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In many sci-fi movies and TV shows, orbiting spacecraft take up “stationary” positions over New York, London, or some other major landmark. That’s not possible, though. It is possible to assume a stationary position — but only if the spacecraft is above the equator.
Such a spot is called a geosynchronous orbit. It’s achieved at an altitude of about 22,000 miles. At that distance, a craft’s orbital speed carries it around Earth in exactly one day. So the craft appears to “stand still” above a single spot on the equator.
Hundreds of craft have been placed in such orbits. Most of them are communications satellites. A geosynchronous orbit allows them to stay in constant range of the same tracking stations, so they provide unbroken service. Others are weather satellites, which can keep a constant eye on the same patch of ground.
A craft can’t remain stationary away from the equator because it always orbits the center of Earth’s mass — the center of the planet itself. So if you tried to place a satellite above New York, at a latitude of about 40 degrees north, its orbit would carry it an equal distance south of the equator as well. From New York, the craft would return to the same spot in the sky at the same time every day. Between those moments, though, it would drop southward for 12 hours, then return north. So most cities don’t have to worry about alien spaceships keeping a constant watch from high overhead.
Script by Damond Benningfield