I check JPL's Planetary Photojournal every day to see what's new from their various spacecraft, and there have been some particularly pretty images from Cassini lately. This one really caught my eye. In a rare view, we see one of Saturn's icy moons (this one is Rhea) nearly fully lit, what scientists refer to as "low phase." But the background is odd, grading from dark in the upper left to less dark at lower right. Turns out that's Saturn in the background. Because Rhea and Saturn are lit at the same angle -- nearly full phase -- that means Rhea must be perched near the edge of Saturn's disk, where the terminator (or day-night boundary) barely peeks over the edge. You can see the geometry with help from JPL's Solar System Simulator.