Check out this series of images, taken a bit more than a minute apart, of Saturn’s moon Epimetheus passing in front of its sister moon, Janus, from the perspective of the Cassini spaceprobe, which took these images on February 14, 2010. Cassini has photographed Saturnian moons passing in front of one another before; what’s neat about this is that Epimetheus and Janus are rather close in size — Janus is 179 km across, Epimetheus 113 km — and distance: they were only 15,000 km away from another at this point. They essentially share the same orbit, swapping the higher and lower positions every four years or so as they exchange momentum. The Planetary Society has a page explaining the complicated orbital mechanics of Epimetheus and Janus’ shared orbit.