Retrograde Motion Demonstrated

Too bad I haven't been keeping up with my feeds lately or I could have pointed to this Astronomy Picture of the Day earlier. It shows a short film (not made entirely from photographs, it actually continues a little ways into the future) showing Saturn's changing position in our sky. The apparent reversal is caused by Earth, in our smaller and faster orbit, catching up and passing Saturn as he continues his steady and stately tour of Sol's domain.

This is the kind of effect which Copernicus found to be much more elegantly explained by a heliocentric universe rather than the epicycles of a geocentric one. Not that he was right, mind, but he was less wrong.

Click a day back to see Saturn's axis in action too!

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