The German astronomer Johannes Kepler (1571–1630) formulated three laws of planetary motion that also apply to the motion of satellites around Earth. According to Kepler's third law, the orbital period of a satellite is proportional to its distance from Earth. Satellites in low orbits, altitudes of a few hundred to a thousand kilometers, have orbital periods less than two hours; in contrast, the moon, at an altitude of about 380,000 kilometers, has an orbital period of about 27 days.
Between these two extremes is an altitude that corresponds to an orbital period of one day. A satellite in a circular orbit at such an altitude revolves around Earth at the same speed as Earth's rotation. This altitude is 35,787 kilometers, and the orbit is called a synchronous or geosynchronous orbit.
If the orbital plane of a satellite is not coincident with Earth's equatorial plane, then the orbit is said to be inclined, and the angle between the orbital plane and the equatorial plane is known as the orbit's inclination. In a geosynchronous orbit, the point on the Earth directly below the satellite moves north and south in a narrow figure-eight pattern with northern and southern latitude limits corresponding to the inclination.
If the inclination of a geosynchronous orbit is zero (or near zero), then the satellite remains fixed (or approximately fixed) over one point on the equator. Such an orbit is known as a geostationary orbit. An advantage of the geostationary orbit is that antennas on the ground, once aimed at the satellite, need not continue to rotate. Another advantage is that a satellite in this type of orbit continuously sees about one-third of Earth.
One disadvantage of the geostationary orbit is that the gravity of the sun and moon disturb the orbit, causing the inclination to increase. The satellite's propulsion can counter this disturbance, but since the amount of fuel a satellite can carry is limited, increased inclination may remain a problem in some scenarios. The geostationary orbit's finite capacity is another disadvantage; satellites using the same frequencies must be separated to prevent mutual interference.