Some synchronised satellites trace a figure eight across the sky each day
A geosynchronous satellite circles Earth once every 23 hours, 56 minutes and 4 seconds, matching the planet's spin. Only a perfectly circular one over the equator stays fixed overhead. Tilt or stretch the orbit and the satellite still returns to the same spot daily, but loops through a figure eight along the way.
Tilting the orbit relative to the equator makes a satellite drift north and south in the sky, while stretching it into an ellipse makes it swing east and west. Ground stations following such satellites need steerable dishes. Engineers exploit this deliberately. The Tundra orbit is stretched and tilted at 63.4 degrees, an angle at which the orbit barely shifts on its own, letting a satellite linger for hours over one far northern or southern region. Japan's Quasi-Zenith Satellite System uses four satellites at a 42 degree tilt, each hanging high above Japan so signals can drop into streets between tall buildings, then racing quickly over Australia.
Even satellites meant to sit still will not stay put unaided. The solar wind, the pressure of sunlight, lumps in Earth's gravity and the pulls of the Moon and Sun all nudge them, so thrusters make regular corrections. Left alone, an orbit's tilt swings between 0 and 15 degrees over a 55-year cycle, and satellites slide toward one of two stable longitudes, 75 and 255 degrees. Near the end of a mission, when fuel runs short, operators sometimes skip the costly tilt corrections.
Getting there usually involves a transfer orbit: the rocket drops the satellite on a long ellipse whose high point reaches the final altitude, and its own engine then rounds the path out. Launch sites near the equator help, since a satellite cannot start with less tilt than the latitude it leaves from. Once in place, a spacecraft can move along the equator by briefly speeding its orbit up or slowing it down, drifting east or west.
One idea remains hypothetical: a statite, pushed by sunlight on a solar sail, that would hover over Earth's night side near 30 degrees of latitude and reappear in the same place every 24 hours.
Source: Geosynchronous orbit
