Saturn gets edgy

Saturn gets edgy

Saturn gets edgy

Bad Astronomy
The entire universe in blog form
Dec. 25 2008 11:05 PM

Saturn gets edgy

So you got a telescope for Christmas/Hanukkah/Newtonmass/whatever... or you've had one for awhile. Either way, you get a treat this week. Or a lack of one. Saturn's rings are going away.

Hubble image of Saturn in 1996
Hubble caught Saturn with edge-on rings in 1996. Image courtesy Erich Karkoschka (University of Arizona Lunar & Planetary Lab) and NASA/ESA.

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!  


Well, kinda. Saturn, like the Earth, is tipped a bit compared to the plane of its orbit; we're canted at a 23.5 degree tilt, and Saturn is off from being vertical by about 26.7 degrees. Saturn's magnificent rings are aligned with its equator, so that means that roughly twice every Saturn orbit we cross the "ring plane". In other words, from Earth we see them edge-on.

And the rings are thin. Incredibly thin. Despite being over 200,000 km across, the rings are typically at most only a few dozen meters thick. To scale, that's far thinner than a piece of paper.

So when we pass through the plane of the rings, they practically disappear from sight. I've seen it once through a telescope, when we were near (but not quite at) that point, and Saturn looks pretty weird when it goes commando. We're used to it wearing these big gaudy rings, and there it was, nearly nude. It's maybe not the best time to show the planet off to friends and family, but it's still pretty cool.

The Earth actually doesn't pass through the ring plane until September 2009, but at that time Saturn will be on the other side of the Sun, and pretty much unobservable. You'd think that a month or two before then would be the best time to observe the narrowly thinning rings, but in fact the best time is right now! Due to the vagaries of our mutual orbits, the rings are actually at a minimum right now, the last week of 2008, when they are inclined just 0.8 degrees to our line of sight.

If you have a telescope, get out and take a look! Saturn will be nearly ringless for the next few months, and then the rings will start to open up once again. After that, you'll have plenty of time to soak in the phenomenal view of the solar system's best showpiece -- the next ring plane crossing isn't until March of 2025.

Right now, Saturn is in the constellation Leo and shines fairly brightly at about magnitude 1, about the same brightness as the star Regulus which marks the heart of the lion. It rises around midnight local time right now, and is high enough to observe a couple of hours later. You can find sky maps at Your Sky and Heavens Above, and you can read more about the ring plane crossing on the NASA news page, and on Alan Dyer's astronomy page.

And I have to add: this isn't merely a curiosity; there is scientific value to this event. Telescopes can focus on the planet and see things otherwise hidden in the glare of the very bright rings. Faint moons, the existence of material above and below the ring plane, features on Saturn itself: all these can be easier to see without the icy, reflective ring particles blasting out light. It's funny. Saturn is the most beautiful planet in the solar system through a telescope because of those rings, but it may be the most scientifically interesting when we can't see them at all.