Midnight on a ringed world

The entire universe in blog form
Aug. 24 2010 8:05 PM

Midnight on a ringed world

Check. This. Out. What an amazing shot from 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!  


Holy Haleakala! How gorgeous is that?

This picture really threw me for a minute. I couldn't figure it out! Obviously, you're seeing the night side of Saturn; the planet itself is an almost entirely dark disk. The bright curve you see at the edge is sunlight scattered by the upper atmosphere -- the break in the curve near the top is from the shadow of the rings!

But you can also see the moon Enceladus, too: the plumes of water geysers off the southern pole (at the bottom) are obvious. When this shot was taken, Cassini had to look past Saturn to see Enceladus; that is, the moon was farther from Cassini than the planet was.

OK, cool enough, but the problem is Enceladus looks like it's full, as if we're seeing the side completely lit by the Sun. How can that be? If we're seeing the dark part of Saturn, the Sun must be on the other side -- in other words, we're facing toward the Sun in this picture, and it's blocked by Saturn itself. But if that's true, there's no way Enceladus can be fully lit. The Sun would have to be between the planet and the moon! Draw yourself a picture if that helps. I was honestly baffled about this, until I read the caption for the picture:

Enceladus and its plumes have been brightened by a factor of two relative to the planet and rings.

Aha! The astronomers artificially brightened the part of the image with Enceladus to make it easier to see. That's why it looks so bright. And that also explains why it looks full: it's being illuminated by Saturn itself! If you were standing on Enceladus in the middle of the part we see here, it would be midnight, and you'd see a full Saturn directly overhead. It's a big, bright planet, so it would illuminate the ground just like a full Moon lights up the ground here on Earth.

Phew! I thought I was losing it for a second. The geometry of the Cassini pictures can be pretty confusing sometimes, and I thought I was totally lost with this one. Nice to know my confusion was earned honestly.

Related posts:



More Than Scottish Pride

Scotland’s referendum isn’t about nationalism. It’s about a system that failed, and a new generation looking to take a chance on itself. 

iOS 8 Comes Out Today. Do Not Put It on Your iPhone 4S.

Why Greenland’s “Dark Snow” Should Worry You

Three Talented Actresses in Three Terrible New Shows

The Human Need to Find Connections in Everything

It’s the source of creativity and delusions. It can harm us more than it helps us.


Happy Constitution Day!

Too bad it’s almost certainly unconstitutional.

What Charles Barkley Gets Wrong About Corporal Punishment and Black Culture

My Father Was James Brown. I Watched Him Beat My Mother. Then I Married Someone Like Him.

  News & Politics
Sept. 17 2014 12:02 PM Here It Is: The Flimsiest Campaign Attack Ad of 2014, Which Won’t Stop Running
Business Insider
Sept. 17 2014 1:36 PM Nate Silver Versus Princeton Professor: Who Has the Right Models?
Atlas Obscura
Sept. 17 2014 1:46 PM A Salute to Defiant Scots on the Eve of Their Possible Secession
  Double X
The XX Factor
Sept. 17 2014 1:26 PM Hey CBS, Rihanna Is Exactly Who I Want to See on My TV Before NFL Football Games
  Slate Plus
Slate Fare
Sept. 17 2014 9:37 AM Is Slate Too Liberal?  A members-only open thread.
Brow Beat
Sept. 17 2014 1:01 PM A Rare, Very Unusual Interview With Michael Jackson, Animated
Future Tense
Sept. 17 2014 12:35 PM IOS 8 Comes Out Today. Do Not Put It on Your iPhone 4S.
  Health & Science
Bad Astronomy
Sept. 17 2014 11:18 AM A Bridge Across the Sky
Sports Nut
Sept. 15 2014 9:05 PM Giving Up on Goodell How the NFL lost the trust of its most loyal reporters.