Date: August 10, 2009

Title: What Happened to Saturn’s Rings?

Podcaster: Adam Fuller from Columbia Astronomy

Organization: Columbia University Astronomy
http://outreach.astro.columbia.edu

Description: 400 years ago Galileo turned his telescope towards Saturn and saw what he thought were three separate round bodies. For the next 50 years astronomers hotly debated the true nature of Saturn. Then, in 1659, Christiaan Huygens radically changed the debate with his book, System Saturnium. In it he provided the correct explanation of Saturn’s companions: that they were actually part of a large ring wrapping around the planet. In this podcast we’ll delve into the history of this groundbreaking insight and explain why Saturn is ringless tonight.

Bio: Adam Fuller is currently a graduate student in the Earth and Planetary Sciences department at Johns Hopkins University. He graduated from Columbia University with a B.S. in Astrophysics this past May. He also has a B.A. in Journalism from the University of North Carolina at Chapel Hill (How ‘bout them Heels!). His research interests include planetary science, meteorology, and astrobiology. Outside of school he is an avid marathoner, an undrafted NBA free agent, and a dedicated uncle to three proto-astrophysicists.

Today’s sponsor: This episode of The 365 Days of Astronomy Podcast is sponsored by Clockwork Active Media Systems. Clockwork invents, designs, develops and maintains web applications that market, sell, streamline, automate and communicate. Visit http://www.clockwork.net or email inquiries at clockwork dot net to get started on your web project.

Transcript:

Hello everyone, and welcome to Columbia Mondays! My name is Adam Fuller, and I’m a recent graduate of the undergrad astrophysics program at Columbia University in the City of New York. I’m currently a first year grad student in the Earth and Planetary Sciences department at Johns Hopkins University.

In today’s 365 Days Of Astronomy podcast I’m going to talk about the discovery of Saturn’s rings. There are two motivations for this. The first is that Saturn is my favorite planet. Some of my undergraduate research at Columbia focused on the evolution of Saturn’s rings, and I’ve given a couple of public talks and slideshows this past spring about this “beautiful stranger” in our solar system. The second motivation is that today is Saturn’s equinox, a day when its rings disappear from Earth’s view.

Saturn’s spin axis is tilted almost 27 degrees relative to it’s orbital plane. Its orbital period, one Saturn year, is almost 30 Earth years long. This means Saturn’s equinox, when the sun is directly over its equator and each hemisphere’s day is the same length of time as its night, happens once every 15 years. During Saturn’s equinox, Saturn’s rings will be viewed edge-on from Earth; they’ll completely disappear! This isn’t a sudden event, however. The rings have been gradually getting thinner for the past seven and a half years—since Saturn’s solstice—but it’s become dramatically more noticeable to us here on Earth for just over the past year. After today, they’ll gradually reappear.

So in today’s podcast we’re going to celebrate Saturn’s rings the way we celebrate friends just before they go away on a long trip. We’ll reminisce over the good times and the bad. We’re sad to see them go, but we know they’ll be back, so there’s no reason to feel down.

Like almost everything else in astronomy, the story of Saturn’s rings starts with Galileo Galilei. 400 years ago he turned his telescope towards the night sky and recorded his first observations of the Moon. Seven months later he made his first observations of Saturn. He wrote, “Saturn is not alone but is a composite of three, which almost touch one another, nor do they move with respect to one another nor change…”

That was July 25th, 1610. Two years later Galileo noticed that Saturn’s two “moons” on either side of Saturn had disappeared. He predicted they’d reappear the following summer for just a couple of months, and he was right—about them reappearing. But instead of sticking around for just a couple of months, they remained visible for several years. Then, in 1616, he noted that Saturn’s “two companions are no longer two small perfectly round globes as they were before, but are at present much larger bodies and no longer round in shape, but as seen…[as] two half-ellipses.”

These “moons” had begun to change shape!

For the next 40 years many astronomers from across Europe turned their telescopes towards Saturn and began recording what they saw. Every astronomer saw something different. Galileo saw moons. Christopher Wren thought it was a thin corona that rotated about Saturn. Johannes Hevelius believed that these were two appendages physically attached to the planet, like giant arms or cup handles. Everybody had a different explanation, but nobody had one explanation for all of the different shapes that had been observed.

That is, no one until Christiaan Huygens began building his own telescopes. Back in the 17th Century, if you wanted a telescope, you couldn’t order one off the web. You either had to make your own, or you had to be a nobleman who received one from a scientist who wanted you to fund his research. Huygens made his own, and his scopes were so good he quickly realized that Saturn’s “companions” were actually “a thin flat ring, nowhere touching, and inclined to the ecliptic.” This is what he encoded in an anagram that he included in a treatise he published in 1656 that also announced his discovery of Titan. During the next three years he made continuous observations with his superior telescopes and, using the heliocentric model of our solar system, eventually worked out how the “thin flat ring” affected Saturn’s “shape” over the years. He published his results in his ground-breaking book, Systema Saturnium, in 1659, 350 years ago.

But let’s quickly step back 1,700 years before Huygens for a moment. Around 250 BC, a Greek astronomer, Aristarchus of Samos, proposed a radically new model of the universe, a heliocentric model. Instead of having the Earth at the center of the universe, Aristarchus proposed that it was actually the Sun. All the other known planets moved around the Sun along the same plane. Aristarchus even deduced the correct order of the planets: Mercury, Venus, Earth, Mars, Jupiter, and Saturn. (Uranus and Neptune weren’t discovered until 1781 and 1846.) It was a revolutionary idea that was immediately forgotten until Copernicus resurrected it in 1543. It was this heliocentric model of the universe that allowed Huygens to figure out the true reason behind the changing appearance of Saturn’s disk.

When Huygens looked through his scopes, he saw a disk that seemed to wobble up and down between Saturn’s hemispheres over the years. He could tell which side of the disk he was looking at based on the orientation of the ring as it bent in front of Saturn. If the solar system followed the heliocentric model—which still wasn’t fully accepted by the scientific mainstream in 1659—Huygens realized the shape of the disk was simply a matter of visual orientation. Prior observations disagreed wildly, he said, because of the inferior telescopes his contemporaries were using to observe Saturn.

Because Saturn’s spin axis is at a fixed incline relative to its orbital path, as Saturn orbits the Sun only the angle at which we see the rings changes. During Saturn’s equinox, we see the rings edge-on. Something astronomers didn’t know in the 17th Century, however, was that the densest, most visible rings are barely 100 feet thick. When Saturn and Earth are lined up on the same side of the sun, they’re still 800 million miles away from each other. We’re trying to look across 800 million miles at something that’s thinner than a third of a footfall field. No wonder they seem to disappear! During Saturn’s solstice, however, when we’ll get the best view of one of its poles, we’ll also see the rings at their maximum inclination visible from Earth. Again, Saturn’s spin axis is fixed, so the rings aren’t changing, just our viewing angle of the rings.

Huygens revealed his discovery in 1659 when he published his book, Systema Saturnium. 300 years later this book was celebrated as one of the top ten most important books in the formation and development of Western science and technology. This wasn’t the case, however, when the book was originally published. The heliocentric model of the solar system wasn’t widely accepted yet, and the Church still carried a lot of influence in astronomical circles. Less than a year after its publishing, Systema Saturnium came under attack by Eustachio Divini, an Italian telescope maker, and Honoré Fabri, a Jesuit astronomer in Rome. They published a treatise that criticized Huygens observations, his telescopes, and his heliocentric arguments. They also put forth a theory that Saturn had four satellites: two nearby dark ones and two far out bright ones. The dark one, they said, occasionally eclipsed the bright ones, creating the optical illusion of Saturn having handles.

Huygens responded to Divini and Fabri a few months later with another treatise that discussed damning problems with their observations. To fix these issues, Divini and Fabri needed to add two more satellites, but the damage was done. A series of experiments were conducted at Italy’s prestigious Accademia del Cimento shortly afterward that showed that Huygens’ ring theory explained Saturn’s appearance much better than Divini and Fabri’s. In fact, by 1665, Fabri conceded that Huygens’ ring theory was correct after he used a telescope with much better optics to observe Saturn. Of course, he still refused to accept the heliocentric model. By 1670, Huygens’ ring theory was generally accepted by almost all astronomers and academics.

So what is it again that’s happening today? Aside from Bert Ma’s birthday, it’s Saturn’s equinox; the Sun is shining directly on Saturn’s equator. This means each hemisphere’s day is the same length of time as its night. (Earth’s next equinox is September 22nd.) This also means that from our vantage point on Earth the rings are edge on; they’ve completely disappeared. But they’ll reappear and should be observable again within a couple of months as spring begins in Saturn’s northern hemisphere.

But a ringless Saturn isn’t something to avoid tonight. The disappearance of Saturn’s rings means it should be easier to pick out Saturn’s moons—Titan, Tethys, Rhea, and Dione—before and after they transit Saturn. Enceladus (en-SELL-e-dus’) may even be observable.

Well, I hope you’ve enjoyed this latest Columbia Monday podcast about the discovery of Saturn’s rings. For more information about the public events at Columbia Astronomy visit outreach.astro.columbia.edu. Our next Columbia Monday podcast will be by Holly Larson on Monday, August 24th entitled “The Re-emergence of Planetary Science: Revisiting Percival Lowell.” I’m Adam Fuller. Have a great day and keep listening.

End of podcast:

365 Days of Astronomy
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