radius of its orbit must change too. A faster satellite must be in a smaller orbit, closer to the planet, where the pull of gravity is stronger.
