A glistening yellow gem encircled by hoops of gold, Saturn is one of the sky's few genuine showstoppers. Even people who have never bothered to play connect-the-dots with the constellations inevitably gasp in wonder when they see the ringed planet through a telescope. And there is no time like the present to experience this joy: Saturn hasn't looked this good for three decades.
Three types of unusual planetary orientations coincide in December to bring Saturn to its peak. First there is the tilt of Saturn's rings, which consist of billions of chunks of water ice organized into a disk that is incredibly thin, roughly 300 feet from top to bottom. The rings open up twice to us during Saturn's 29.5-year orbit, as the planet's north pole alternately appears tipped toward us and away from us. When the rings are aimed edgewise toward Earth, which last happened in 1995, they are nearly impossible to see. Earth is now in an opposite position in this cycle, so the rings appear tipped at their maximal 27-degree angle. Shinier than the planet, the rings nearly triple Saturn's total brilliance when they are viewed at their most open.
The Hubble Space Telescope captures Saturn's rings at their most revealing angle. Photograph courtesy of NASA/STSCI/AURA.
Second, there is Saturn's position in the sky. As it circles the sun, the gas giant trudges slowly through the zodiac, spending a gloomy decade buried in southerly constellations. From the United States, Saturn hangs low on the horizon, blurred by atmosphere and often hidden behind trees and buildings. After another few years of so-so visibility, the planet ascends for a decade of high-flying prominence. Saturn is currently near its apex, residing in Taurus. There the planet is up for 15 hours a day, and it reaches about as high in the sky as the June sun. It has not attained such an elevated position since 1975.
The third factor favoring us this month is Saturn's location along its orbit. Earth speeds past the ringed world every 378 days, each time overtaking it at a slightly different spot. Because Saturn's orbit is oval, not circular, the gap between Earth and Saturn during those approaches can vary by 94 million miles. Now everything meshes. When Earth swings by Saturn on December 17, it will be the nearest meeting in 27 years.
At nightfall this month, Saturn will be the brightest "star" in the sky, shining at a magnitude of -0.3 in the east. The planet's steady, tawny glow is lovely even to the naked eye. Through even a modest telescope, however, a far greater spectacle is easily seen. Two prominent rings—the wide inner B ring and the narrower, slightly darker A ring—pop into view with as little as 30x magnification. The two are separated by the Cassini Division, an improbable dark gap largely swept clean by the pull of nearby Mimas, one of Saturn's 30 known satellites.
Images from the Voyager spacecrafts reveal that the Cassini Division is the largest among thousands of subtle, groovelike voids caused by gravitational interactions between ring particles and small moons orbiting within and outside the rings. Recent studies indicate that the particles will eventually spiral in toward the planet. In a few hundred million years, the rings will probably fade away. Conversely, they cannot be more than a couple of hundred million years old—created, perhaps, when a passing asteroid deteriorated. NASA's upcoming Cassini mission will clarify the rings' origin and fate.
Earth may once have possessed a set of rings like Saturn's. One recent theory proposes that an asteroid impact 35 million years ago lifted debris into orbit around our planet. The resulting rings would have persisted a few hundred thousand years, causing climatic havoc by casting shadows.
Meanwhile, the great observing season will run until early spring, and the planet will reappear next December under perfect circumstances again. After that the rings will seem to grow narrower, and the planet will sink lower and move farther away. Then it will be another three decades until the next prime performance of the lord of the rings.
For a good overview of the current science on Saturn, go to the wonderful Views of the Solar System site: www.solarviews.com/eng/saturn.htm.
To find out what we will soon be learning about Saturn, take a preview of the Cassini-Huygens mission, currently en route to the ringed planet: saturn.jpl.nasa.gov/cassini/index.shtml.