Although the two movies were prepared from the same images, they show the Pluto-Charon pair from different perspectives. The top movie is “Pluto-centric.” Charon is shown as it moves in relation to Pluto which has been digitally centered in the movie. (The North Pole of Pluto is at the top.) Pluto makes one turn around its axis every 6 days, 9 hours and 17.6 minutes—the same amount of time that Charon rotates in its orbit. The shifts in Pluto’s brightness result from the brighter and darker terrains on its differing faces.
The second movie is barycentric. Pluto and Charon are shown in motion around the binary’s barycenter, the shared center of gravity between the two bodies. Because Pluto is much more massive than Charon, the barycenter (“X” marks the spot) is much closer to Pluto than to Charon.Image Credit: NASA