Throughout 2009 stargazers the world over are celebrating the International Year of Astronomy, which marks 400 years since the great Italian scientist Galileo Galilei first turned a telescope to the sky. Galileo's discoveries launched a revolution in our understanding of the cosmos, one that's still going strong four centuries later.

While it's true that Galileo first glimpsed the Moon and stars with his little refractor in late 1609, he didn't make the discovery for which he is arguably most famous until January 1610. That was when, as he wrote in his book Sidereus Nuncius (The Starry Messenger), "Jupiter presented itself to me."

Viewing Jupiter at a magnification of 30×, Galileo was amazed to see it form a straight line with three stars, two on one side of the planet and one on the other. Night after night, the stars' positions right or left of Jupiter changed, and eventually he noticed a fourth star in the lineup too. Was Jupiter zigzagging against the starry background from one night to the next? That didn't make sense. Galileo soon realized that the planet's shifting companions aren't stars — they're moons!

On this manuscript page Galileo recounts the observations that led to his realization that Jupiter is circled by four moons at different distances from the planet

It's hard to overstate the significance of this early telescopic finding. In 1610, nearly everyone thought that Earth was the center of all heavenly motion. Polish astronomer Nicolaus Copernicus had proposed 67 years earlier that the planets — including Earth — orbit the Sun, but most scientists thought this was ridiculous. After all, they reasoned, if Earth were whipping around the Sun, it would lose the Moon! Yet here was Galileo showing that a planet could orbit something — it didn't matter whether it was the Sun or the Earth — and hang onto its moons.

Galileo's discovery of what we now call Jupiter's Galilean satellites didn't prove that Earth goes around the Sun, but it definitely made the idea less outlandish. And sure enough, within a matter of decades, Copernicus's heliocentric, or Sun-centered, solar system achieved widespread acceptance — especially after the great English scientist Isaac Newton identified gravity as the force that keeps moons orbiting planets and planets orbiting the Sun. (Incidentally, Galileo's German contemporary, Johannes Kepler, figured out in 1609 that celestial orbits are ellipses, not circles. The International Year of Astronomy appropriately commemorates Kepler's contributions to science as well as Galileo's.)

Author Rick Fienberg captured this image of Jupiter and its four Galilean satellites on the evening of October 10th from his observatory in New Hampshire. From left to right, the moons are Callisto, Europa, Io, and (to the right of Jupiter) Ganymede.

As it did to Galileo four centuries ago, Jupiter is presenting itself to us right now. Just go outside after dark, look low in the south if you live in the Northern Hemisphere or high in the north if you live in the Southern Hemisphere, and there it is: the brightest point of light in the sky these evenings. If you know the constellations in this part of the starry dome, you'll recognize that Jupiter shines in Capricornus, the Sea Goat. The planet is truly dazzling — much brighter than any star.

If you own any telescope at all, I think I can safely say it's more powerful than Galileo's. It surely has a bigger and better main lens or mirror, and it offers higher magnifications than 30×. This means you can easily see Jupiter's moons for yourself. In fact, you may be able to see them in ordinary binoculars.

As Galileo himself noticed, the moons orbit so fast that you can see their positions change appreciably over just a few hours. Io, the innermost of the Galilean satellites, orbits Jupiter in 1.8 days — that's a pretty short month! Then come Europa (3.6 days), Ganymede (7.2 days), and Callisto (16.7 days). Since they shift positions continuously, how can you tell which is which in the eyepiece? My favorite way is to call up Sky & Telescope's Jupiter's Moons applet. Any of the popular desktop-planetarium programs will identify the moons for you too.

The Cassini spacecraft captured this stunning view of Jupiter, with the Great Red Spot at lower right, as it flew by at a distance of 48 million miles on October 8, 2000, en route to Saturn. Courtesy NASA, JPL, and the University of Arizona.

Sometimes you'll see a moon's inky black shadow drift across Jupiter's cloud tops, and sometimes you'll see a previously hidden moon pop out from behind the planet's disk or shadow. Planetarium software and the Jupiter's Moon applet can alert you to such events. I've been using telescopes for more than 40 years, and I still find myself watching Jupiter's moon-dance every chance I get.

In a modern backyard telescope, you can get a better view of Jupiter itself than Galileo ever did. He never mentioned the planet's cloud bands, which are visible in even a 2-inch-diameter telescope as dark and light stripes. And you can see the Great Red Spot, a gigantic cyclone that's been raging in the planet's southern hemisphere for several centuries. Keep in mind, though, that you can see the spot only when it's on the Earth-facing side of the planet, which is true for only a few hours at a time since Jupiter spins once every 9.8 hours. (Sky & Telescope has another applet you can use to find out when the Red Spot is crossing the center of Jupiter's disk.) And the spot's really not so great — it's a subtle reddish-brown oval, hardly worthy of its name. But it's still worth looking for.

So get outside this season and spend some quality time with Jupiter. Think of it as your Galileo moment!

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