### Geostationary satellites

A geostationary satellite orbits around the equator with an orbit time of 24 hours, so that it maintains the same position above the earths surface. These satellites are essential for communication so that receivers on the Earth (like the satellite dishes on houses) can always point the same way and are always in line of sight.

Thinking of satellite orbits in terms of Newtonian mechanics, the force of gravity on the satellite in orbit will be less than at the Earth's surface. The force is given by Newton's Law of Gravity; in a formula that is GMm/r^{2}

Where M is the mass of the Earth, m the mass of the satellite, r is the height of the satellite from the **centre** of the Earth and G is the universal gravitational constant.

If we wish to put a satellite into a geostationary orbit then it must orbit in the same 24 hours (86,400 seconds) that the Earth takes to spin once on its axis. We can calculate the orbit height in calculation using either the linear speed or the angular velocity:

mv^{2}/r= GMm/r^{2} = mrω^{2} ( see notes and video on circular motion)

### Polar Satellites

As the name suggests polar satellites orbit the Earth from pole to pole, often in a low down with an orbit time of 90 minutes. These satellites are often taking regular photographs and are widely used for such things as weather forecasting, crop surveys, oceanography and of course for military spying. They can photograph the whole of the surface of the Earth in 24 hours as it rotates beneath them.

Below is a short summary video lesson.

Other pages of notes and video on astronomy which may be useful are:

**Units of distance notes and video****Measuring distance by parallax/triangulation notes and video****Life cycle of stars****Big Bang theory and evidence****Development of the Universe after the Big Bang****Real and apparent magnitude****Hubble's Law and measuring distance notes and video****The age of the universe notes and video****Using Hertzsprung Russell diagrams notes and video****Cepheid variable stars****Type 1A supernova**