There Goes Mars - Answer to "Quiz"

Eleven images - from Sept. 15, 2005 to Jan. 18, 2006 - were made of Mars using a digital camera mounted on a tripod and imaged on the Mars' region of the Loading large  Students Robin Gallagher, Julia Travis, and Brandon Kelley each contributed to the photographs as well as myself.  These images were aligned so that all the stars appear at the same position in each frame, then the images are displayed in succession - a time lapse movie.  The resulting composite animated result is quite large so it may take quite awhile to download. 

Notice that Mars starts the season moving east (left) a little, then for most of the sequence it moves west (right) relative to the stars, finally reversing direction again back to the east.  This westward motion (somewhat counterintuitive because the planets orbit to the east) is called "retrograde" motion.  It is essentially caused by the fact that the earth, closer to the sun than Mars, orbits faster than Mars and "passes" the outer planet.  The outer planet, like a slower car on an expressway, appears to the faster car to be moving backwards.  This explanation seems very simple.  However, in the pre-copernican days for thousands of years, people thought the earth, naturally, was the center of the universe and the sun and planets all orbited the earth.  Mars' retrograde motion (shown in the animated photo) was very difficult to explain, and the scientists at the time (Ptolemy AD 85 - 165) invented elaborate complicated orbits of the planets around the earth to account for the retrograde motion.

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