11/13/2023 0 Comments Planetary retrograde motionThe accompanying diagram illustrates a case of retrograde motion exhibited by a superior planet such as Mars. In reality, the truth was much simpler and the heliocentric model of Copernicus, with the Sun at the centre of the solar system, showed the way. This explanation was so successful that it held sway for centuries. However, the ancient astronomers were nothing if not inventive and they devised a complicated system involving epicycles (circles within circles) to explain the occasional backward motion of the planets. Retrograde motion simply could not exist in such a universe. In the geocentric model where Earth was at the centre of the universe and everything - the Moon, the Sun, the planets, the stars - moved around it in circular orbits. Retrograde motion was a major headache for ancient astronomers. This change in direction is an optical illusion caused by the differences in orbital velocities of the Earth and the other planet. ![]() Of course, the planet is not really changing directions in space. This east-to-west motion is known as apparent retrograde motion or simply retrograde. However, occasionally a planet's track across the celestial sphere slows, stops and then reverses direction for a period of time. This is termed direct or prograde motion. The planets normally move from west to east against the background stars. ![]() Possibly the most familiar use of the term relates to the apparent motion of a planet amongst the background stars. It might refer to the direction in which a planet spins on its axis or the direction in which an object orbits another. Retrograde motion can mean several different things to astronomers.
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