If you plot the observed position of Mars night after night, you'll see that Mars appears to move backward occasionally—drawing loops and curves against the background stars over time. This "apparent retrograde motion" happens because the Earth-based observer is also in motion, orbiting the Sun almost twice as fast as Mars. Every time Earth passes Mars, mars appears to move backward in a loop or s-shaped curve. The shape of Mars' retrograde curve is caused by differences in the orientation of the orbits of Earth and Mars.
In the video, we draw a line from Earth through mars, and to the background stars, plotting the apparent position of Mars against the background stars over time.













