A satellite is said to be in synchronous orbit about a planet if the period of the satellite’s orbit around the planet is exactly equal to the rotation period of the planet (i.e. the length of its sidereal day: one day plus one night), and the direction of the satellite’s orbit is the same direction as the rotation of the planet.
The term geosynchronous orbit is a more specific term referring to satellites in synchronous orbit around our Earth, the prefix geo-indicating this, geo being the Greek root for earth. A satellite in geosynchronous orbit will be above the exact same point, or the exact same time, each day. The semi-major axis for all geosynchronous orbits is 42,164 km. Our Moon is obviously not in synchronous, or more specifically geosynchronous orbit about the Earth. The period of its orbit around the Earth is not the same as our sidereal day; in fact, it takes the Moon about 27.3 of our days to complete one orbit of our Earth.
