Tracking a Vanishing Act : Even amateurs can start to appreciate Saturn during a nine-month astronomical event in which the rings will seemingly ‘disappear.’
What would you do if you possessed a spectacular set of golden rings that suddenly vanished?
Look for them, of course.
But what if the rings disappeared every 13 years . . .
Obviously, this is no average set of rings. These are the rings of Saturn. But the answer is still the same. Because if you don’t scope out these rings between now and next February, you’ll be missing a celestial vanishing act that won’t repeat itself until the next century.
WHAT’S HAPPENING
Astronomers liken Saturn’s rings to a snowstorm in orbit: composed of trillions of floating, frozen particles reflecting light from the sun. The rings are held together by gravity and also include gaps, the largest of which is called the Cassini Division. The entire ring system circles the equator of Saturn--a massive ball of hydrogen and helium second only in planetary size to Jupiter.
Saturn’s ring system is also similar to a music album: thin but broad in diameter. In fact, Saturn’s rings have an immense diameter: they stretch an estimated 180,000 miles from tip to tip and can often be brighter than the planet itself. But they are very thin--less than a mile thick. In fact, they appear invisible edge on.
Now imagine that you are holding the album at a slight angle as you walk around a lamp in the center of the room. As you circle the lamp, you note that the top and bottom parts of the album are illuminated at different times with respect to your position with the lamp.
The same thing happens with Saturn: the northern and southern sides of Saturn’s rings take turns being illuminated every 13 to 16 years during the planet’s 30-year orbit of the sun.
This year marks one of those transitions.
Since 1980, Earth-based observers have viewed the illuminated northern side of the rings, as the north pole of Saturn is pointed toward the sun. But on Nov. 19, 1995, Saturn’s rings go edge-on with the sun, marking a new phase of Saturn’s orbit that swings the south pole toward the sun. As a result, the southern side of Saturn’s rings will be visible from February, 1996, to the year 2009.
Because of our orbit around the sun, the Earth will plunge into and out of Saturn’s ring plane (not the rings themselves) during this year’s transition, giving us the rare chance to view the rings edge-on as they seemingly “disappear.”
And it all begins Monday--the first of three dates on which the Earth passes through the ring plane.
WHEN, WHERE AND WHAT TO OBSERVE
Saturn is currently the only bright planet in the eastern sky just before dawn. Tony Cook, astronomical observer at the Griffith Observatory in Los Angeles, suggests using a small telescope (eight-inch diameter or more) to observe any detail, although binoculars will reveal that the rings have, indeed, vanished from sight.
But with a larger telescope, the rings are still visible through Monday, when the Earth crosses Saturn’s ring plane from north to south.
“It’s fun to watch night after night to see how much harder the rings are to see,” said Cook. “They do become very faint, like an edge-on view of a razor. They are so thin that they actually disappear from sight.”
In late June to July, earthbound observers may see two things: first, the heavy, black shadow of the rings on Saturn’s cloud tops. Second, we may faintly see the dark side of the rings.
Said Cook: “Even though the illuminated side of the rings is not visible, there is light that gets scattered through, kind of like light hitting cigarette smoke. But you need at least a 12-inch-diameter telescope to see it.”
Then, on Aug. 10, the Earth will pass again through the ring plane, only this time going from south to north, so the illuminated northern face of the rings will again become visible.
By Sept. 14, Saturn will be at its closest point to the Earth, about 800 million miles, and will be visible all night long.
That all changes by Nov. 19, when Saturn’s rings go edge-on to the sun. According to Cook, the visible side of the rings gradually changes from daylight to shadow as the sun illuminates the southern side of the rings.
“This will allow us to see the structure of what scatters light in the rings,” Cook said. “Things that are ghostly in the normal view of the rings, like the Cassini Division, become the brightest parts of the ring, so the rings look very different than we normally see.”
The Earth passes a final time through the ring plane on Feb. 11, 1996, when we will again look at the south side of rings that are illuminated. For the next 7 1/2 years, the rings will be more open and illuminated at a wider angle.
POTENTIAL FOR DISCOVERY
The three ring-plane crossings offer a rare opportunity for professional sky-watchers. Between 1655 and 1980, 11 of Saturn’s moons have been discovered during times when the rings disappear.
“Because the glare of the rings [isn’t] an issue, it’s a good time for searching for other previously unknown moons,” Cook said.
Even more important, perhaps, this nine-month astronomical event gives even the most inexperienced amateur time to appreciate Saturn.
Now is the time to sign up for a summer or fall astronomy course at your local community college, join an astronomy club or make plans to be at Griffith Observatory this summer.
After all, this is one vanishing act you may never get another chance to follow.
