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In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, or, equivalently, the angle between its equatorial plane and orbital plane.〔 〕 It differs from orbital inclination. At an obliquity of zero, these lines point in the same direction i.e. the rotational axis is perpendicular to the orbital plane. Axial tilt differs from inclination. Because the planet Venus has an axial tilt of 177° its rotation can be considered retrograde, opposite that of most of the other planets. The north pole of Venus is "upside down" relative to its orbit. The planet Uranus has a tilt of 97°, hence it rotates "on its side", its poles being almost in the plane of its orbit.〔 Over the course of an orbit, the obliquity does not change, and the orientation of the axis remains the same relative to the background stars. This causes one pole to be directed toward the Sun on one side of the orbit, and the other pole on the other side — the cause of the seasons on the Earth. Earth's obliquity oscillates between 22.1 and 24.5 degrees on a 41,000-year cycle. It is currently 23.44 degrees and decreasing. == Standards == There are two standard methods of specifying tilt. The International Astronomical Union (IAU) defines the north pole of a planet as that which lies on Earth's north side of the invariable plane of the Solar System;〔''Explanatory Supplement 1992'', p. 384〕 under this system, Venus' tilt is 3°, and the right hand rule does not apply. NASA defines the north pole with the right hand rule,〔 which has Venus tilted 177° ("upside down"). The results are equivalent and neither system is more correct. 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「Axial tilt」の詳細全文を読む スポンサード リンク
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