Saturn is at opposition this week.

That means relative to Earth, the sun and Saturn are directly opposite one another.

Last year, Earth crossed through Saturn's ring plane, making the rings virtually disappear from our perspective.

It takes Saturn almost 30 years to complete one orbit around the sun. We pass through the ring plane every 15 years, as Saturn reaches its vernal and autumnal equinox points. Each time we pass through the ring plane, our view of Saturn alternates toward either its northern or southern hemisphere. Up until late 2009, the south side of Saturn's rings were illuminated by the sun. Now, for the first time in 15 years, the north side of the rings are sunlit.

Today, the fact that Saturn has rings is almost taken for granted. But in 1610, as Galileo raised his telescope to look at Saturn, he was astonished by the sight. He noted in his log book, "I have observed the highest planet to be tripled-bodied. This is to say that to my very great amazement Saturn was seen to me to be not a single star, but three together, which almost touch each other."

Two years later, Galileo looked at Saturn again and the "triple-bodied star" had become one. He wrote "I do not know what to say in a case so surprising, so unlooked for and so novel."

What Galileo had actually witnessed was Saturn's ring plane shift to edge-on. It wasn't until nearly 50 years later that Christiaan Huygens was able to explain that Saturn was surrounded by a ring system.

Right now, Saturn's rings are tilted just a few degrees from edge-on giving us a good look at its moons. At other times, when Saturn's rings are more inclined, it can be hard to see the moons as they pass in front of the sunlight glare of the ring system.

While an edge-on view of the rings only happens every 15 years, another interesting effect is seen each time Saturn reaches opposition. It is called the Seelinger Effect. When opposite us from the sun, Saturn's rings are bathed in direct sunlight. The rings become very bright, with an almost starkly white appearance. Once past opposition, the rings become more subdued and with a more muted appearance.


Saturday is this year's observance of Earth Hour. Earth Hour is a worldwide community event designed to highlight growing concern about global climate change.

Governments around the world will demonstrate solidarity by extinguishing lights on famous monuments and buildings. In Paris, the Eiffel Tower will go dark. In Athens, lights on the Acropolis will be turned off. In Australia, the Sydney Opera House will have its lights powered down.

Across the United States, leaders in large cities including Chicago, New York, Las Vegas, Nev., and San Francisco are joining in with more than 681 cities in 76 nations around the world.

Earth Hour executive director Andy Ridley states, "Earth Hour is driven by citizens and grassroots groups thinking globally and acting locally. No matter how big or small your organization, I urge you to get involved in Earth Hour and really make a difference in your community and in the world."

Earth Hour begins at 8:30 p.m. To learn more about Earth Hour activities worldwide visit http://earthhour.org.

Peter Lipscomb operates Astronomy Adventures guided night sky tours and advocates for sensible and energy-efficient lighting practices through public outreach and education. Contact him at peter@astronomyadventures.com.