Retrograde Motion
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Transcript: The word “planet” comes from the Greek root for the word “wanderer.” The planets move through the fixed stars from night to night. This motion was known to ancient people for the five planets that can be seen with the naked eye. Mercury and Venus are never seen very far away from the Sun. Their orbits of the sun are interior to the Earth’s, so they always appear within about twenty-five and about forty-five degrees from the Sun respectively. The planets in orbits exterior to the Earth’s display what is called retrograde motion occasionally. That is, for a period of weeks or months at a time, their systematic motion around the stars will reverse and then change again. This backward, or retrograde, motion comes because the Earth is on an interior orbit and moving faster in its orbit. And so the Earth appears to overtake on the inside a planet like Mars causing Mars to apparently move backwards for a period of time. Retrograde motion is in principle observable on all the planets with exterior orbits to the Earth’s, but retrograde motion is most apparent for the planet nearest to the Earth, Mars.
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