If a satellite orbits Earth at an average altitude of 35,786 kilometres (22,236 miles) above sea level, it will complete an orbit every 23 hours, 56 minutes and four seconds – exactly the same time that the planet itself takes to rotate. Satellites whose orbital periods match Earth’s rotation are said to be ‘geosynchronous’. Their orbits are often used to make sure a satellite is permanently visible from a particular point on the surface (like a transmitter), or that the satellite keeps a constant watch over a certain area.
A special kind of geosynchronous orbit is a circular orbit above Earth’s equator – in this case a satellite remains above exactly the same spot on the surface, and travels in the same direction in the sky. Austrian rocket engineer Herman Potocnik discovered this ‘geostationary’ orbit in 1928, but surprisingly it was science-fiction author Arthur C Clarke who, in 1945, first pointed out how geostationary satellites could be used to bounce radio signals around the globe.
The first geosynchronous satellite, the Boeing-built Syncom 2 (pictured), finally reached orbit in 1963, and the first truly geostationary satellite, Syncom 3, followed in 1964. Today, over 200 satellites hover in geosynchronous orbits; they are mostly used for communications (eg satellite TV and telephony), though sometimes also for weather observation and reconnaissance.