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The Sciences

Disappearing Rings


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You might think it would be hard to hide something that’s 40,000 miles wide. But that’s exactly what Saturn will pull off this fall, when in a rare sort of celestial shell game its giant rings will vanish from sight. For years those myriad chunks of water ice encircling Saturn have been angling toward an increasingly edgewise slant. Now the action reaches a climax.

Saturn’s axial tilt is responsible for the pageant. The planet’s rings tip toward or away from us as the planet swings lazily in its 29-and- a-half-year path around the sun. Since that orbit is itself inclined 2.5 degrees to our own, we view it from slightly different angles over the years. Most of the time the tiny difference between the way the sun and Earth face Saturn has no effect on how we see Saturn illuminated. Only now, with the rings so close to edgewise, is that minute discrepancy enough to make the rings wink out like a snuffed candle.

Imagine what the scene looks like from Saturn itself. Since 1980, creatures gazing up from that planet’s northern hemisphere would have seen sunlit rings, while those below Saturn’s equator would have viewed the dark side of the rings. Then, on November 19, sudden drama: The sun will hover directly over the rings’ edge, casting their shadows as a thin straight line on Saturn’s equator. We on Earth see the north face of the rings illuminated until that day. But suddenly the enormous rings go completely dark as we see the north side in shadow. Now the sun shines on the southern side of the rings.

As we circled the sun this year, we crossed above the plane of Saturn’s rings on August 10. That week they vanished through the largest Earthly telescopes. But the Hubble Space Telescope may resolve them as a hair-thin line. And the Hubble will be watching, since this is the best chance to discover any tiny satellites orbiting above Saturn’s equator that would otherwise be hidden in the rings’ glare. That’s how astronomers spotted the moon Janus in 1966, one Saturnian year ago. What will Hubble’s peerless resolution uncover as it observes a ring-plane crossing for the first time?

Amateur telescopes cannot perform such useful science. But they can get a good look at the immense world of chilled hydrogen when it comes closest to Earth--and shines its brightest--on September 14. Unfortunately, this year, instead of blazing at its usual magnitude and becoming the brightest star in the autumn sky, it’s outshone by Arcturus, Vega, Capella, and Rigel. That’s because Saturn sans rings will appear only half as bright as it normally does. Nonetheless, any small telescope can reveal the delicate spectacle of rings poised just two degrees from edgewise. Everything changes again in November when, after 15 years, the sun finally sets on the north face of the rings. When the lights go out, observers will be left with a ringless planet throughout the winter, its oval disk bisected by a black pencil line. Finally, in February, with Saturn lost in glare on the far side of the sun, we cross the ring plane once again, heading downward to join the sun in facing the south side of Saturn’s rings for the next 15 years. The rings turn edgewise again in 2009, but that, too, will happen behind the sun. So this is the last ring-plane crossing we’ll see until 2024, when another Saturnian year will have passed.

Reason enough to point a telescope Saturn’s way. Or just to gaze with the naked eye at that steady light shining brightly against the dim midnight stars of Aquarius, halfway up the southern sky.

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