Saturn’s iconic rings will seem to “disappear” for a couple of days starting this weekend — at least from our vantage point on Earth.
The rings won’t actually vanish, but for a short time, the angle between Earth and Saturn will create something of a cosmic illusion, making the planet’s rings appear invisible from our point of view.
The reason this occurs has to do with the specific tilts of both planets. Saturn’s axis is tilted 26.73 degrees from vertical as it orbits the sun, which is similar to Earth’s 23.5-degree tilt, according to NASA. So as each planet rotates on its axis and orbits the sun, Saturn’s appearance, including its rings, can differ when viewed through telescopes or high-powered observatories on Earth.
Occasionally, Saturn’s angle of tilt toward Earth turns the rings precisely edge-on, making it seem like the thin bands are gone.
“They literally almost disappear,” said Sean Walker, an associate editor at Sky & Telescope, a monthly magazine about science and amateur astronomy. “Normally you see the rings around Saturn, but when you have an edge-on view, it looks like a hair-thin line of light just cutting across.”
In recent months, Saturn’s vast rings have appeared to progressively thin when viewed through ground-based telescopes. By Sunday, the rings will reach that precise edge-on angle.
The illusion is temporary, lasting only about a day or two before the rings slowly start coming into view again.
And the event is relatively rare: Ring plane crossings — as the phenomenon is known — typically occur twice during the 29.4 years it takes Saturn to make one orbit around the sun. This means they can be seen from Earth about once every 13 to 15 years.
Unfortunately, however, Saturn will be too close to the sun this weekend for skywatchers to see how it looks without its rings. But another opportunity will come later this year, on Nov. 23, when the rings will be nearly edge-on (though not precisely).
Walker said he witnessed a ring plane crossing in 1995, using a backyard telescope.
“It was fantastic,” he said. “It looked like a different planet. I spent a good part of the night just drinking in that view.”
It was around that time, he said, when he was just starting to become interested in astronomy.
In addition to being a quirky product of geometry, a ring plane crossing also happens because of the structure of Saturn’s rings. The iconic accessories are made up of billions of swirling chunks of rock and ice but are relatively flat, which is why the rings can seem to disappear into a thin line when tilted edge-on to Earth.
The edge-on view also makes it easier to see more of Saturn’s moons, Walker said.
“When we have these ring plane crossings, the light that normally reflects off Saturn’s rings is no longer glaring back toward Earth,” he said. “That means you can detect a lot more of the smaller moons.”
With a regular backyard telescope, Walker said, people can often spot four to six moons around Saturn. During a ring plane crossing in the dark sky, people may be able to detect nearly a dozen.
“The really faint ones become apparent because they line up in almost a perfect line,” he said.
After this weekend’s ring plane crossing, the next full one will take place in 2038. That event should be visible from Earth, according to Walker.
“I’d love to see it again,” he said. “It’s just going to be a little while to wait.”
