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Floating Saturn

Week of September 15, 1997

image of Saturn Last week's snack talked a bit about what ancient astronomers missed. This one remarks about what was known before the invention of the telescope. If you watch the night sky long enough, you'll note that some stars appear to wander along a line in the sky. These stars were called "planets" by the Greeks (which means "wanderers" in Greek) and were used to represent their gods.

In more modern times, the planets were studied in earnest by Johannes Kepler in the early 1600's. Kepler, basing his work on past astronomers Copernicus and Tycho Brahe, figured out mathematical relationships between the planets that could accurately predict their position years in advance. He did the bulk of his work largely before Galileo first turned his telescope to the starry sky, but it was still accurate enough that his tables of positions were in use for many years. Largely, we still use his mathematical equations today to predict the planets' motions.

One thing Kepler found, for example, was that the speed at which a planet orbited the Sun depended on its distance from the Sun; a slower moving planet was inevitably farther from the Sun. By representing this using math, astronomers were able to determine the actual distances to the planets without ever having actually left the Earth! Further, his relationship also applied (with some small changes) to the moons around planets as well. This turned out to be pretty important. Why, you ask?

Look at Saturn, or example (I'm sure you were wondering just when I'd get to the actual planet in today's snack!). We know its distance from the Sun, and therefore from the Earth, using Kepler's laws. However, we also can measure and time the orbits of Saturn's moons as well. These two relationships can let us figure out the mass of Saturn. Now, we can also measure Saturn's diameter because we can measure how big it appears, and we can compare that to how far away it is. From this we can determine Saturn's actual size (it's about 115,000 kilometers in diameter, or about 9 times the size of the Earth).

The density of an object is defined by its mass divided by its volume. When you calculate the density of Saturn, you find that it is the least dense of all the planets. In fact, it is less dense than water! You may know that things less dense than water float (ice, for example, or cork). So it is a common analogy to say that if you could find a tub of water big enough, Saturn would float in it! But it would leave a ring. Haha.

animation of Saturn rotation

Actually, astronomers may have known Saturn was not terribly dense before this calculation was ever done. When you look at Saturn through a telescope, it appears fairly flattened (Jupiter does too). That is because it rotates so quickly; centripetal force (the same sort of force that makes you lean over when a car makes a turn) flattens the spinning planet out. The Earth is a bit flatter this way (the equatorial radius is somewhat larger then the polar radius), but the effect for the Earth is small because the Earth is so stiff. But Saturn is mostly gas, and the spinning makes it look noticeably flattened. Check out a picture of Saturn and see for yourself! The picture above is an animation made up of several images of Saturn taken through a telescope. You can see how flat Saturn looks! (Animation courtesy of Antonio Cidadao.)

Want to know more about Saturn? The Nine Planets is loaded with info. You can skip right to the Saturn pages too.

©2008 Phil Plait. All Rights Reserved.

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