Motion

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Motion

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The act, process, or instance of change of position. Also called movement, especially when used in connection with problems involving the motion of one craft relative to another. </dd>
Absolute motion is motion relative to a fixed frame of reference. Actual motion is motion of a craft relative to the earth. Apparent or relative motion is change of position as observed from a reference point which may itself be in motion. Diurnal motion is the apparent daily motion of a celestial body. Direct motion is the apparent motion of a planet eastward among the stars; retrograde motion, the apparent motion westward among the stars. Motion of a celestial body through space is called space motion, which is composed of two components: proper motion, that component perpendicular to the line of sight; and radial motion, that component in the direction of the line of sight. </dd>

References

This article is based on NASA's Dictionary of Technical Terms for Aerospace Use