One orbit, with a radius of 42,000 kilometers, has a period...

One orbit, with a radius of 42,000 kilometers, has a period of exactly 24 hours. A body in such an orbit, if its plane coincided with that of the Earth's equator, would revolve with the Earth and would thus be stationary above the same spot on the planet. It would remain fixed in the sky of a whole hemisphere ... [to] provide coverage to half the globe, and for a world service three would be required, though more could be readily utilized. (1945) [Predidicting geosynchronous communication satellites]