What is retrograde motion? via Shareaholic

Retrograde motion of Jupiter or Mars or Saturn in our sky is an illusion, caused by Earth's passing these slower-moving outer worlds. But there's a real retrograde motion, too.

What is retrograde motion? | EarthSky 2/6/17, caused by Earth’s passing these slower-moving outer worlds. But there’s a real retrograde motion, too. A photomosaic from Voyager 2 of Neptune’s largest moon, Triton. The moon orbits Neptune opposite the direction that the planet rotates. Does this mean that Triton came from the Kuiper Belt and was eventually captured by the ice giant? NASA / Jet Propulsion Lab / U.S. Geological Survey.

What is retrograde? Sometimes the planets appear to change direction in the sky. This retrograde motion is entirely an illusion caused by the Earth passing the slower moving outer planets.

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What is retrograde motion and why does a planet like Mars appear to change directions in the night sky? Is it playfully looping about in its orbit around the sun or is something less strange going on?

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The dazzling “star” near the waxing moon on January 1, 2 and 3, 2017 is Venus. Mars is also along a line between the sunset location on your horizon and the moon and Venus in your sky.

Venus is one unique planet when it comes to rotational motion. It takes Venus 243 Earth days to completely rotate on its axis, but just 225 days to orbit the sun. Venus also is one of two planets that rotates in reverse, a phenomenon called retrograde motion, which according to most theories is due to an ancient planetary collision.

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