From Earth’s point of view, Mars is unique. It’s the most Earthlike world we know; backyard telescopes can sometimes show polar caps, surface markings, seasonal white clouds, and windblown dust. Mars also behaves uniquely in our sky. It spends most of its time far away as a tiny blob in a telescope, then every 2.1 years it swings much closer for just a few months around opposition.

Moreover, Mars comes almost twice as close at some oppositions than at others (because Mars has a significantly elliptical orbit that is also near Earth’s orbit). The near and far oppositions (“perihelic” and “aphelic”) come and go in a 16-year cycle.

We’re now near the bottom of that cycle. Mars reaches opposition and makes its closest approach to Earth in late January 2010. But it appears only 14.1 arcseconds in diameter at the time. Mars will appear larger than 12″ through the end of February, and larger than 10″ through late March (see the graph on the bottom). That’s pretty small.

And next time around, in March 2012, Mars will be only 13.9″ wide at closest approach. Not until July 2018 will it peak out again, at 24.3″.

But astronomy is all about making the most of very distant, difficult views. And at least Mars will cross the night sky high for Northern Hemisphere observers. It spends the next five months in and near Cancer, at declinations +16° to +23°.

In a good 4-inch or larger telescope on a night of steady air, you may first make out the bright north polar cap. In February or March you may notice the cap shrinking in the Martian northern spring. Dark surface markings may be harder to discern, depending on which side of Mars is facing Earth when you look. Watch also for bright limb hazes, occasional white clouds, and possibly the obscuring bright patch of a dust storm moving from day to day. But don’t expect impressive views. Every bit of what you see will be a hard-won prize.

It’s one thing to detect a vague smudge or two on Mars’s tiny disk. The smudges become much more exciting if you can identify and name them. Use our Mars Profiler to find out the names of the features on the side of Mars that's facing you.

Right now I'm daydreaming of being on the other side of the world — on the tiny island of Malé in the Maldives, to be exact. More than being someplace that's lots warmer and more picturesque than my native Boston is this time of year, tomorrow the 100,000-odd Maléans will, weather permitting, see an unusually long annular (ring) eclipse of the Sun.

In fact, notes eclipse guru Fred Espenak, January 15th's event is the longest annular eclipse until the year 3043. Were I in the right place (in the Indian Ocean midway between Madagascar and Sri Lanka) at the right time (7:06:33 Universal Time), I'd be able to watch the Moon's disk completely engulfed by the Sun for 11 minutes, 8 seconds.

Not much of a long-distance swimmer, I'd certainly settle for Malé, where annularity will be nearly as long (10m 45s). Or I might choose the 6m 53s available from Nairobi, the capital of Kenya, or even Chongqing in China (7m 53s). On paper, at least, I could pick a viewing site anywhere along the where the event's 200-mile-wide, 8,000-mile-long track. This eclipse's exceptionally broad path results from the Moon being relatively distant, two days before reaching the apogee of its orbit, and Earth being relatively close to the Sun, having passed perihelion on January 3rd.

The annular path begins at sunrise over central Africa (at 5:14 UT), then crosses open water until it slices directly between the southern tip of India and northern Sri Lanka (at 7:51 UT). The shadow then makes landfall again in Myanmar (formerly Burma) at 8:33 UT before crossing southern China and concluding at sunset just offshore in the Yellow Sea (8:59 UT).

Most everyone in Africa and Asia will see some degree of partial eclipse. In Calcutta and Beijing, for example, the eclipse will become about 75% complete.

If, like me, you're nowhere near the eclipse path, you can witness it vicariously by logging on to one of several online streams provided by dedicated umbraphiles.

You can also check out this year's other eclipses, including a tempting total solar eclipse in July that crosses bits of French Polynesia and Easter Island.

Yesterday I kept daydreaming about Galileo. Exactly 400 years ago, on January 7, 1610, the famous Italian astronomer spied the moons of Jupiter for the first time and, once he realized what he'd seen a few days later, created a revolution in astronomy that reverberates to this day.

Backyard observing has changed tremendously since Galileo's time. Our telescopes are so much better, our ability to appreciate what we see vastly improved. But we're still limited by local circumstances — by the time and place we choose to set up our scopes, and by the light pollution that almost certainly degrades our view.

Fortunately, this weekend you'll have not one but two chances to view the universe with first-rate equipment and modern detectors from pristine, dark-sky sites. The good folks at Astronomers Without Borders (AWB) have teamed up with the Virtual Telescope project and Global Rent-a-Scope to provide a remote-observing experience for those of us lacking good skies or too busy to drag our own scopes outside.

The effort, called Big Dipper to Southern Cross, features robotic scopes in two hemispheres. Using your computer, you'll watch as experienced observers slew from one object to the next and show how they capture the wonders of the night sky. You'll be able to chat with other participants and with the telescope operator. This event builds on a successful remote-observing effort held by AWB last September.

So if you've ever wondered what remote observing was like, here's your big chance to try it, along with like-minded amateurs from around the world, and at no cost! All you have to do is make sure your computer has Adobe's Flash plug-in and then go to the Big Dipper to Southern Cross event website.

The northern-sky tour takes place today, January 8th, from 20:00 to 22:00 Universal Time (3 to 5 p.m. Eastern Standard Time). The southern-sky tour is Sunday, January 10th, from 12:30 to 14:30 UT (7:30 to 9:30 a.m. EST).

Although I've seen my share of solar eclipses, I have a soft spot for coverups of the Moon. Maybe it's because lunar eclipses often occur in the dead of night, when I can contemplate the umbra's progress serenely instead of being embedded in a frenzied, sweaty crowd waiting for the Sun's corona to make a fleeting appearance. Or maybe it's because I know that more than half of the globe can enjoy a lunar eclipse along with me.

Whatever the reason, I'm going through a little withdrawal right now. February 2008 marked the third total lunar eclipse within a 12-month span and the last time anyone saw the Moon completely awash with umbral glow. Get this: four lunar eclipses happened during 2009, and three of those were merely penumbral — meaning the Moon skirted Earth's shadow so shallowly that even someone paying close attention likely missed these.

So it was with only modest enthusiasm that I reminded myself about a partial lunar eclipse that would take place on New Year's Eve. The eclipse's midpoint came at 19:23 UT, when a slight umbral nick (amounting to only 8% of the Moon's diameter) covered the southern limb.

Skywatchers in the Eastern Hemisphere had the best seats to view this subtle event, and Web postings from across the Atlantic suggest that many of them enjoyed clear skies for the eclipse. But I missed it, because North America was still awash in daylight at the time. (Besides, it was snowing in Boston at eclipse time.)

One of those who opted to watch instead of (or maybe in addition to) ringing in the new year was Anthony Ayiomamitis. Observing from Athens, Greece, he took a series of images that showed the slow progress of the Moon's bright disk through the bit of umbra that smudged its surface. I like the way this series shows the penumbral shading, which can be notoriously difficult to judge by eye.

I asked Ayiomamitis about the thin, bright rims around some of the images in his sequence — a processing artifact perhaps? "No," he replied. "We had some thin high-level clouds, and I am convinced this is the source of the problem."

In any case, I tip my sequin-spangled New Year's hat to him and to everyone who looked skyward last night to watch more than just celebratory fireworks.

S&T.com's guide to eclipses for the coming year is here. Looking that over, you'll note that a total eclipse of the Moon — the first in nearly three years — is coming on the night of December 20–21, 2010.

Only 354 days to go!

Industry stats show that by late December the end-of-the-world disaster flick 2012 had grossed $730 million worldwide. This suggests that lots of you have seen it.

But not me. I've got more important things to worry about than this market-driven piece of trumped-up hysteria. After all, an even more alarming calamity awaits us on New Year's Eve: a full Moon — the second one in December.

I couldn't believe that doomsayers had overlooked this dread portent, so I double-checked my facts. Yep, it's all right there on page 52 of December's Sky & Telescope: full Moons occur on December 2nd at 7:30 Universal Time, and again on the 31st at 19:13 UT. Running the numbers, I calculate that those two events take place 29.488 days apart — amazingly close to the Moon's average synodic month of 29.531 days.

And did I mention that late on December 31st there'll also be a partial lunar eclipse, visible from Europe and Asia? And for all this to occur on the final day of 2009, the end of the dread decade of the 00s, the Uh-ohs? Can this all be mere coincidence?

Seriously, I doubt the world will grind to a halt on New Year's Eve. After all, the circumstances were the same 19 years ago, on December 31, 1990 — and there were no global consequences (apart from the debut of the Sci-Fi Channel on cable television).

In modern usage, the second full Moon in a month has come to be called a "Blue Moon." But it's not! This colorful term is actually a calendrical goof that worked its way into the pages of Sky & Telescope back in March 1946. There author James Hugh Pruett wrote how two full Moons fall in a single month seven times every 19 years. He then stated, "This second in a month, so I interpret it, was called Blue Moon."

Pruett's interpretation might have faded into history and been forgotten, had my old friend Deborah Byrd not picked up on it in January 1980 script for the Star Date radio program. She's since moved on to Earth and Sky and set the record straight. But by then this bit of faux folklore had taken on a life of its own.

It's now clear that "Blue Moon" appeared in a 1937 edition of the Maine Farmer's Almanac to denote an extra full Moon in a given season. You're probably familiar with terms like "Harvest" and "Snow" to describe the full Moons at various times of year. But when a fourth one intrudes in the three-month interval between, say, September's equinox and December's solstice, a gap occurs in this naming scheme. That's why editor Henry Porter Trefethen inserted a Blue Moon (as the third of the four) all those years ago in his almanac.

For the numerologists among you, this month's doubletake is the first since May 2007, and the next won't come until August 2012 (there's that scary date again). As for me, if skies are clear when I'm out celebrating, I'll take a peek at that brilliant orb as it rises over the Boston skyline to see if it's an icy shade of blue. Or maybe I'll just howl.

The strongest and most reliable meteor showers are the Perseids of August and December's Geminids. Balmy weather and summer vacations have made the Perseids well known and popular, but the Geminids are actually easier to view from mid-northern latitudes. For one thing, nights are much longer in December. And while the Perseids are best viewed just before dawn (as most showers are), you can easily get an eyeful of the Geminids during the evening hours.

This year the Moon will be nearly new when the Geminids peak on the night of December 13-14. The shower's radiant, the point in the sky from which they all seem to originate, is near Castor and Pollux. It's well up in the east by 9 or 10 p.m. and crosses near the zenith (for mid-northern observers) around 2 a.m.

The shower should peak around 5:00 Universal Time on the morning of the 14th, corresponding to midnight EST and on the 13th at 9 p.m. PST — excellent timing for North America and Western Europe. Under dark-sky conditions you might see as many as 120 medium-speed meteors per hour. (Light pollution reduces the numbers.) The shower is active to a lesser extent for at least a day or two beforehand and about one day after.

The Geminid meteor shower is extremely unusual in that its parent object isn't a comet. Instead, it's an asteroid, a chunk of rock roughly 3 miles across called Phaethon (pronounced FAY-uh-ton). How can an asteroid produce meteoroids? Nobody knows for sure. Many scientists believe that Phaethon is the core of a comet that's been baked completely dry. Maybe a smaller asteroid collided with it long ago. In any case, a ribbon of debris lines Phaethon's orbit.

Meteor watching couldn't be easier. Lie back in a reclining lawn chair, relax, and watch the sky overhead. Ideally, you want nothing but sky in your field of view — not trees, and certainly not the ground. That means that you should either lie flat on your back or recline so that you face at least 45° above the horizon. Also remember that December nights are cold at mid-northern latitudes. Normal winter clothing won't even come close to keeping you warm after you've been lying still for a couple of hours. The best solution is to use a sleeping bag. Second best is plenty of blankets over your warmest clothing. And don't forget a hat and gloves!

The arriving Geminids will cover the whole sky, so it doesn't really matter which way you're pointed. If you look straight at the radiant, you'll see meteors coming directly toward you, bright but with short trails. Look the opposite way, and you'll see lots of long meteors moving away from you.

Careful counts of meteors have scientific value. Click here to learn how to conduct a scientific meteor count and how to report your results to the International Meteor Organization.

Update: It was indeed a great year for the Geminids! The shower peaked at about 150 meteors per hour (the Zenithal Hourly Rate: the rate as would be seen under ideal conditions) during the late hours of December 13th Universal Time, according to this preliminary activity profile posted in near-real time by the International Meteor Organization.

Most meteor showers vary from year to year, but the Leonids are particularly capricious. Many years they chug along producing just 5 or 10 meteors visible per hour. But at the Leonids' historical greatest, in 1833, meteors were seen to fall "like snowflakes in a blizzard," with estimated rates of several dozen per second!

This year is expected to be better than average. The "traditional," most reliable part of the shower should peak around 4 a.m. EST (1 a.m. PST) on the morning of Tuesday, November 17th. You might see 20 or 30 meteors per hour under ideal dark-sky conditions. (Remember, if you want to stay up late instead of getting up early, you'll be staying up Monday night. It's easy to get the date wrong for events that happen after midnight!)

A second, briefer, but very intense outburst is expected about 12 hours later — during the early-morning hours of November 18th in Asia. (See "Will the Leonids Roar Again?".) There's only an off-chance that some activity from that burst will still be going on by the time the Earth turns halfway around and the Leonids become visible in the Americas on the morning of the 18th.

But if the sky is clear, why not go out again that morning — and also before the predicted peak, on the morning of the 16th? The Leonids have surprised the theorists before, and they surely will again.

Wherever you are, no Leonids will be visible before the shower's radiant point (in Leo) rises around local midnight. And peaks and bursts aside, the number of visible meteors increases steadily from radiant-rise until Leo is highest, just as the sky is starting to get light.

Be sure to bundle up warmly; meteor-watching is always colder than you expect. Ideal meteor-watching equipment is a comfortable lounge chair, a warm sleeping bag, and a pillow. If you live in a city or suburb, consider traveling to a dark location far from city skyglow. In any case, find a spot where no lights glare directly into your eyes.

The direction to watch is wherever your sky is darkest. Notice the meteors' flight paths; only those streaking away from the direction to the constellation Leo are Leonids.

Another, less-known meteor shower is going on simultaneously — the Taurids. They're sparse but tend to be very bright. If you see a slow, bright meteor heading away from the direction to Taurus, that's a Taurid.

And you're bound to see a few sporadics that aren't associated with any major shower.

For more information, read our article Basics of Meteor Observing. (Be sure to click on "Next Page" below the ad.) And if you already know the basics, take a look at Advanced Meteor Observing.

This could be a particularly good year for the Orionid meteor shower, which runs from roughly October 17th to 25th with one or more peaks around the morning of the 21st. The Moon is out of the sky during the good meteor-watching hours from midnight to dawn. Moreover, in years past the Orionids have shown a 12-year cycle in their strength — and based on this we should be seeing a bumper crop of Orionids in 2009, with peak rates of up to 30 meteors visible per hour by a single person under ideal observing conditions.

The Orionids have an illustrious parentage. Like the Eta Aquarids of May, they are bits of debris shed long ago by Halley's Comet. The two showers are essentially one and the same; Earth intersects a single, broad stream of meteoroids at two places in its orbit on opposite sides of the Sun.

Like the Eta Aquarids, the Orionids tend to be faint and swift — only the Leonids hit Earth's atmosphere faster — and they often leave briefly glowing trains. The shower is actually a complex of several components with different maxima spread over several days. These radiants (the points in the sky from which the meteors appear to radiate) are grouped near Orion's Club, as shown on the accompanying chart.

For observers around 40° north latitude, these radiants rise high in the eastern sky (at least 45° up) by about 2 a.m. daylight saving time. So that's about when the meteor activity gets pretty good. The first light of dawn begins stealing into the east about four hours later.

Halley's Comet last came through the inner solar system in 1985–86, and at that time its nucleus shed a layer of dirty ice about 20 feet (6 meters) thick on average. This has been happening every 76 years for many millennia. During that time the dirt bits have spread all around Halley's elongated orbit and a fair distance from it sideways, which is why some of the particles now intersect Earth even though the comet's orbit does not. (The orbits of Halley and Earth are separated by 22 million km, or 15% of the average Earth-Sun distance, at their closest point.) No one knows how long it took the Orionid meteoroids to drift so far off track — one estimate is 4,000 to 10,000 years — but it's clear that as shower meteoroids go, the Orionids are old.

They've been seen for a long time too. The first known Orionid shower was recorded by the Chinese in AD 288, when "stars fell like rain." The shower has been well observed ever since astronomers first recognized its radiant in 1864.

Few celestial events are as dramatic as occultations, when the Moon (or another body) passes in front of a bright star or planet. Everything else in the sky happens on a time scale of minutes (for Jupiter's moons) to hours to millennia. But occultations of stars — aside from our own Sun — are almost instantaneous.

Four first-magnitude stars experience occultations by the Moon: Aldebaran, Antares, Regulus, and Spica. Antares is my favorite of those, for several reasons. It's the most colorful of the four, and it's set among several other bright stars. It has one of the biggest angular diameters of any stars — 0.045" — so it actually takes about 1/10th of a second to disappear — long enough to be perceptible, at least in theory. And it's a double star whose very faint companion was actually discovered during an occultation, shining briefly next to the Moon's limb after the bright star had disappeared.

If its orbit were fixed in space, then the Moon would occult the same stars each month. But in fact, the Moon's orbit wobbles like a top, with a period just under 19 years. So stars undergo long series of monthly occultations, followed by long periods with no occulations.

On January 11, 2010, Antares experiences the last occultation of the current series that's practical to observe from any populated place on Earth. This one won't be easy, either, but it should be visible through a telescope if the atmospheric conditions are good.

The good news is that the occultation is visible over a densely populated area, including the northeastern half of New England, a bit of New York and Ontario, and essentially all of Quebec and the Maritime Provinces. The bad news is that the sky will be extremely bright — far too bright to see Antares without a telescope. In the best locations (northern New York and Vermont, Montreal, Ottawa), the occultation starts a few minutes before sunrise. Over most of the area, it starts after the Sun has cleared the horizon.

In either case, the thing to do is to set up your telescope at least a half hour before sunrise, when Antares is still easy to see. Then you can keep tabs on the star's position as the Moon creeps up on it and the sky grows brighter.

For more information on observing occultations, see our online article Occultations: The Fastest Things in the Sky.

The late, great statesman Tip O'Neill once famously quipped, "All politics is local," and much the same can be said about light pollution. Case in point: A few years ago I moved to a new house that was less than 5 miles from my old one — and in doing so my night sky got at least two magnitudes darker.

So how starry is your starry night sky? You can find out easily, thanks to a sky-awareness campaign called Great World Wide Star Count. It'll take just 20 minutes or so, and you'll be joined by thousands of equally-curious skygazers around the globe. Do it on your own, with your family, or as part of a larger group.

All you'll need are a clear evening sky sometime between October 9th and 23rd, your own two eyes, and a set of simple star charts. First, download the handy five-page activity guide (available in eight languages) and print the star charts. If you live in the Northern Hemisphere, you'll be looking high up for the constellation Cygnus, and its Northern Cross asterism. If you're south of the eqautor, the target area surrounds the Teapot in Sagittarius. Each of the seven maps shows stars down to a different magnitude limit, plus one for a cloudy sky.

Then, after stepping out under the early-evening sky and letting your eyes adjust to the darkness, match one of the charts to what you see overhead. Step back inside and report what you've found online. You're done! (Unlike many contests, you can enter more than once! You might be surprised by how much the sky's darkness can vary from night to night.)

GWWSC is a managed by UCAR's Windows to the Universe project. The 2007 and 2008 efforts netted more than 8,000 observations from 65 countries. This year, as a component of the International year of Astronomy, it's helping to raise dark-sky awareness in every corner of the globe.

Go ahead — participate in the Great World Wide Star Count and become a "citizen scientist"!

Sending a spacecraft crashing into the Moon is nothing new. On September 12th, for example, I'm sure you were all celebrating the 50th anniversary of the Luna 2's crash landing in Palus Putredinis. NASA started its smashups a few years later with the Ranger series, followed by an assortment of Apollo parts. Lunar Prospector carried a bit of super-scientist Gene Shoemaker's cremated remains when it struck the Moon a decade ago. Japan's Kaguya orbiter had a flashy finale last June 10th that was captured by a large telescope on Earth.

Early on Friday morning, October 9th, gravity and momentum will conspire to draw NASA's Lunar Crater Observation and Sensing Satellite (LCROSS) and its Centaur rocket into the Moon. They will strike an obscure crater named Cabeus that's 60 miles (100 km) across and close to the Moon's south pole.

It's the latter characteristic that will have countless telescopes on Earth — and a few off of it — locked in on Cabeus. (Impact site finder charts.) Based on NASA's latest (Oct. 7th) predictions, the rocket body will strike at 11:31:19 Universal Time (7:31:19 a.m. EDT, 4:31:19 a.m. PDT). The smaller, instrumented LCROSS shepherding craft will crash a few minutes later, at 7:35:45 UT, just after flying through and sampling the debris plume from the first strike with nine onboard instruments.

NASA has a great animation showing the sequence of events. I hope that the real event looks half this good!



So why pick this dark and hard-to-observe site? Parts of the floor of Cabeus lie in permanent shadow — the Sun never shines there — and there's lots of circumstantial evidence that frozen water lies mixed into the dusty rubble inside Cabeus and numerous other craters near the Moon's poles. Slam a big enough bullet into such a deposit, the thinking goes, and the resulting plume of debris should be infused with water vapor that can be detected and quantified spectroscopically.

If any frozen water is really there.

Mission officials picked the time and place for LCROSS's demise such that the waning gibbous Moon will be high in the night sky for the phalanx of giant telescopes atop Mauna Kea in Hawaii. The viewing geometry also favors big observatories in California, Arizona, and New Mexico. Locations in North America west of the Mississippi River should see the Moon in a dark predawn sky, but those of us on the East Coast are out of luck — the Moon is still up at impact time, but so is the Sun.

So if you're among the geographically fortunate, what might you see? That depends.

First, it depends on how big a plume the impacts make. LCROSS's two main components — its bullets — are the 2.2-ton Centaur rocket that propelled the spacecraft to the Moon and a smaller, 0.6-ton "shepherd" that will guide both craft to the target. Shortly before the impact, the shepherd will separate and fall far enough behind the rocket's lifeless hulk (about 400 miles) to fly through the plume created by the Centaur before crashing itself at 1½ miles (2½ km) per second.

Theorists predict that the rocket's strike should trigger a distinct flash perhaps as bright as 6th magnitude in visible light. But terrestrial telescopes won't see that because it'll be hidden by the rim of the target crater — unless the aiming is off badly! You'll have better odds trying to spot the plume of debris lofted into the sunlight above ground zero following the hit. It's unclear how big and bright that transient curtain of dust will be. According to recent estimates by LCROSS scientists, the dense core of each plume might appear about as bright as the surface of Mars — not dazzling but perhaps detectable. A plume 6 miles (10 km) tall would be 5 arcseconds tall as seen from Earth — very tiny. (The whole Moon, by comparison, will appear 1,900 arcseconds in diameter.)

Second, how big is your telescope? Odds are you'll need an aperture of 10 to 12 inches to record the impact plume with a camera, more still if you want to see it by eye. NASA has a website with many more details and suggestions for would-be observers and imagers. Amateurs planning on watching for the event have also set up a Google group.

Just remember, warn the mission's plume gurus: This event is short (each plume may be visible for 20 to 120 seconds), low (1 to 20 miles in height), and dim (magnitude 6 to 11 for its brightest part).

And what about those of us who lack a light bucket? Check the LCROSS website for exact impact times and other late updates. See also NASA's tips on when and how to watch, including a list of events open to the public.

You can also watch the Centaur's rocket booster's crash from the perspective of the doomed shepherd spacecraft. Watch on NASA TV; coverage begins 1 hour 15 minutes beforehand, at 6:15 a.m. EDT (10:15 UT). If you have trouble with your browser plugins, you can watch in your favorite media player:
Stream for Windows Media Player
Stream for Real Player
QuickTime

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More links:

Great zoom-in visualization of what the impact plume may actually look like, from a new NASA/Goddard press release (Oct. 8).

NASA's Impact Party Toolkit

Webcast from The Exploratorium in San Francisco

SLOOH telescope live feeds from Arizona (in darkness) and New Hampshite(daylight).

LCROSS Citizen Science site

LCROSS imaged in space by an amateur

Apart from the nearly full Moon, Jupiter, the King of Planets, has the evening sky pretty much all to itself. It's easy to spot above the southern horizon as night falls.

But I'd like to draw your attention to some happenings in the morning sky. Hey, most of you are already getting up before dawn anyway — or you can if you really try — so poke your head out for a few minutes during your early-morning routine. If you do, you'll be rewarded by views of three planets that are giving Jupiter stiff competition for stargazers' attention.

Venus is the bright "Morning Star" low in the east near dawn. A couple months ago this planet was higher up and quite a spectacle, but now it's past its prime. Day by day this month you'll see Venus slide a little closer to the horizon in the twilight. What's really exciting, however, is its changing arrangements with sibling planets Mercury and Saturn in the days ahead.

Mercury is best seen in the first half of October. On the 6th it appears farthest from the Sun, and therefore its highest before sunrise. Look for it that morning several degrees to the lower left of Venus. And if you continue along that imaginary line a bit farther, you should spot dimmer Saturn.

After the 6th, Mercury sits lower during each successive dawn. But Saturn creeps higher, passing impressively close to Mercury on October 8th and then Venus on the 13th. For a finale, on October 16th our planet trio is joined by a slender crescent Moon.

The best viewing for all of this should come about 60 to 40 minutes before your local sunrise time. And when exactly is that? You can always find your sunrise and sunset times (and much else) once you put your location into our online almanac. (If you're on daylight saving time like most of North America, make sure the Daylight Saving Time box is checked.)

Looks like I'll be getting up early and often this month!

Few stargazers bother to track down asteroids in the night sky, even though many of them are quite easy to spot. For example, in 2007 asteroid 4 Vesta passed unusually close to Earth and could be glimpsed with the unaided eye from a dark location.

Right now you can look for another famous minor planet. Asteroid 3 Juno is having an especially favorable apparition, making it about a magnitude brighter than it usually gets. It's magnitude 8.2 on September 1st, 7.7 at opposition on September 21st, 7.8 on October 1st, 8.2 on November 1st, and 8.9 on December 1st.

According to Don Yeomans, manager of the Near-Earth Object Program at NASA's Jet Propulsion Laboratory, "This is going to be as bright as it gets until 2018."

Juno is positioned beneath the Great Square of Pegasus near the intersection of Pisces, Cetus, and Aquarius. This region of the sky is low in the east after evening twilight but higher up and better placed for viewing around midnight.

As the chart at right shows, Juno is currently passing not far from Uranus (itself easy to see at magnitude 5.7), as the chart at right shows. Click on the chart for a larger, black-on-white chart that you can use with binoculars or a small telescope.

German astronomer Karl Ludwig Harding first glimpsed Juno in September 1804, only three years after the discovery of the first asteroid, 1 Ceres. Yet it was a lucky find. With a diameter of 145 miles (234 km), Juno is only the 10th largest asteroid, and it's by far the smallest of the four found from 1801 through 1807.

After that no others turned up until 1845, long enough for the idea of "four minor planets" to become entrenched in astronomy literature. In fact, astronomers of that era considered Ceres, Pallas, Juno, and Vesta part of the Sun's planetary retinue and even assigned them planetary symbols.

The spectrum of Juno suggests that it's composed of silicate rock, and dynamicists believe it is the source of many of the meteorites that rain on Earth. Several years ago astronomers used adaptive optics to record Juno at several wavelengths with the 100-inch Hooker telescope atop Mount Wilson. It's an intriguingly lumpy body, and apparently parts of it have been chipped away over the eons by collisions with other asteroids.

A NASA spacecraft named Dawn is en route to Vesta and Ceres, but it might be a very, very long time before we get close-up views of Juno. So take advantage of this window of opportunity to add another famous celestial object to your "life list."

The curtain is rising on a one-act drama in the night sky that was last performed in the early 1980s and won't come round again until 2036. Fortunately, backyard stargazers will not only have the best seats in the house — they've also been invited on stage to participate!

Our star: Located less than 4° from brilliant Capella, Epsilon Aurigae is a seemingly normal F-type supergiant with about 300 times the Sun's diameter and 15 times its mass. It's about 2,000 light-years away. Right now Auriga rises in the northeast about 9 p.m. and is high enough for quality viewing by midnight.

The plot: In 1821 astronomers discovered that Epsilon had unexpectedly dimmed to about half its normal brightness, from magnitude 3.0 to 3.8. This mysterious dimming occurred again in 1847, 1874, and 1903, by which time astronomers had figured out that the star was part of an eclipsing binary system.

The plot thickens: Spectroscopic observations revealed that the eclipse-causing companion must be nearly as massive as Epsilon — yet astronomers have yet to detect any light from it. As best they can tell, the dimmings are due to an oblong, opaque disk that's cloaking the companion star (or stars).

Your role: The latest eclipse from this is just getting under way, and Epsilon Aurigae should gradually dim until early winter in the Northern Hemisphere. (Sorry, its declination makes for poor viewing for southern observers.) It will remain faint throughout 2010 before slowly brightening to its normal luster by mid-2011.

Modern observatories aren't equipped to study stars this bright — but many backyard astronomers are. So the American Association of Variable Star Observers has just launched a program that allows amateurs of all stripes to help solve this centuries-old enigma.

Unlike other "citizen science" efforts, the AAVSO's Citizen Sky lets you experience all aspects of scientific research. Your visual or electronic estimates of Epsilon Aurigae's brightness are of course welcome — that's been the bread and butter of the AAVSO's work since its founding in 1911. But you can also use the online tutorials to learn how to analyze data, create and test your own hypotheses, and write up findings for publication in astronomy journals. You can work alone, join a team of observers, or form your own.

Funded by the National Science Foundation, Citizen Sky is a collaboration of the AAVSO, Denver University, Chicago's Adler Planetarium and Astronomy Museum, Johns Hopkins University in Maryland, and California Academy of Sciences. Its lead astronomer, Robert "Dr. Bob" Stencel (University of Denver), studied Epsilon Aurigae extensively during its previous eclipse.

"Our goal is to introduce the public to authentic science and at the same time use this talent to help astronomers," notes AAVSO director Arne Henden.

As it turns out, I've just started teaching observational astronomy to a small class of high-school students, and "Citizen Sky" will be a perfect project for them. I know they'll enjoy getting a taste of real-world science — I'm sure you will too.

Right now the planet Jupiter is oriented such that its equator and the orbits of its four big moons are almost exactly edge-on to the Sun and Earth. This alignment happens every six years, on opposite sides of Jupiter's 12-year orbit around the Sun.

At such times its four Galilean satellites undergo mutual phenomena: they often get occulted and eclipsed not just by big Jupiter and its shadow but also by one another. Amateur Christopher Go recently recorded a striking example of the latter.

During a mutual occultation, for example, you can watch two satellites appear to merge and, in the middle of the merger, slightly dim. During an eclipse, a lone moon fades and rebrightens as it's crossed by the shadow of one of its siblings.

Although these little encounters are great fun simply to watch, anyone who records their light curves accurately with a photometer can help to refine the satellites' orbits.

Listed below are the mutual occultations and moderately deep eclipses through the end of 2009, when you'll find Jupiter conveniently placed for viewing in the evening sky. (Thanks to Belgian astro-calculator Jean Meeus for providing the list!)

Events are listed by their date and Universal Time, which is 4 hours ahead of Eastern Daylight Time. The next column tells which satellite occults (o) or eclipses (e) another. For example, 4o3 means that satellite IV (Callisto) occults satellite III (Ganymede) by passing in front of it. Similarly, 1e2 means that satellite I (Io) casts its shadow onto satellite II (Europa). Sometime these notations are followed by A for annular or T for total; otherwise the event is partial. The six possibilities are shown schematically here.

Event size indicates how much of the more distant satellite's diameter is obscured during an occultation, measured as the percentage drop in its light. The table lists only those events for which the dropoff is at least 25% (0.3 magnitude).

Although this table lists only those events easily observable from North America, Sky & Telescope has prepared a comprehensive tabulation of all pairings for the entire year. So find the events that occur at night for you when Jupiter is up, and mark your calendar!

Mutual Events of Jupiter's Satellites for North America
Date (UT)	Start (UT)	End (UT)	Event type	Event size
Aug. 7	5:12	5:19	1e2	53%
7	5:33	5:43	1o2	79%
12	1:46	2:01	3e2T	100%
12	2:05	2:17	3o2	31%
14	7:49	8:01	1o2	92%
14	7:49	7:58	1e2	55%
15	0:26	1:04	1o2	31%
15	23:50	23:56	1e3	27%
19	5:32	5:48	3o2	41%
19	6:04	6:22	3e2	99%
21	10:11	10:25	1o2T	100%
21	10:38	10:49	1e2	52%
22	4:00	4:17	1o2	47%
23	3:19	3:27	1e3	26%
24	23:24	23:40	1o2T	100%
25	0:08	0:22	1e2	47%
26	9:16	9:37	3o2	48%
26	11:00	11:25	3e2	82%
29	6:42	6:55	1o2	53%
29	7:38	7:46	1e2	28%
30	5:44	5:51	1o3	7%
30	7:47	8:09	1e3	30%
Sep. 1	2:04	2:26	1o2	81%
1	3:56	4:29	1e2	25%
5	9:10	9:21	1o2	55%
5	10:21	10:29	1e2	50%
16	0:41	0:51	1o2	53%
16	2:12	2:19	1e2	86%
23	3:01	3:08	1o2	51%
23	4:41	4:47	1e2	96%
30	5:18	5:25	1o2	49%
30	7:07	7:13	1e2	81%
Oct. 7	7:35	7:41	1o2	47%
17	23:01	23:07	1o2	46%
22	23:12	23:17	3o1	40%
24	0:31	0:40	3o2	61%
25	1:19	1:24	1o2	47%
30	2:04	2:10	3o1	49%
31	3:55	4:03	3o2	68%
Nov. 1	3:38	3:43	1o2	51%
6	5:06	5:13	3o1	56%
8	5:57	6:02	1o2	56%
13	2:04	2:08	2o1A	86%
17	1:19	1:26	2o3A	59%
20	4:19	4:23	2o1A	86%
24	4:39	4:46	2o3A	59%
25	23:47	23:52	1o2	83%
Dec. 3	2:09	2:13	1o2	99%
15	0:26	0:30	2o1	54%
22	2:48	2:52	2o1	41%
27	22:28	22:31	1o2	35%