Geosynchronous

Geosynchronous describes an orbital period that matches the rotation of the body it orbits. For Earth, a geosynchronous orbit has a sidereal period of about 23 hours 56 minutes. A satellite in such an orbit does not have to stay fixed in the sky; its ground track depends on the orbit’s inclination and eccentricity.

The height required for a geosynchronous orbit around Earth is about 35,786 kilometers (22,236 miles) above the equator, corresponding to a radius of roughly 42,164 kilometers (26,199 miles) from Earth’s center for a circular orbit. If the orbit is perfectly circular and equatorial (inclination of 0 degrees), the satellite remains fixed above a single point on the equator, an arrangement commonly called a geostationary orbit. With any nonzero inclination, the satellite appears to move in the sky, tracing a three- or eight-shaped path over the course of a day, which makes maintaining a fixed ground position more complex.

Geosynchronous orbits are widely used for communications and meteorology because their constant orbital period allows ground antennas to stay directed toward a fixed region of the Earth. Geostationary orbits are a subset of geosynchronous orbits, offering a stationary position relative to the Earth's surface. In practice, natural and solar-system perturbations, along with solar radiation pressure, cause gradual drift in inclination and longitude, necessitating periodic station-keeping maneuvers to maintain the desired position.

The concept has historical significance; the first geosynchronous satellite was successfully launched in the early 1960s, validating the idea that a satellite could stay aligned with Earth’s rotation.