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In orbital mechanics, mean motion (represented by ) is the angular speed required for a body to complete one revolution about the primary body, assuming constant speed in a circular orbit which completes in the same time as the variable speed, elliptical orbit of the actual body.〔 〕 While nominally a mean, and theoretically so in the case of two-body motion, in practice the mean motion is not typically an average over time. It is rather the instantaneous value which satisfies the above conditions as calculated from the current gravitational and geometric circumstances of the body's constantly-changing, perturbed orbit. ==Definition== Define the time period for the body to complete one orbit about the primary as , with dimension of time. Then in angular measure : or with dimensions of radians per unit time, degrees per unit time or revolutions per unit time.〔 〕〔 〕 The value of mean motion depends on the circumstances of the particular gravitating system. In systems with more mass, bodies will orbit faster. Bodies closer together will also orbit faster. 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「mean motion」の詳細全文を読む スポンサード リンク
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