![]() Similarly, the "north rotational pole" of a body is defined by the direction of the thumb if one were to wrap the fingers around the body's equator in the direction it spins. According to this definition, the north pole is the one that points north of the invariable plane.) (The International Astronomical Union has defined a different convention for planetary bodies in the solar system. The "north orbital pole" of a celestial body is defined by the right-hand rule: If one curves the fingers of the right hand along the direction of orbital motion, with the thumb extended parallel to the orbital axis, the direction the thumb points is defined to be south. Inherent retrogradation is defined by motion relative to an axis of rotation or orbit. The word "retrograde" derives from the Latin words "retro", backwards, and "gradus", step Prograde was first seen in an abstract of an astronomy-related professional article in 1963 (J. While the terms direct and prograde are equivalent in this context, the former is the traditional term in astronomy. In the case of celestial bodies, such motion may be real, defined by the inherent rotation or orbit of the body, or apparent, as seen in the skies from Earth. Retrograde motion is motion in the opposite direction. Direct motion is the motion of a planetary body in a direction similar to that of other bodies within its system, and is sometimes called prograde motion.
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