Two moons align perfectly in a picture from Saturn.

Saturn’s Bull’s-Eye Moons

Saturn’s Bull’s-Eye Moons

Bad Astronomy
The entire universe in blog form
Jan. 5 2016 9:30 AM

When Worlds Elide

Tethys and Enceladus

Photo by NASA/JPL-Caltech/Space Science Institute

When it comes to jaw-dropping and eerily alien portraits of distant worlds, you just can’t beat Cassini.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

I can’t stop staring at this picture. It’s ridiculously amazing.


What you’re seeing are two moons of Saturn, Tethys (the bigger one) and Enceladus (the smaller one). The Cassini spacecraft was in just the right place at the right time to see them perfectly aligned, Enceladus superposed on Tethys like rings on a dartboard. And to make it cooler, the rings of Saturn slice across below, backlit by the Sun, dramatic and moody.

The geometry of this image almost sings. The big moons of Saturn orbit the planet in almost exactly the same plane (more on that in a sec), which is also the plane of the rings. Cassini orbits at an angle to that plane, so sometimes it’s above it, and sometimes below. For this shot, it was just below the plane, by a scant 0.34°. If it had been exactly in the plane, the rings would be a straight geometric line one pixel high, cutting across the picture.

But Cassini was below the ring plane by a scosh, so they appear as elongated ellipses. Not only that, but they appear grayish instead of blinding white. That’s because we’re looking at the underside of the rings when the Sun is shining down on them. If Cassini had been above the ring plane, the rings would look far brighter, the ice crystals sending that light more directly toward the camera.

As it happens, while Enceladus orbits Saturn almost exactly in the ring plane, Tethys’ orbit is inclined by a little more than a degree. That’s enough that a superposition like this is pretty rare; from the geometry it looks like Cassini caught it as it neared the “top” of its orbit, when it was above the ring plane.

a closer look at the moons
Zoom and enhance.

Photo by NASA/JPL-Caltech/Space Science Institute


But there’s more. Inset here is a zoom on the moons, with the brightness enhanced. Look at Tethys; see how the “dark” part is actually softly lit? That illumination is coming from Saturn itself; the planet is sunlit and very bright, so from Tethys it would fill the sky and light up the part of Tethys not illuminated by the Sun (the “night side” of the moon). The same thing happens on our own Moon when it’s new, and you can see the “dark” part faintly illuminated; this is called “Earthshine,” and it’s from the Earth lighting up the dark part of the Moon. In this case, it’s Saturnshine!

But why isn’t the dark part of Enceladus lit then as well? Why is it so much darker? Both moons are icy and extremely reflective; they should both reflect Saturnlight equally well.

I have to admit, this threw me for a few minutes. But then I realized something important: Just because the two moons appear to be close together in the shot (right on top of each other!) they are in fact very far apart in space. Here’s a diagram that’ll help.

Saturn and moons
This took me a while to figure out.

Photo by NASA/JPL/Space Science Institute/Gordan Ugarkovic/Phil Plait

I put Saturn there in the middle, with Enceladus and Tethys in approximately the correct positions (their sizes are exaggerated). The Sun shines from the lower right, and Cassini is way, way off to the left, well outside the frame of this diagram. It was more than 2 million kilometers from Enceladus when the shot was taken; for comparison, the rings are roughly 300,000 kilometers across.


If you were on the night side of Tethys (more or less the upper left side of the circle in the diagram) you’d see Saturn nearly full in your night sky, that immense planet lighting up everything. But from the night side of Enceladus, Saturn would be a crescent, much less bright—and for most of the night side of Enceladus Saturn wouldn’t even be in the sky!

That’s why the night side of Enceladus is so much darker than that of Tethys. Saturn isn’t lighting it up.

Now look, if you have a hard time grokking this, I hear ya: I had to draw myself several diagrams, and in my first try I had the geometry messed up (I put both moons on the sunward side of Saturn, before realizing they were on opposite sides of the planet).

Trying to figure this stuff out can be pretty complicated, and actually a little maddening. But the end product is something wondrous: understanding. Once I got the players all in their right places, the setup of this shot made sense, and I understood what I was seeing.

And that’s the point, isn’t it? I do love the beauty of this image, the seriously spectacular interplay of light and shape.

But once I added the science behind it, the math, the dance of gravity and geometry, the portrait took on a depth—literally—I hadn’t seen at first glance.

It shows, as it always does, that knowing the science behind a photo only adds to its profundity. That, to me, is one of the best things science does.