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ure out the orbital periods for Europa and Ganymede. By the way, Io's distance from Jupiter isn't too much greater than the moon's distance from Earth. Yet, Io flies around Jupiter in 1.769 days whereas Earth's moon circles our planet in 27.322 days. The difference in the orbital periods is due to the fact that Jupiter is 318 times more massive than Earth. If Earth equaled the king planet Jupiter in mass, then our moon's orbital period would be a little over 1.5 days.
• Ever since the early 1600s - when Galileo peered at Jupiter with one of the first telescopes - Jupiter's four largest moons have played a major role in the history of astronomy. At that time, Earth was thought to be the center of the universe. Galileo's observation that Jupiter's moons revolve around Jupiter - and not Earth - suggested the possibility that Earth isn't the center of everything, after all. That observation was the beginning of our modern-day cosmology.
• Jupiter's Great Red Spot is spinning up. Your backyard telescope should show you Jupiter's famous Great Red Spot. This is a huge storm on Jupiter, twice as wide as the planet Earth. Recently, astronomers have noticed that Jupiter is "spinning up," or rotating faster. How do they know? The Great Red Spot has been seen to shrink slightly. Astronomers compare it to an ice skater pulling her arms in to spin faster. As the size contracts, the spin rate increases.
• Bottom line: Use the moon to find Jupiter, the fifth planet outward from the sun, this Tuesday evening, December 25, 2012! The Christmas conjunction of this bright pair will be seen throughout the world.
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