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 
sky.
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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