Mercury and Venus, the inferior planets, exhibit apparent retrograde motion when at inferior conjunction. The effect is similar to passing an automobile on a highway observers in the faster car see the slower car apparently moving backwards as they overtake it. (Of any two planets, the one closer to the sun has the greater orbital speed.) As a consequence, a superior planet's progress through the zodiac is interrupted by annual loops or switchbacks. The superior planets, whose orbits lie outside that of the earth, appear to move backward at opposition, because the earth is overtaking and passing them. All the planets exhibit apparent retrograde motion when they are nearest the earth i.e., they appear to move backward (east to west) against the background of stars. With the exception of the rotation of Venus, there is no real retrograde motion among the planets, although the plane in which Uranus rotates and its five satellites revolve is tilted slightly more than 90° to the plane of the ecliptic, so that these motions are technically retrograde. Bodies in the solar system with real retrograde orbits include certain moons of the outer planets, and some asteroids and comets. The most common direction of motion in the solar system, both for orbital revolution and axial rotation, is from west to east (counterclockwise as seen from the north celestial pole) revolution or rotation in the opposite direction is actual retrograde motion. Retrograde motion, in astronomy, real or apparent movement of a planet, dwarf planet, moon, asteroid, or comet from east to west relative to the fixed stars.
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