Cassini catches four little moons in motion

I've posted animations from Cassini before in which there are multiple moons moving around, but this is one of the coolest such sequences I've seen yet. It features four of Saturn's smaller moons, and yet it manages to get a view from sufficiently close to actually resolve features on the surfaces of all but one of them. (Atlas is too small.) You can see Prometheus, the space yam, with two lobes and a narrow waist between them; and you can see Epimetheus' "nose," and craters on Janus. Neato!

The moons are:

It's a challenge to catch so many moons in one photo, but sometimes Cassini gets help from orbital mechanics. In fact, I had a difficult time figuring out when exactly these images were taken on July 27, because the four moons are bunched somewhat close to each other in their orbits, especially Atlas and Prometheus; there were three or four times on July 27 when both of the moons could be found near each other near the east or west edge (or "ansa") of the rings.

This was a fun and relatively easy animation to assemble. As I usually do, I loaded the stack of images into a single file in Photoshop, with each frame a separate layer, then I cropped it close around the part of the frame where all the action was happening, and then I went through and adjusted the levels to set the background space to appear black. I also erased the worst of the cosmic ray hits at the same time (though I was probably also erasing background stars in doing so).

Then I had a decision to make. The original frames were taken with the camera pointed at Prometheus -- Prometheus was always in the center of the frame. Animated like that, the rings moved across the screen during the animation, which I found very distracting. (You can see a version of that animation aligned that way here.) So instead of aligning the images on Prometheus, I elected to align them on the edge of the A ring. That held the rings still throughout the animation, and allowed me to focus on the relative motion of the moons.

As a result, you can see Prometheus' motion appears to accelerate from frame to frame, even though the motion of the other moons (especially Epimetheus and Janus) appears pretty steady in its speed. What causes that? Can you figure it out?

That's going on is that Prometheus is located very close to the F ring. And you can see the ansa, or edge, of the F ring in this frame. The F ring is basically circular in shape, but we are looking at it almost edge-on, seriously foreshortening it. When Prometheus is apparently moving slowly, that's because most of its motion is actually in the direction pointing in and out of your computer screen. It appears to accelerate as it moves around the rings because its direction of motion is going increasingly parallel to the plane of your computer screen. Fun with vector geometry! Also, animating it this way holds the F ring's position still, so you can see lumps moving around in the F ring behind Prometheus.

By the way, the only other animation I could find in my image library containing this many moons is this one, from back in January of 2006; and that one used the wide-angle, not the narrow-angle camera. Getting four moons at once is hard to do!