Retrograde Motion


Marilynn Stone

This SMART website is hosted by the Illinois Institute of Technology

         Why do the planets sometimes appear to be moving backward when viewed from the earth?

The outer planets are farther from the sun than the earth and when viewed from the earth, the earth moves farther in its orbit during one month than an outer planet does.  By making a handout and simply drawing straight lines from the earth through the planet orbit to the stars, one can see that the planet appears to slow down and move backward when viewed against the background of stars.





Handout with the lines drawn:










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