Orbit means travel round, and a moon, planet or other space object may be held within a larger space object’s gravitational field and orbit it.
Orbits may be circular, elliptical (oval) or parabolic (conical). The orbits of the planets are elliptical.
An orbiting space object is called a satellite.
The biggest-known orbits are those of the stars in the Milky Way, which can take 200 million or more years.
Momentum is what keeps a satellite moving in space. How much momentum a satellite has depends on its mass and its speed.
A satellite orbits at the height where its momentum exactly balances the pull of the larger object’s gravity.
If the gravitational pull is greater than a satellite’s momentum, it falls in towards the larger space object.
If a satellite’s momentum is greater than the pull of the larger object’s gravity, it flies off into space.
The lower a satellite orbits, the faster it must travel to stop it falling in towards the larger space object.
Geostationary orbit for one of Earth’s artificial satellites is 35,786 km over the Equator. At this height, it must travel around 11,000 km/h to complete its orbit in 24 hours. Since Earth also takes 24 hours to rotate, the satellite spins with it and so stays in the same place over the Equator.