2009.03.12 04:05:51

Rick Fienberg

I have good news and bad news. The good news is that Saturn is now well up in the evening sky, making it an obvious telescopic target for 100 Hours of Astronomy in early April. The bad news is that Saturn's rings are tilted nearly edge-on to Earth, causing the planet to appear less bright in our skies — and less spectacular in our telescopes — than seems fair during the International Year of Astronomy.

But even the bad news is good news, because it offers an opportunity for a "teachable moment" at the telescope. It turns out that among his many accomplishments, Galileo was the first to observe Saturn's rings change from wide open to edge-on and back again. Curiously, though, he never realized it!

Here's how the great Italian scientist described his initial view of what was then the outermost planet known in the solar system: "Saturn is not a single star, but is a composite of three, which almost touch each other, never change or move relative to each other, and are arranged in a row along the zodiac, the middle one being three times larger than the lateral ones, and they are situated in this form: oOo." That was in July 1610, shortly after the publication of his book Sidereus Nuncius, in which he announced his earlier discoveries of the craggy lunar surface, four moons orbiting Jupiter, and countless stars too faint to be visible without a telescope.

Were Saturn's companions moons? If so, they must be very different from the ones around Jupiter: they were a considerable fraction of the size of the planet itself, and they didn't change position from night to night. Soon the mystery deepened: in December 1612 Galileo noted in a letter that Saturn was now a solitary sphere! He predicted that its two attendants would return, and indeed they did. But now they took on yet another shape: crescents, as shown in Galileo's 1616 drawing at right.

Unfortunately for Galileo, he never managed to build a telescope good enough to give a really clear view of Saturn. It wasn't until the 1650s that Dutch astronomer Christiaan Huygens finally figured out what was going on: the planet is encircled by "a thin, flat ring, nowhere touching, and inclined to the ecliptic."

It's that last part that explains Saturn's changing appearance, as illustrated in Huygens's diagram below. The ecliptic is the plane of Earth's orbit around the Sun. Just as Earth's equator is tilted about 23½° out of this plane, Saturn's is tipped by about 27°. This means that for half of Saturn's 29-year circuit around the sky we see the rings' north face, and for the other half we see their south face. Every 14 to 15 years, during the switch from north to south or south to north, the rings briefly appear edge on, all but disappearing from view because they're so very thin — only 10 meters (30 feet) or so. The last time this happened was 1995-96, when the Hubble Space Telescope began snapping the sequence of images that appears near the top of this page. And here we are in 2009, with another ring-plane crossing happening right on schedule.

Galileo watched Saturn's rings disappear, but he couldn't fathom what he was seeing. Four hundred years later, you can watch Saturn's rings disappear too — and know why. That's exactly what we're celebrating during IYA2009: the advancement of our understanding of the universe brought about by the telescope, from Galileo to Hubble and beyond.

Want to see Saturn for yourself? The ringed planet is currently just a little south of Leo the Lion's hindquarters. If you're not sure how to find Leo in the sky, don't worry. During 100 Hours of Astronomy, the first-quarter Moon comes along to show you the way. As illustrated here using the Stellarium freeware planetarium program, Leo is just left of the Moon on the evening of April 3rd (as seen from midnorthern latitudes), with Saturn nearby, about halfway up the sky — just begging you to have a look.

By the way, we designed the IYA2009 Galileoscope with Saturn in mind. Even at the telescope's modest 25x magnification, you can detect the rings. Add the included 2x Barlow lens to double the magnfication to 50x, and you can't help but be impressed with the view — much better than Galileo himself ever enjoyed.