![]() It is more common to keep the period positive and let the inclination vary between 90° and 180° for retrograde motion, and between 0° and 90° for direct motion, but when this inclination isn't listed, a negative period is the only indication that an orbit or rotation is retrograde. The choice between these two notations is largely arbitrary. Similarly, a moon spinning backwards on an axis inclined by 10° from the axis of its orbit can instead be described as being flipped upside-down and spinning forwards. 170° (upside-down) and +6 h (forwards), in which case no period would ever be negative.In which case no inclination would ever exceed 90° (anything more than 90° would be upside-down), or of: 10° (rightside-up) and −6 h (backwards),. ![]() ![]() For example, a moon in a retrograde orbit that is inclined from the pole of its planet by 10°, and with a 6-hour orbital period, could be said to have the orbital parameters of: There are two notations for retrograde motion that are mathematically equivalent: The body can be considered to orbit backwards, or it can be considered to orbit forwards, but with its orbit upside-down. 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 invariant plane of the solar system.) (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 north. 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.
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