Retrograde motion is the movement that a celestial body possesses when it moves in the opposite direction to the direct motion, that is, in the opposite direction to that normally expected.
The definition can concern both rotational movements around its own axis and revolutions around another body.
The retrograde voice derives from the combination of two Latin words: the adverb "retro", meaning "back" and the verb "degrees", meaning "to walk". Retrograde, therefore, means "walking backward".
Definition of direct motion
In the solar system almost all celestial bodies rotate, on themselves or around the Sun, in the same sense that the Sun also rotates on itself: any motion in this direction is called direct or prograde. By convention this direction has been chosen as counterclockwise. This implies that the upward direction of the solar system, that is the positive semi-axis perpendicular to the plane of the ecliptic, is the one from which the motion of the planets around the Sun appears to be counterclockwise. This direction is roughly that of the Earth's North Pole - in other words, an observer looking at Earth's North Pole from above sees the planets of the solar system spin counterclockwise around the Sun.
All the planets of the solar system revolve around the Sun counterclockwise: the same happens with the orbital motions of the Moon, the moons of Mars and the larger moons of Jupiter and Saturn.
In addition to spinning counterclockwise around the Sun, all planets, except Venus and Uranus, spin counterclockwise on themselves. The counterclockwise rotation of the Earth is the cause of the fact that the Sun rises in the East (area around the East cardinal point). Instead on Venus the Sun rises in the West (area around the West cardinal point).
The rotational motions of a planet on itself as well as those orbital around another body that occur with the opposite direction to the direct one are called retrograde.
Retrograde rotational motions
Venus and Uranus, unlike the Sun and the other planets, rotate clockwise on themselves and are examples of retrograde rotation. One of the hypotheses that has been made to explain why these two planets are retrograde is that they originally had a direct rotation and that following a violent impact they have become retrograde.
Venus after the impact would have kept its axis almost unchanged but would have started to rotate clockwise. Instead, Uranus, hit on one side, would have overturned for just over 90 °, starting to "roll" as it travels its orbit around the Sun, but becoming in fact, if we scrupulously adhere to the definition given, a planet with retrograde rotation .
As for the motions of revolution, some small moons, rather than counterclockwise as would be the norm, orbit their planet clockwise and are therefore called retrograde satellites. It is a situation that particularly concerns gas planets: these are asteroids or Kuiper Belt objects that have been captured by one of the gas giants or original moons of the planet itself and have "irregular", highly eccentric and / or "irregular" trajectories. or inclined. Taking into consideration only the irregular satellites, i.e. those presumably captured, we find 48 retrograde versus 7 directed relative to Jupiter, 18 to 8 relative to Saturn and 8 to 1 relative to Uranus.
A separate discussion deserves Neptune, which has captured a retrograde satellite, Triton, from the Kuiper belt: the six "irregular" satellites, with highly eccentric and / or inclined orbits, more external than Triton, are, unlike the cases above in which there is a conspicuous imbalance in favor of the retrogrades, equally divided, 3 against 3, between direct and retrogrades, advancing the hypothesis