The geostationary satellite
 
An orbit of particular interest for an artificial satellite is a circular one at a height above the Earth’s surface of some 35 000 km.
Every orbit has its own period of revolution, T . For a circular orbit of an approximate height of 35 000 km, the period is 23^h 56^m, in other words 1 sidereal day. A satellite placed in a circular equatorial orbit at such a height will, therefore, remain above a particular point on the Earth’s equator. Such geostationary satellites are used for communications purposes, relaying radio and television signals over most of the Earth. These now numerous satellites are said to occupy the Clarke belt, so named after the science fiction writer, Arthur C Clarke, who first suggested the use of stationary satellites for communication purposes. It may be noted that because of the relative proximity of stationary satellites, their apparent position in the sky suffers from a substantial parallax.