According to NASA:
This movie shows Ganymede, Jupiter's largest moon, as it ducks behind the giant planet. Astronomers combined a series of images taken with the Wide Field Planetary Camera 2 aboard NASA's Hubble Space Telescope to make the 18-second movie. The 540 movie frames were created from Hubble images taken over a two-hour period on April 9, 2007.
Check it out!
Credit: NASA, ESA, E. Karkoschka (University of Arizona), and G. Bacon (STScI)