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Retrograde Motion
© Copyright 2000, Jim Loy
To most
of us the planets just seem to go across the sky with the stars, in a 24 hour
cycle, as the earth spins. But the planets are moving, against the background
of stars. Planet means wanderer in Greek. If you map a planet's apparent path
through the stars, you will see it appear to stop and go backward, every once
in a while. Ptolemy deduced very complicated orbits for the planets, in order
to explain this "retrograde" motion. The triumph of Copernicus'
theory of a sun-centered solar system was that it explained retrograde motion
quite simply. On the left we see two planets orbiting the sun. We are on the
blue planet, looking at the red planet. When the two planets are closest
together (at opposition), the red planet seems to go backward (See that the
direction that the arrow is pointing moves backward for a moment). This is just
because we are speeding past the other planet. When you pass another car, that
car seems to be going backward.
In the above paragraph, we have the situation that we are on the inferior
planet (the one closer to the sun) and we are watching the superior planet. We
also see retrograde motion (against the background stars, not with respect to
the sun) when we watch an inferior planet, for the same reason.
The above animation was done with Cinderella (which produced 36 separate
images), Paint Shop Pro (which captured the images, cropped them, and saved
them as 36 gifs) and Ulead GIF Animator (which combined them into an animated
gif). The whole process took me about an hour and a half.
My thanks to Ulrich Kortenkamp for correcting a bug in the gif animation.
Apparently Ulead GIF Animator put the bug there.
See Ptolemy's Epicycles, a more elaborate
Cinderella animation which requires Java.
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