Figure 2-2Paths of Mars (a) the retrograde motion of Mars as it would be seen in a series of images taken on the same photographic plate. (b) to help visualize this motion on the celestial sphere, astronomers often plot the position of Mars (or other body in retrograde motion) on a star chart. the retrograde path is sometimes a loop north, as shown in (a) and (b), or south of the normal path, and sometimes an s-shaped path across the ecliptic. (c) in the middle of 2016, Mars will undergo an s-shaped retrograde motion.