Feb 19, 2002

Vanishing Saturn




The breathtaking ringed planet will vanish behind Earth's moon on Wednesday, Feb. 20th. Some astronomers will be watching carefully for Saturn's "lost ring."




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February 19, 2002: Seeing Saturn through a telescope is wonderful. Seeing it disappear is even better.


Sky watchers in parts of North America will have a chance to do both on Wednesday, Feb. 20th, when the Moon glides in front of the ringed planet -- an event astronomers call a "lunar occultation."

"Observers in the northeastern United States and eastern Canada will have the best view of the occultation," says NASA astronomer Mitzi Adams. "There, Saturn will vanish behind the Moon just after sunset and reappear about an hour later."

Above: Photographer Peter Paice of Northern Ireland captured this photo of Saturn disappearing behind the Moon on Nov. 3, 2001. See more from




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The International Occultation Timing Association (IOTA) has prepared tables of local disappearance times and reappearance times for more than 300 North American cities. A useful map is also available from Sky & Telescope magazine.

Across much of the United States and all of Mexico, the occultation will occur (in part or entirely) during daylight hours. Astronomer Clay Sherrod says that shouldn't stop sky watchers who want to observe the occultation. "Saturn is one of the toughest daylight objects," he cautions. "Its color and contrast are almost identical to that of a fairly hazy day. The best way to find Saturn is to find the Moon first -- easily done -- then trace along its dark eastern edge until you spot the planet through your telescope."

"Even small telescopes will reveal Saturn's rings," adds Adams. "They're lovely -- especially when they are disappearing behind lunar mountains and craters."

In fact, a six-inch telescope trained on Saturn after dark when "seeing is good" will reveal two broad rings -- the A ring and the B ring -- separated by a narrow dark gap.


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Above: Saturn's rings and inner moons. The A ring and the B ring are easy to spot through a small telescope. [



That gap," explains Jeff Cuzzi, a ring theorist at the NASA Ames Research Center, "is the Cassini Division. Ring particles within the division were long ago cleared out by gravitational tugs from Saturn's moon Mimas." The B ring lies between Saturn and the Cassini Division; the A ring lies outside.

The Moon's gliding limb will take about 2 minutes to cover Saturn's rings from tip to tip -- an event filled with anticipation for some astronomers who will be on the look-out for Saturn's so-called "lost ring."

In 1973, during a similar lunar occultation of Saturn, Canadian astronomer Glen Reed was waiting expectantly for the outer edge of the A ring to emerge from behind the Moon. He was surprised when a faint glow (like "a campfire on the other side of a treeless hill on a dark night," he later wrote) appeared seconds before the A ring was due.


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Reed's observation was the latest of several sightings of a possible faint ring lying just outside Saturn's main rings. Beginning in 1908, astronomers -- many of them skilled observers -- have from time to time claimed to spot something like Reed saw. The matter remains controversial, though. The lost ring seems to come and go, and not all observers can see it.


In fact, Saturn does have faint rings beyond the A ring. They are called the F ring, the G ring, and the E ring. NASA spacecraft have photographed them from close range, and they are about where the lost ring ought to be.

Above: Using an opaque disk to block the bright planet, astronomers at the University of Hawaii captured this image of Saturn's ultra-faint E ring. [more]

But there's a problem: these rings are widely thought to be too faint to see through amateur telescopes on Earth.

Perhaps, says Cuzzi, when the Moon is blocking the glare from Saturn, amateurs using averted vision might be able to see faint outer rings through a high-power telescope. But it wouldn't be easy.


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Another possibility is that the outer rings were once brighter than they are now. Cuzzi explains: "Saturn's rings are not static and changeless. For example, we think that tiny moonlets within the rings sometimes collide and produce fresh batches of ring material. The process can work in reverse, too, when ring particles accumulate to form tiny moon-like clumps." Indeed, the Voyager spacecraft and the Hubble Space Telescope have spotted such new clumps in the F-ring.


Above: A Voyager 1 image of Saturn's clumpy F ring. [more]

"I doubt, however, that the 'lost ring' is the F-ring," says Cuzzi. "It's so close to the bright A-ring that visual observers would have trouble separating the two." More likely candidates, he says, are the G and E rings. "They are broader than the narrow F ring and farther removed from the A ring's glare."

The E ring, in particular, is an intriguing possibility, he says. "The source of the E ring seems to be Saturn's 250-km wide moon Enceladus." Dust particles are dislodged from the moon's surface by small meteorite impacts. Some of the dusty bits escape Enceladus and join the E ring, which is brightest near the moon's orbit.


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That process accounts for the faint E ring that exists today. Cuzzi notes that occasional strikes by larger meteorites might brighten the ring noticeably. But there's another possible source of material perhaps more dramatic than meteorite impacts: volcanoes.


Right: An artist's rendering of icy volcanoes (conjectured but not yet observed) on Saturn's moon Enceladus. [more]

Parts of Enceladus photographed by the Voyager spacecraft appear curiously smooth -- as if they had been recently resurfaced by watery volcanoes or geysers. "An erupting geyser on Enceladus could eject more material into the E ring and, for a while, brighten it."

Many astronomers will monitor Saturn this week to see what appears when the bright A ring vanishes behind the quarter Moon's darkened limb. Probably nothing! But you never know what you will see ... unless you look.



Editor's note: On Wednesday the Moon will lie in a beautiful part of the night sky. Not only will Saturn be nearby, but also Jupiter, Sirius (the brightest star in the sky), the Pleiades, and Orion the Hunter.

of Saturn, the Moon, and their surroundings. Web Links


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Feb 20, 2002, Saturn Occultation Links:


  • A Saturn-Moon Spectacle (Sky & Telescope)
  • Spectacular Saturn Occultation (ASO)
  • History of Saturn's Lost Ring (IOTA)
  • Disappearance Times (IOTA)
  • Reappearance Times (IOTA)

Right: This image, courtesy of Sky & Telescope (copyright 2001, all rights reserved) shows where Saturn will disappear behind the Moon and reappear again for a sampling of North American cities. [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.

More Saturn Links -- Saturn Fact Sheet (JPL); Frequently Asked Questions About Saturn's Rings (JPL); Saturn's Ring System (NASA AmSaturn (The Nine Planets); Saturn (Windows to the Universe); Timeline of Saturn Discoveries (JPL)

What are the Names of Saturn's Rings? (Yahoo!)

Circumplanetary Dust Grains: from Birth to Death -- learn more about the formation Saturn's E ring.

Rings around other planets: Jupiter (NASA Ames); Jupiter (JPL); Uranus (NASA Ames); Uranus (JPL); Neptune (NASA Ames); Neptune (JPL)


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