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A geosynchronous orbit (sometimes abbreviated GSO) is an orbit around the Earth with an orbital period of one sidereal day, intentionally matching the Earth's sidereal rotation period (approximately 23 hours 56 minutes and 4 seconds).〔V. Chobotov, ed., (1996) ''Orbital Mechanics, 2nd edition'', AIAA Education Series, p. 304.〕 The synchronization of rotation and orbital period means that, for an observer on the surface of the Earth, an object in geosynchronous orbit returns to exactly the same position in the sky after a period of one sidereal day. Over the course of a day, the object's position in the sky traces out a path, typically in a figure-8 form, whose precise characteristics depend on the orbit's inclination and eccentricity. A special case of geosynchronous orbit is the geostationary orbit, which is a circular geosynchronous orbit at zero inclination (that is, directly above the equator). A satellite in a geostationary orbit appears stationary, always at the same point in the sky, to ground observers. Popularly or loosely, the term "geosynchronous" may be used to mean geostationary.〔C. D. Brown (1998), ''Spacecraft Mission Design, 2nd Edition,'' AIAA Education Series, p. 81〕 Specifically, geosynchronous Earth orbit (GEO) may be a synonym for ''geosynchronous equatorial orbit'', or ''geostationary Earth orbit''.〔 Communications satellites are often given geostationary orbits, or close to geostationary, so that the satellite antennas that communicate with them do not have to move, but can be pointed permanently at the fixed location in the sky where the satellite appears. A semi-synchronous orbit has an orbital period of 1/2 sidereal day, i.e., 11 h 58 min. Relative to the Earth's surface it has twice this period, and hence appears to go around the Earth once every day. Examples include the Molniya orbit and the orbits of the satellites in the Global Positioning System. == Orbital characteristics == Circular Earth geosynchronous orbits have a radius of . All Earth geosynchronous orbits, whether circular or elliptical, have the same semi-major axis.〔 〕 In fact, orbits with the same period always share the same semi-major axis: : where ''a'' is the semi-major axis, ''P'' is the orbital period, and ''μ'' is the geocentric gravitational constant, equal to 398600.4418 km3/s2. In the special case of a geostationary orbit, the ground track of a satellite is a single point on the equator. In the general case of a geosynchronous orbit with a non-zero inclination or eccentricity, the ground track is a more or less distorted figure-eight, returning to the same places once per sidereal day. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Geosynchronous orbit」の詳細全文を読む スポンサード リンク
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