Saturn’s Rings: 400 Years of Engagement

Today scientists with the Cassini mission to Saturn released yet another glorious picture of the gassy planet’s icy rings—in this case, a shot of the shepherd moon Prometheus carving arcs in the thin, outer F ring.


—Image courtesy NASA/JPL/Space Science Institute

Cassini has been in orbit around Saturn since June 2004, and it’s sent back scads of pictures over the years of the planet’s amazing ring system, revealing, for example, how tiny moons like Prometheus herd ring matter and create gaps between the rings.

Even from Earth, Saturn’s rings pop into view through modern binoculars or backyard telescopes, making the loops a well-known feature in the night sky.

But 400 years ago, those familiar rings created quite a puzzle for one of the world’s most famous astronomers.

Although Galileo Galilei trained his telescope on the heavens for the first time in the fall of 1609, it wasn’t until July 25, 1610, that he took a gander at Saturn.

His homemade telescope allowed Galileo to see the planet in unprecedented detail … but with the technology of the time, the view wasn’t exactly crystal clear.

What’s more, the mere concept of a planet with rings of orbiting debris would have been totally foreign to the great man’s mind.

So, already familiar with four of the larger Jupiter moons, Galileo’s first thought was that Saturn had a couple moons of its own—two closely orbiting bodies almost as big as the planet itself.

“… to my very great amazement Saturn was seen to me to be not a single star, but three together, which almost touch each other,” he wrote at the time.


Saturn and its “moons,” as drawn by Galileo.

—Image courtesy NASA

Not a bad guess: Lots of bright stars that we see as points of light with the naked eye (i.e., Alpha Centauri) actually turn out to be two or more stars orbiting each other on closer inspection.

Luckily Galileo looked at Saturn again in 1612 and was promptly blown out of the water to see that the two megamoons had disappeared.

“I do not know what to say in a case so surprising, so unlooked for, and so novel,” he wrote.

Another peek in 1616 revealed that the “moons” had returned, but they were no longer round—the shapes had shifted to look more like “two half ellipses with two little dark triangles in the middle of the figure and contiguous to the middle globe of Saturn, which is seen, as always, perfectly round.”

This cosmic head-scratcher baffled the best scientific minds until 1655, when Dutch astronomer Christaan Huygens took a look through his own beefed up telescope and proposed that the morphing moons might actually be “a thin, flat ring, nowhere touching, and inclined to the ecliptic.”

Ah, that’s better.

The reason the rings occasionally “vanish” is because, the way we orbit in relation to Saturn, we see the loops at different angles over time—about every 15 years, the wafer-thin rings turn edge-on to our field of view.


Saturn, with the ring plane facing us in 1994 (top) and edge-on in 1995.

—Top image courtesy Reta Beebe (New Mexico State University), D. Gilmore L. Bergeron (ST ScI) and NASA; bottom image courtesy Amanda S. Bosh (Lowell Observatory), Andrew S. Rivkin (Univ. of Arizona/LPL), the HST High Speed Photometer Instrument Team (R.C. Bless, PI), and NASA

Now, after 400 years of observations, we have a pretty good picture of Saturn’s many and diverse rings. We even know that other gas giant planets have rings of their own, albeit much thinner and fainter than Saturn’s.

(Tee-hee: “Hubble Reveals New Moons, Rings Around Uranus.”)

Today’s nod to history is a nice reminder that, although certain things about the universe might seem like cold, dry facts, there’s still room for some completely unexpected discoveries.

As the Man in Black said:

“Fifteen hundred years ago everyone *knew* the world was the center of the universe. Five hundred years ago, eveyone **knew** the earth was flat. … Imagine what you’ll know tomorrow.”


—Image courtesy NASA/JPL/Space Science Institute

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