Sun-synchronous orbits keep a satellite positioned so the time remains constant across any latitude, while geostationary orbits keep a satellite in the same position night or day. Sun-synchronous and geostationary orbits keep satellites in a constant position relative to the Earth.
Sun-synchronous orbits operate at 700 to 800 kilometers from Earth and can keep a satellite in either constant brightness or darkness, depending on the mission. Satellites used to study the Earth’s surface maintain constant illumination for imaging devices, while satellites used to measure radio waves or other phenomena maintain constant darkness. Sun-synchronous orbits require adjustment to maintain position over a 365-day year, given the orbit is 360 degrees and uses the Earth’s equatorial bulge to maintain position. Engineers have found that perfectly round planets, such as Venus, make sun-synchronous orbits nearly impossible to maintain.
Geostationary orbits operate at approximately 35,780 kilometers from Earth. At this distance, the satellite and the Earth orbit at the same speed, so the satellite maintains a constant position relative to an observer on Earth. Most geostationary orbits align with the equator where gravitational forces remain constant. Geostationary satellites also employ Lagrange points where the gravitational pull from the Earth and the sun are equal. Most geostationary satellites carry communication equipment, since their orbit provides coverage for a full hemisphere of the Earth.