Press play to let time advance automatically. When paused, use the slider to move time manually.
Note the position of mars (the reddish dot) along the celestial sphere, (the large circle). Usually, it's moving in the same direction (CCW) but occasionally it switches and moves backwards (CW). The motion of the planets during these periods of time is called retrograde motion.
Link to stand-alone sim: Here is a link to view the sim on its own, with no distractions.