Dec 13, 2002

Breathtaking Saturn

On Dec. 17th, Earth and Saturn will have their closest encounter in nearly 30 years.


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December 13, 2002: Thirty years ago, Earth and Saturn had an extraordinary close encounter. The ringed planet was only 1.2 billion km from Earth--about as close as it can get--and its rings were tipped toward us. The view through a telescope was simply breathtaking.

Next week it's going to happen again.

"On Dec. 17th, Saturn and Earth will be unusually close together--about the same as thirty years ago," says NASA astronomer Mitzi Adams. And once again the planet's rings are tilted in our direction for maximum effect. "So get out your telescope." says Adams. "Even a small one will do."

Above: Photographer Ed Grafton captured this image of Saturn from Houston, Texas, on Dec. 11, 2002. He used a 14-inch telescope and a CCD camera. [



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Observers very often gasp when they first see Saturn through a telescope. Adams recalls her own first look: "It was Nov. 1969, and I was a volunteer at the Fernbank Science Center in Atlanta. We were tracking the Apollo 12 spacecraft for an NBC news crew using the center's 36 inch telescope. It was a very cold night, about 9o F. For fun we rolled out a 10-inch telescope, too, and pointed it at Saturn. The sight of the planet and its rings took my breath away. Or maybe that was the cold!" she laughs. "Seriously, I don't think I'll ever forget that image. Now, whenever I show Saturn to 'first-timers,' I really enjoy watching their reactions, remembering my own experience."

Dec. 17, 2002, is special because that's when Saturn and the sun are on opposite sides of the sky. Astronomers call this "opposition." When the sun sets, Saturn rises and it's up all night. Saturn at opposition is close to Earth (see the diagram below) and therefore bright.

Below: Saturn is "at opposition" when it and the sun are on opposite sides of Earth. The size of Earth's orbit is exaggerated for clarity. Saturn is 9 1/2 times farther from the sun than Earth.

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Oppositions of Saturn come every 13 months or so. This one is the best in nearly 30 years because Saturn is also near perihelion--its closest approach to the sun. Adams explains: "Saturn's 30-year orbit is not a perfect circle. It has the shape of an ellipse with one side 6% closer to the sun than the other. When Saturn is closer to the sun it's also closer to Earth ... and we get a great view."

Finding Saturn is easy, notes Adams. Look east after sunset. Saturn will be there, rising, among the bright stars of the constellation Taurus. Saturn is yellow in hue and doesn't twinkle like a star. Shining at visual magnitude -0.5, it's one of the brightest objects in the winter evening sky. At midnight, adds Adams, Saturn will be almost directly overhead.

On Dec. 18th, says Adams, you can use the Moon to find Saturn because the pair will be close together. Glare from the full Moon will impair your night vision, but not nearly enough to wipe out Saturn.

Saturn is so bright, she explains, in part because its vast rings are tipped toward us. They reflect sunlight very well. The rings are 274,000 km wide--twice as wide as the planet Jupiter. They are only a few tens of meters thick, however, which is why they vanish when seen edge-on.

Such was the situation 7 years ago: the rings were edge-on and practically invisible. This happens because Saturn is tilted 27o with respect to its own orbit, so the rings appear to wobble as the planet goes around the Sun. Sometimes, like now, they are easy to see, but not always.

Below: The eastern sky at 9:30 p.m. local time on Dec. 17 and 18, 2002, as seen from mid-northern latitudes. The blue circles denote the changing position of the Moon. Southern hemisphere sky watchers should invert this map and look northeast. [


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Galileo himself was vexed by Saturn's wobble. He discovered Saturn's rings in 1610. Altissimum planetam tergeminum observavi, he wrote in Latin. "I have observed the highest planet triple: o0o." That was how the rings looked through his primitive telescope. Galileo's attempts to understand what he saw were frustrated when, two years later, the blobs vanished. The rings had tipped edge-on, but didn't know that. The coming and going of Saturn's rings puzzled researchers for many years thereafter, and indeed it wasn't until 1656 that Dutch astronomer Christiaan Huygens figured out the correct explanation.

Astronomers have been observing Saturn's rings ever since--nearly 400 years. Even so, there's much we don't know about them. The rings are made of icy chunks ranging in size from dust to large houses. It looks like debris, but from what? A shattered moon? A passing asteroid torn apart by Saturn's gravity? No one knows. There is mounting evidence that the rings are much younger than Saturn itself. Some researchers believe the rings appeared only a few hundred million years ago--a time when the earliest dinosaurs roamed our planet. (For more information about this intriguing possibility, read the Science@NASA story "The Real Lord of the Rings.")

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Right: The Cassini spacecraft will reach Saturn in 2004. [more]

A spacecraft called Cassini--a joint mission of NASA, the European Space Agency and the Italian Space Agency--is en route to Saturn now. It will arrive in July 2004 and become the first spacecraft to orbit rather than fly by Saturn. (Six months later Cassini will release its piggybacked Huygens probe for descent through the thick atmosphere of Saturn's moon Titan.) Bristling with scientific instruments and with plenty of time to use them, Cassini may tell us a great deal about the ringed planet.

Meanwhile, the only instruments you need to enjoy Saturn are your eyes and a small telescope. "Saturn and Earth will be close together for many weeks," notes Adams. If you miss it on Dec. 17th, she says, don't worry. The planet can take your breath away for some time to come.

Credits & Contacts
Author: Dr. Tony Phillips
Responsible NASA official: John M. Horack
Production Editor: Dr. Tony Phillips
Curator: Bryan Walls
Media Relations: Steve Roy
The Science and Technology Directorate at NASA's Marshall Space Flight Center sponsors the Science@NASA web sites. The mission of Science@NASA is to help the public understand how exciting NASA research is and to help NASA scientists fulfill their outreach responsibilities.
more information

The Real Lord of the Rings (Science@NASA) Four hundred years after they were discovered, Saturn's breath-taking rings remain a mystery.

Cassini-Huygens Mission to Saturn and Titan -- (JPL) an international venture designed to explore Saturn, her rings, moons, and the vast surrounding region. Educators, please note the Saturn Observation Campaign.

History of Saturn's Rings -- from the Rice University Galileo Project

more: Saturn (Nine Planets); Saturn's Ring Plane Crossings (STScI); Saturn's Ring System (Ames);

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