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Jovian Moons

January 7, 2010

The planet Jupiter highlights the southwestern evening sky this month. It's the brightest star-like point of light in the early evening, so you can't miss it.

And if you study Jupiter with binoculars or a telescope, you'll see several other star-like points lining up beside it: the planet's four largest moons.

Three of them were first beheld by human eyes 400 years ago tonight, when Galileo Galilei aimed his telescope at the planet. He saw three little points of light lining up around Jupiter, and assumed they were stars. The alignment was so intriguing, though, that he looked again the next night, and several more nights after that.

Although the exact alignment changed, the little stars always stayed close to Jupiter -- something that wouldn't happen if they really were stars. What's more, a fourth point of light joined them. Galileo realized that these little lights were really moons in orbit around Jupiter.

It was a stunning discovery. At the time, the leading idea said that everything in the universe revolved around Earth. But the idea that Earth and the other planets went around the Sun was gaining ground. The discovery that celestial bodies were indeed orbiting something besides Earth provided strong evidence for that idea. It helped move Earth out of the center of the universe, while showing that many amazing discoveries awaited a remarkable new instrument: the telescope.

More about the moons of Jupiter tomorrow.

Script by Damond Benningfield, Copyright 2009

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