Spy satellites travel at a velocity high enough that they’re in free fall. They’re constantly falling towards the Earth but at the same rate that the curvature of the Earth is moving away from them.
Communication satellites are no different. The nuance is to understand relative motion. The cosmos is rotating, not the Earth. A communication satellite isn’t stationary in space, it’s traveling through space opposite the direction of the rotation of the cosmos at a velocity equal to the cosmos at its orbital radius.
“Geostationary orbits (GEO) are used for tv, and communication satellites. … To maintain an orbit that is 22,223 miles (35,786 km) above Earth, the satellite must orbit at a speed of about 7,000 mph (11,300 kph). That orbital speed and distance permits the satellite to make one revolution in 24 hours.”– Space 13 Things, Brown University
The principle is the same, it has to maintain velocity so that it’s constantly in free fall but the curvature of the Earth means that it never hits the ground, it enters a free fall orbit.
- The Earth is completely stationary, no daily rotation.
- The sun rises and sets every day,
- the stars arc across the sky every night,
- Obviously the cosmos is rotating.
- The communication satellite has to travel at a very high velocity in order to
- be in free-fall orbit and
- appear to be stationary.