One of the biggest stories in 21st-century astronomy is the discovery of hundreds of planets around other stars. Thanks to NASA's Kepler planet-hunting satellite, it's only a matter of time (probably less than 3 years) before astronomers find an Earth-like world orbiting a Sun-like star. But you don't have to wait. If you own a telescope, you can look out into space and scrutinize an Earth-like planet every clear evening this season. It's called Mars.

Mars is only about half the diameter and a tenth the mass of Earth, and it has no oceans — though it may have had them in the remote past. Still, in a telescope, it shows numerous features reminiscent of our home planet, including polar ice caps, dark surface markings, bright clouds, and blowing dust. But observing Mars is practical only once every 2 years or so, when it and Earth are relatively close together on the same side of the Sun. The rest of the time, Mars is so far away that it appears as little more than a featureless dot.

Mars (arrowed) gleams with a distinctly ochre hue and outshines all the bright stars with which it shares the sky these evenings. The Red Planet was closest to Earth, shining brightest and appearing biggest in telescopes, in late January. Made with Stellarium.

That "every 2 years or so" has come around again, so Mars is now ideally placed for telescopic enjoyment. It was closest to Earth - about 61 million miles away - on January 27th. Two days later it was at opposition, directly opposite the Sun in our sky. That night it rose at sunset, got highest in the sky at midnight, and set at sunrise. It's easy to see why Mars is called the Red Planet. Go outside on any clear evening and look to the east. There you'll spot a rust-red "star" shining more brightly than any other point in that part of the sky. That, unmistakably, is Mars.

NASA's planet hunter is named for Johannes Kepler, who, in 1609, revealed that planets orbit the Sun in ellipses, not circles. Kepler made this landmark discovery by studying the orbit of Mars, which turns out to be one of the most eccentric, or "out of round," planetary orbits in the solar system. If we happen to catch up to Mars when it's in the part of its orbit closest to the Sun, the Red Planet appears especially bright and big in our telescopes. But this time we've caught up to Mars when it's in the part of its orbit farthest from the Sun, so the planet's disk is at best a disappointingly small 14 arcseconds across. Four oppositions from now, in July 2018, Mars will be much more impressive, spanning 24 arcseconds. (For comparison, Jupiter's disk ranges from about 33 to 50 arcseconds in angular diameter.)

Mars's surface features, clouds, and ice caps are best seen in telescopes of aperture 4 inches or larger. I find that inexperienced observers don't see much of anything on Mars in telescopes smaller than 8 inches and at magnifications less than about 200x. Discerning detail on the disk requires patience and diligence, because unlike Jupiter's main cloud belts and Saturn's rings, Mars's bright and dark markings are remarkably subtle.

What you see when you examine Mars in a telescope depends on when you look. Damian Peach captured these views of the planet's opposite hemispheres through Celestron Schmidt-Cassegrain telescopes. On January 4, 2010 (left), Mars showed its most distinctive dark marking, Syrtis Major, shaped somewhat like Africa. Two weeks earlier, on December 16, 2009 (right), the vast and largely featureless Amazonis desert faced Earth. South is up in both images. Courtesy Damian Peach.

No matter what your level of experience, what you see on Mars depends strongly on when you look. Virtually all of the most contrasty surface features are concentrated in one hemisphere; the other side of the planet is largely blank. Whenever Mars is in the evening sky, as it is now, astronomy magazines publish tables indicating which longitudes are visible at which times. You can then look up the numbers on a Mars map to determine which part of the planet is facing Earth during your observations. Sky & Telescope has a particularly nice utility on its website to make this task easier. S&T's Mars Profiler shows you a map of the side of Mars facing Earth for any date and time, and you can set it to display the map in whatever orientation suits your optics, for example, south up or mirror-reversed. I always have S&T's Mars Profiler open on my laptop when I'm observing the Red Planet. By knowing exactly what to look for and being able to name what I see, I find I can tease out subtle details that I'd otherwise miss.

An armada of robotic spacecraft have visited Mars since the 1960s, and several are operating there now, giving us unprecedented close-ups of the planet's surface. This view of Victoria Crater was snapped by NASA's Mars Reconnaissance Orbiter. Courtesy NASA/JPL-Caltech/University of Arizona.

We no longer have to content ourselves with distant views of Mars from earthbound telescopes, or even from the Earth-orbiting Hubble Space Telescope. The world's spacefaring nations have made the Red Planet a prime target for on-site exploration, mainly because of the slim chance that life may have once gotten a toehold there. There are currently three spacecraft orbiting Mars and mapping its surface in unprecedented detail: NASA's Mars Odyssey and Mars Reconnaissance Orbiter and the European Space Agency's Mars Express. Thanks to the Internet, anyone can browse the extensive image archives from these (and earlier) missions and explore Mars from a perspective just a few hundred miles above its surface.

And if that's not close enough for you, you can virtually descend to the ground itself thanks to the Mars Exploration Rovers Spirit and Opportunity, which have been ambling across the dusty surface and taking pictures for 6 full years now — much longer than anyone expected when the plucky robots were launched on 3-month missions. Someday, probably much farther in the future than space buffs would like, humans will stand on the Red Planet themselves and look back at Earth, the Blue Planet.

NASA's Opportunity rover rolled to the rim of Victoria Crater and captured this shot of the Cape Verde promontory. The dramatic cliff of layered rocks is about 50 meters (165 feet) away from the rover and about 6 meters (20 feet) tall. Courtesy NASA/JPL/Cornell.

While exploring Mars in the eyepiece, it's fun to think of all the spacecraft working there to reveal the secrets of the planet's past and paving the way for human exploration in the future. But it's even more fun, I think, to see Mars with your own eyes, from millions of miles away, and to get to know it not just as a light in the sky, but as another world with places you can name and identify on a map. Enjoy the view now, while you can, because in another few months Mars will recede into the distance, not to strut its stuff again until 2012.

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!