December 16, 2003
The Earth has carried you around the Sun again, steady as ever, while the stars kept their patient watch.
Retrograde Mars
Birthday · December 16, 2003
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APOD published December 16, 2003. Credit: Tunc Tezel. A publication-date match, not a photo taken on your date. Independent of NASA.
Why would Mars appear to move backwards? Most of the time, the apparent motion of Mars in Earth's sky is in one direction, slow but steady in front of the far distant stars. About every two years, however, the Earth passes Mars as they orbit around the Sun. During the most recent such pass in August, Mars loomed particularly large and bright. Also during this time, Mars appeared to move backwards in the sky, a phenomenon called retrograde motion. Pictured above is a series of images digitally stacked so that all of the stars images coincide. Here, Mars appears to trace out a loop in the sky. At the top of the loop, Earth passed Mars and the retrograde motion was the highest. Retrograde motion can also be seen for other Solar System planets. In fact, by coincidence, the dotted line to the right of the image center is Uranus doing the same thing.
Image credit: Tunc Tezel