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The ecliptic is the apparent path of the Sun on the celestial sphere, and is the basis for the ecliptic coordinate system. It also refers to the plane of this path, which is coplanar with the orbit of the Earth around the Sun (and hence the apparent orbit of the Sun around the Earth.)〔 〕 The path of the Sun is not normally noticeable from the Earth's surface because the Earth rotates, carrying the observer through the cycles of sunrise and sunset, obscuring the apparent motion of the Sun with respect to the stars. ==Sun's apparent motion== The motions as described above are simplifications. Due to the movement of the Earth around the Earth-Moon center of mass, the apparent path of the Sun wobbles slightly, with a period of about one month. Due to further perturbations by the other planets of the Solar System, the Earth-Moon barycenter wobbles slightly around a mean position in a complex fashion. The ecliptic is actually the apparent path of the Sun throughout the course of a year.〔 , p. 11〕 Because the Earth takes one year to make a complete revolution around the Sun, the apparent position of the Sun also takes the same length of time to make a complete circuit of the ecliptic. With slightly more than 365 days in one year, the Sun moves a little less than 1° eastward〔 every day. This small difference in the Sun's position against the stars causes any particular spot on the Earth's surface to catch up with (and stand directly north or south of) the Sun about 4 minutes later each day than it would if the Earth did not orbit; our day is 24 hours long rather than the approximately 23-hour 56-minute sidereal day. Again, this is a simplification, based on a hypothetical Earth that orbits at uniform speed around the Sun. The actual speed with which the Earth orbits the Sun varies slightly during the year, so the speed with which the Sun seems to move along the ecliptic also varies. For example, the Sun is north of the celestial equator for about 185 days of each year, and south of it for about 180 days.〔''Astronomical Almanac 2010'', sec. C〕 The variation of orbital speed accounts for part of the equation of time.〔''Explanatory Supplement'' (1992), sec. 1.233〕 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「ecliptic」の詳細全文を読む スポンサード リンク
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