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The equatorial coordinate system is a widely used celestial coordinate system used to specify the positions of celestial objects. It may be implemented in spherical or rectangular coordinates, both defined by an origin at the center of the Earth, a fundamental plane consisting of the projection of the Earth's equator onto the celestial sphere (forming the celestial equator), a primary direction towards the vernal equinox, and a right-handed convention.〔 〕〔 〕 The origin at the center of the Earth means the coordinates are ''geocentric'', that is, as seen from the center of the Earth as if it were transparent. The fundamental plane and the primary direction mean that the coordinate system, while aligned with the Earth's equator and pole, does not rotate with the Earth, but remains relatively fixed against the background stars. A right-handed convention means that coordinates are positive toward the north and toward the east in the fundamental plane. == Primary direction == This description of the orientation of the reference frame is somewhat simplified; the orientation is not quite fixed. A slow motion of Earth's axis, precession, causes a slow, continuous turning of the coordinate system westward about the poles of the ecliptic, completing one circuit in about 26,000 years. Superimposed on this is a smaller motion of the ecliptic, and a small oscillation of the Earth's axis, nutation.〔 ''Explanatory Supplement'' (1961), pp. 20, 28〕 In order to fix the exact primary direction, these motions necessitate the specification of the equinox of a particular date, known as an epoch, when giving a position. The three most commonly used are: *Mean equinox of (a standard epoch, usually J2000.0, but may include B1950.0, B1900.0, etc.) :is a fixed standard direction, allowing positions established at various dates to be compared directly. *Mean equinox of date :is the intersection of the ecliptic of "date" (that is, the ecliptic in its position at "date") with the ''mean'' equator (that is, the equator rotated by precession to its position at "date", but free from the small periodic oscillations of nutation). Commonly used in planetary orbit calculation. *True equinox of date :is the intersection of the ecliptic of "date" with the ''true'' equator (that is, the mean equator plus nutation). This is the actual intersection of the two planes at any particular moment, with all motions accounted for. A position in the equatorial coordinate system is thus typically specified ''true equinox and equator of date'', ''mean equinox and equator of J2000.0'', or similar. Note that there is no "mean ecliptic", as the ecliptic is not subject to small periodic oscillations.〔 〕 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「Equatorial coordinate system」の詳細全文を読む スポンサード リンク
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