The Satellite Story (11 of 27)

The Satellite Story

In order to stay in orbit at a particular height above Earth, you have to travel at the right speed.
As you go further out, the speed has to change, and you go around Earth more slowly.
It turns out that if you get out far enough - about 22,300 miles - then the speed at which you go means the spacecraft will stay above the same point on the Earth.
The spacecraft will go around the Earth once per day, as underneath it, the Earth rotates once per day, so the spacecraft stays in the same place.
This is basically what is done with so-called geostationary satellites.

This new breed of satellite no longer disappeared over the horizon.
And there was an additional benefit.
Typically, one could use three satellites to be able to send communications to any point on Earth, because of the visibility that geostationary altitude provides.
But having solved one set of problems, engineers now faced another.
Their new high-flying satellites had a habit of not staying still.
A geostationary spacecraft, unfortunately, the orbit isn't actually stable.
This is because the sun and the moon disturb the orbit and it moves, it pulls the spacecraft off the ideal position, out of the ideal position in space.
Now, to an observer on the ground, that would...
the spacecraft then appears to move in the 24-hour period against the background.
Now, in an extreme case, that can look like the spacecraft draws a figure of eight in the sky.
That would be a real problem for people - say for direct broadcast television - people with antennas on the side of their houses.
To make them low cost, they're rigidly mounted to the wall of your house.