Retrograde Motion

Yesterday's picture at Astronomy Picture of the Day was an annotated composite photograph of Mars undergoing its retrograde motion between October 2, 2009 and June 5, 2010. The notion of retrograde motion is not always easy to understand (the Minister has tried to explain the concept in various comments around the Internet over the years), but this Youtube video does a good job of explaining the retrograde motion of Mars and predicting the path of the planet against the night sky quite nicely.