Late last year, the Cassini spacecraft caught this amazing show in Saturn's skies. You're seeing possibly waterlogged moon Enceladus crossing in front of much larger moon Rhea - over a period of minutes. A lovely show.
According to the Cassini imaging lab:
The smaller moon Enceladus (504 kilometers, 313 miles across) passes in front of the larger moon Rhea (1528 kilometers, 949 miles across). These three images were each taken a little more than a minute apart. Mutual event observations such as this one, in which one moon passes close to or in front of another, help scientists refine their understanding of the orbits of Saturn's moons.
The view was obtained at a distance of approximately 2.3 million kilometers (1.4 million miles) from Enceladus and 2.7 million kilometers (1.7 million miles) from Rhea.
via Bad Astronomy