Television and weather satellites in geostationary orbit appear to hang motionless above the equator. How many kilometres above Earth's surface do they fly?
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Answer
About 36,000 km
The idea was popularised in 1945 by science fiction writer Arthur C. Clarke, more than a decade before the first satellite of any kind was launched.
How to estimate it
Use Kepler's third law: the square of the period grows as the cube of the orbit's radius. The space station orbits at a radius of 6,800 km in 1.5 hours. A geostationary satellite needs 24 hours, 16 times longer, so its radius is 16^(2/3) = 6.35 times larger: 43,200 km from Earth's centre. Subtract Earth's radius of 6,400 km: 36,800 km. The exact altitude is about 36,000 km.
Source: NASA; ESA
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