Observer's Notebook for December 1999

The Ringed Planet

by John Rummel

It is impossible to discuss the planet Saturn without the discussion being dominated by the beautiful ring system of this huge planet. Though other planets have been found to have rings, none are as spectacular or as visually striking as those orbiting Saturn.

When Galileo first turned his telescope on Saturn in 1610, he was struck by the odd appearance of the planet. His telescope was extremely crude by today's standards - magnifying just 20 times and having fairly primitive optics. Galileo thought he was seeing a three-lobed planet. "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," he wrote. The picture below is Galileo's first known sketch of the planet Saturn, which corresponds perfectly to his description.

It is easy to forgive Galileo for not understanding what he saw. In just a few hours with his new "spyglass," he had turned much of cosmology on its ear, and founded the modern science of astronomy. He was the first to observe features on the moon, the first to see that Jupiter was a giant planet with a system of moons all its own, and the first to see that the milky way was made up of a staggering number of previously unknown stars. Had he identified the strange features of Saturn as a ring, it would have scarcely made more sense than a three lobed star.

His surprise must have been even greater in 1612, for upon further observation of Saturn that year, the two extra lobes had vanished, and Saturn appeared as a featureless disk! The planet Earth had simply entered the plane of the rings, causing them to be temporarily invisible, but Galileo didn't know this, and his wonder only increased, "I do not know what to say in a case so surprising, so unlooked for and so novel."

Saturn's rings are inclined relative to the sun at about 27 degrees. Twice during each orbit around the sun, or about every 15 years (Saturn's year is about 29.5 Earth years), the rings will appear edge-on as viewed from Earth. Galileo was the first to see this phenomenon in 1612. Our next opportunity will come during 2009 (last time was 1995-96).

Currently, the Earth is approaching the position where Saturn's rings achieve their maximum tilt. The more the rings are opened out, the brighter the planet appears visually. Such wide open rings make for great telescopic viewing - the ringed planet's most stunning feature is presented at its best viewing angle.

At the recent Observatory Grand Opening (held on October 27th at Memorial High School), I and several other Madison Astronomical Society members had our telescopes set up out on the school lawn. We were blessed with a clear evening and hundreds of planetarium visitors were lucky enough to get good looks at Jupiter and Saturn through some nice telescopes. It's always a treat to give someone, adult or child, their first glance at the ringed planet. Exclamations of "wow," "cool," and "is that real?" are the most often heard comments.

Pictured above is a view of Saturn I obtained in September of this year. The rings are opened so that the south pole of the planet is presented toward Earth. This picture is comparable to what you can expect in a medium sized telescope at about 100x. Larger scopes will give breathtakingly detailed views of the lovely ring system, and many accompanying moons.

Saturn just passed its "opposition" this month - its closest approach to Earth for the year. That means it's big, it's bright, and it's up all night. Saturn can easily be found any clear evening this fall just to the lower left of Jupiter in the eastern or southeastern sky. Jupiter is the brightest object in the evening sky except for the moon. Saturn is a bit dimmer, but really gives Jupiter a run for its money when viewed through a telescope.

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