The sixth planet from the sun, Saturn, is perhaps best known for its many rings, which consist of billions of particles of ice and rock. But throughout the next several months, if you look at Saturn with a telescope, you’ll see something strange – the rings seem to be disappearing.

The Hubble Space Telescope took this image of Saturn, seen titled edge-on, in 1995.

That’s because about every 14 to 15 years, the tilt of the planet is such that we on Earth see the rings edge-on. In reality the rings are still there, but they appear nearly invisible from Earth.

The phenomenon, which stumped Galileo in the 1600s, is called a “ring plane crossing.” While the Earth has an equinox every six months, Saturn’s are more spaced out — in fact, it orbits the sun once every 29.5 years.

The rings will appear thinner and thinner until September 4, 2009, when they will seem to have vanished. On that day, we will see the sun and Saturn only 11 degrees apart in the sky, says Linda Spilker at NASA, deputy project scientist and co-investigator on the Cassini Mission to Saturn. But beware – Saturn will be in the daytime sky, making it difficult to see so close to the sun.

Unlike a solar eclipse, the Saturn-with-thin-rings phenomenon will be visible from essentially the whole Earth, because it is so far away, Spilker says. But it will be difficult to actually see this around the exact time of the ring plane crossing in September.

The bottom line is that, while the actual ring crossing doesn’t happen until the fall, if you want to see Saturn appear to have thin rings, act fast!

According to the Sky and Telescope star-gazing guide for this week, Saturn’s rings are only 0.8° or 0.9° from edge-on. The planet rises around 10 p.m.

Read more about the ring crossing from NASA.