Moon and Saturn
 Saturn's rings look blue in this recent infrared image from the Cassini spacecraft, which has been orbiting Saturn for almost three years. Big storm systems and bands of clouds highlight the planet itself. Because it is a big ball of gas that rotates quickly, Saturn is a bit flattened. It is about 10 percent larger through the equator than the poles. [NASA/JPL/SSI]
The same thing happens to all the planets. One world where you really can see the difference is Saturn, the second-largest planet in the solar system. It's about 10 percent fatter through the equator than the poles, so it looks like a flattened beachball.
The "squashed" appearance is an important clue to Saturn's structure and composition. When combined with other clues -- like the planet's gravitational pull on its moons -- it shows that Saturn is the least-dense planet in the solar system -- less dense than water.
In fact, Saturn is made primarily of hydrogen and helium. The planet has a small, dense core, but most of its great bulk consists of either gas, or gas squeezed into a liquid or even metallic form. Without a rigid structure, as Saturn spins on its axis its material is forced outward at the equator -- giving the planet a definite bulge around the middle.
Saturn lines up between the Moon and the star Regulus tonight. They're high in the west at nightfall, and remain in view into the wee hours of the morning. Saturn looks like a bright golden star a little to the upper left of the Moon. Regulus is about the same distance to the other side of Saturn. More about this beautiful trio tomorrow.
Script by Damond Benningfield, Copyright 2007
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