Press space to start the animation The animation shows why, to an observer on Earth, Mars' orbit sometimes seems to reverse in a short loop-de-loop. The line of sight of the Earth observer is shown by the white line. Earth travels at approx. twice the speed of Mars, so once every two years Earth catches up to and passes Mars. Because of these planets' relative motion, Mars appears to travel backwards as it is tracked across the night sky, shown by the path of the grey dots.
This is a project 5 years in the making. I first learned about retrograde motion in an 8th grade science class. I tried to create a simulation of it in Scratch, but ran into a number of problems and eventually put the project to the side. I recently started brainstorming for a sci-fi story I want to write and came back to the idea of retrograde motion. I tried once again to simulate it in Scratch, and found I now had the understanding of trigonometry necessary to do so. Kudos to Anil Chopra for first teaching me about retrograde motion back in 8th grade!
