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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Orbital state vectors ： ウィキペディア英語版 Orbital state vectors In astrodynamics and celestial dynamics, the orbital state vectors (sometimes state vectors) of an orbit are cartesian vectors of position ($\backslash mathbf$) and velocity ($\backslash mathbf$) that together with their time (epoch) ($t\backslash ,$) uniquely determine the trajectory of the orbiting body in space. ==Frame of reference== State vectors are defined with respect to some frame of reference, usually but not always an inertial reference frame. One of the more popular reference frames for the state vectors of bodies moving near Earth is the Earth-centered equatorial system defined as follows: *The origin is Earth's center of mass; *The Z axis is coincident with Earth's rotational axis, positive northward; *The X/Y plane coincides with Earth's equatorial plane, with the +X axis pointing toward the vernal equinox and the Y axis completing a right-handed set. This reference frame is not truly inertial because of the slow, 26,000 year precession of Earth's axis, so the reference frames defined by Earth's orientation at a standard astronomical epoch such as B1950 or J2000 are also commonly used. Many other reference frames can be used to meet various application requirements, including those centered on the Sun or on other planets or moons, the one defined by the barycenter and total angular momentum of the solar system, or even a spacecraft's own orbital plane and angular momentum. ==Position vector== The position vector $\backslash mathbf$ describes the position of the body in the chosen frame of reference, while the velocity vector $\backslash mathbf$ describes its velocity in the same frame at the same time. Together, these two vectors and the time at which they are valid uniquely describe the body's trajectory. The body does not actually have to be in orbit for its state vector to determine its trajectory; it only has to move ballistically, i.e., solely under the effects of its own inertia and gravity. For example, it could be a spacecraft or missile in a suborbital trajectory. If other forces such as drag or thrust are significant, they must be added vectorially to those of gravity when performing the integration to determine future position and velocity. For any object moving through space, the velocity vector is tangent to the trajectory. If $\backslash hat\; =\; v\backslash hat\{\backslash mathbf\{u\}\}\_t$ 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「Orbital state vectors」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.