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June 17, 2024

Backyard Astronomy: Saturn

By Cheryl Haimann

Even if you know next to nothing about astronomy, you could probably pick Saturn out of a planetary lineup. It's the one with the rings, right? Right.

It wasn't always known by that distinction. As the second largest planet in the solar system, Saturn is easily visible to the naked eye. When Galileo first turned his telescope toward it in 1610, he was puzzled. The disc had protrusions on either side, unlike anything he had ever seen. Not only that, but the protrusions became smaller over time, and even disappeared briefly. For the next fifty years or so, astronomers debated the nature of the unusual planet. Did it have handles? Ears? Two other nearby objects?

Telescopes, and astronomical observation, became more sophisticated during the 17th century, and by mid-century, Christiaan Huygens [HOI-gens] was able to identify a flat ring, separate from the planet and able to cast shadows. He also saw that, like Earth, Saturn was tilted on its axis, accounting for the varying shape of the rings. As Earth and Saturn orbited the sun, our perspective constantly changed. A few years later, Giovanni Domenico Cassini discovered that there was not one ring, but two, with a gap between them that is now known as the Cassini Division. This gap is easily visible in most modern telescopes.

Contemporary studies have shown that there are even more rings (the count is currently seven), and that even the gaps have some matter in them. The rings are composed of chunks of water ice, ranging from pebble sized to a few meters in diameter. The rings are not very deep, only a few kilometers at the most, but the purity of the ice makes them very reflective.

It is likely that the rings are the remnants of moon that disintegrated. Whether the destruction was caused by a meteor strike or by the moon being ripped apart by the tidal force of Saturn's gravity is still a matter for speculation.

In 1980 and 1981, Voyager 1 and Voyager 2 flew by Saturn on their way to the far reaches of the solar system. They confirmed that Saturn is largely composed of hydrogen and helium. It is the only planet in our solar system that is less dense than water. Given that you could find a large enough lake, Saturn would float in it.

More recently, the Cassini spacecraft spent four years exploring Saturn. After its original mission ended in 2008, it was still performing well, so the mission was extended. The new mission, Equinox, is studying how seasonal changes affect Saturn, the rings, and its moons.

Saturn takes 29 years to orbit the Sun, and twice during that cycle, when Earth passes through the plane of the rings, the rings disappear from our view. 2009 is one of those years. The rings appear quite slender now, and the disappearing act will occur on September 4. In the spring, though, you can still see the rings, and possibly some of the detail, through a telescope.


GLOBE at Night is an annual event to raise awareness of light pollution. Between March 16 and 28, 2009, participants go out on any clear night and observe Orion. They compare their observations to a set of magnitude charts on the website, and report the results, which are then compiled and made available on the website. GLOBE at Night is kid-friendly, and offers information and projects for students, parents and teachers.


Backyard Astronomy updates are available on Twitter!

Article © Cheryl Haimann. All rights reserved.
Published on 2009-03-16
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