![]() ![]() ![]() Copyright © 2023, Columbia University Press. The Columbia Electronic Encyclopedia, 6th ed. In the geocentric Ptolemaic system, the retrograde motion of the planets was explained, using epicycles, as real retrograde motion the modern heliocentric theory satisfactorily explains these motions as apparent, due to the relative speeds of the planets in their orbits about the sun. As measured against the fixed stars, planets typically appear to move eastward. ![]() They are then passing between the earth and the sun, overtaking the earth, and thus seem to move east to west, relative to both the sun and the background stars. Retrograde motion can be real or illusory Some retrograde motion is an illusion. as illustrated in Figure 4.2.i Thus, the deferentepicycle model can reproduce the general eastward, and occasional retrograde, motion of the planets. Retrograde Motion The apparent motion from east to west (looking / viewing to the south apparent left to right) motion of a planet or other object as. 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. The retrograde motion is caused by the Earths own motion around the Sun. 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. Apparent retrograde motion is the apparent motion of a planet in a direction opposite to that of other bodies within its system, as observed from a. 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. Anyone performing any part of these demonstrations, even with revisions, knowingly and voluntarily assumes all risks associated with them.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. Retrograde motion is the motion of a celestial body that is opposite to its apparent orbit. These risks include, without limitation, bodily injury (and possibly death), including risks to health that may be temporary or permanent and that may exacerbate a pre-existing medical condition and property loss or damage. Performing all or any portion of any of these demonstrations, with or without revisions not depicted here entails inherent risks. This reversal of direction is a phenomenon that all the solar systems outer planets periodically undergo, a few months before they reach opposition. The demonstrations included here are within the public domain and can be found in materials contained in libraries, bookstores, and through electronic sources. Saturn will enter retrograde motion, halting its usual eastward movement through the constellations, and turning to move westwards instead. ![]() The University of Iowa is not responsible for demonstrations performed by those using their own equipment or who choose to use this reference material for their own purpose. The distance from the Earth to the Sun is small compared to the distance from the Sun to the stars. Retrograde motion of the planets is explained by the Earth's motion. Disclaimer: These demonstrations are provided only for illustrative use by persons affiliated with The University of Iowa and only under the direction of a trained instructor or physicist. The Earth has three motions: daily rotation, annual revolution, and annual tilting of its axis. ![]()
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