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.

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%

When most of us look at Jupiter through a telescope, we're content to see a few of the planet's dusky bands, its Great Red Spot, and the pinpoint sparkle of its four Galilean satellites.

But not Christopher Go. Armed with his Celestron C11 scope, Astrophysics AP900GTO mount, and Imaging Source DMK21BF04 camera, he prowls the solar system for photoworthy prey and usually finds it. I don't know if there's something about the air over Cebu City in the Phillipines, where he lives, or something in the drinking water, but Go is among the world's elite when it comes to planetary photography.

As evidence, I submit the sequence shown below. On August 16th, from 16:38 to 17:39 Universal Time, Go carefully recorded the passage of little Io past big-brother Ganymede — and a transit of Io's shadow across the larger moon's disk. Note that the two moons are not only resolved, but also that detail is apparent on their surfaces!

"I discovered this event 2 weeks ago while playing with WinJUPOS," Go tells me. "For
this event, I used a green filter and recorded in monochrome. There was no time for RGB integration. I also couldn't use a color cam as Jupiter was too low and atmospheric dispersion would be a problem. Each frame is a product of a 30-second video integration. The setup is the same as when I'm imaging Jupiter — in fact, I imaged the planet in between the transits (except during the shadow part)."

Ordinarily, these moons would not appear so close together. But right now we're enjoying a long sequence of such mutual phenomena because Jupiter passed through an equinox on June 22nd, which makes the orbits of the Galilean satellites appear edge-on from the Sun's perspective. This occurs every six years. It also helps that Jupiter reached opposition on August 14th, placing it especially close to Earth.

Coincidentally, the geometric conditions are much the same on Saturn, which had its "spring equinox" on August 12th. However, right now Saturn is hopelessly lost in the Sun's glare, so it's up to NASA's Cassini orbiter to show us all the interesting comings and goings that are taking place on and around that wonderfully ringed planet.

The annual August Mars Hoax is back yet again.

Actually, "hoax" is the wrong word, unless some joker is now spreading it knowingly (quite possible). It's an e-mail chain letter claiming that Mars is about to come closer than ever in history and will look as big and bright as the full Moon with the naked eye. If your well-meaning great-aunt or your cousin's brother-in-law's dog hasn't sent it to you yet, it's probably just a matter of time.

What's going on is this. Back on August 23, 2003 (that's 2003 with a 3, folks), Mars had an especially favorable opposition, coming close enough to Earth to appear 25 arcseconds wide. That's still pretty tiny even in a telescope — smaller, for instance, than Jupiter always appears.

Back then, someone somewhere pointed out that at a magnification of 75× in a telescope, Mars would appear as big in the eyepiece (½° wide) as the Moon does unmagnified. True enough. But two things happened, as often do with chain letters. First, it got rewritten bit by bit to improve the story as people passed it around, so that the "75×" was downplayed or, in some versions, left out. Second, the chain letter kept going and going, with the same breathless excitement, long after August 2003 receded into the mists of history.

In 2009, for instance, there's no opposition of Mars. Its next one will come on January 29, 2010, and even that will be a poor one, with Mars appearing no larger than 14 arcseconds wide.

Every year I give members of the news media, when they phone Sky & Telescope, the following quote:

"The Mars chain letter is not a bad thing, it's a good thing. It is basically harmless, so it is an immunization. That is, if you make a fool of yourself to your family and friends by sending it to them, you may be embarrassed enough that you won't send them the next e-mail chain letter you get, which could be a lot less benign."

P. S.: The first place to check for facts about any internet rumor, hoax, or urban legend is www.snopes.com. Bookmark it.

Last night's full Moon was accompanied by a lunar eclipse — but no one noticed! Even amateurs who were aware of the event couldn't tell by eye that something was going on.

That's because the Moon skirted through the outermost fringe of Earth's shadow, the penumbra, and only about half of the lunar disk was involved.

Photography proved the only sure way to confirm that an eclipse occurred, as these two images by Turkish amateur astronomer Tunç Tezel show. He took identical 1/500-second pictures at ISO 100 using a Meade 8-inch LX10 telescope and a Canon EOS 5D camera. One was snapped well before the eclipse began and the other at mid-eclipse. You can see a subtle change in shading at lower left.

Meanwhile, the Moon appears to nod noticeably between the two frames, mostly due to Tezel's changing perspective as Earth rotated through the night.

This weak lunar eclipse comes just two weeks after a spectacularly deep and long total solar eclipse on July 22nd, which was preceded by another weak penumbral eclipse on July 7th. And these two barely there events continue an extended drought of lunar eclipses. The last total lunar eclipse occurred in February 2008 (the third of its kind within a 12-month span), and the next won't happen until late December 2010.

Observers in the Eastern Hemisphere can look forward to a partial lunar eclipse this coming New Year's Eve. It won't be much of a visual treat either — but at least it'll be noticeable! Click here to keep tabs on all of this year's solar and lunar eclipses.

The little bits of interplanetary grit making up the Perseid meteoroid stream orbit the Sun with a period of about 130 years, like their object of origin, Comet 109P/Swift-Tuttle. The richest part of the stream is strung out near the comet itself, which last dipped through the inner solar system in 1992. So the shower's annual sky show has waned of late — gone are the great Perseid meteor displays of the early 1990s.

This year's standard Perseid peak is predicted to come around 18^h Universal Time (2 p.m. Eastern Daylight Time) on August 12th. That's good timing for the Far East, but for North America it splits the difference between the nights of August 11-12 and 12-13. Flip a coin — or watch the evening weather forecast — to decide which night to watch for them. The shower is also active to a lesser degree for many days beforehand and several days afterward.

The waning Moon is nearly at last quarter those two nights. It rises an hour or two after dark and will brighten the sky somewhat during the best Perseid-activity hours, from 11 p.m. until dawn. Nevertheless, this is a pretty reliable shower, and some Perseids should be there for the catching.

Moreover, meteor specialists Esko Lyytinen and Mikhail Maslov suggest we may encounter a ribbon of very old debris (ejected by the comet in 1610) on the morning of August 12th near 9^h UT (4 a.m. CDT; 2 a.m. PDT). This could up the count for an hour or so. Both researchers also think that Earth's proximity to the stream's core might produce an additional surge four hours earlier, around 5^h UT (1 a.m. EDT).

You may also see occasional meteors from two lesser showers that are also active: the Delta Aquarids and Kappa Cygnids. These move noticeably slower than Perseids, and they travel in different directions as if originating from their respective constellations.

Meteor watching is great "eyeball astronomy." Find a spot with an open view of the sky, wrap up warmly in winter clothes or a sleeping bag, and use mosquito repellent where you're not wrapped. Lie back in a lounge chair and watch whatever part of your sky is darkest. Be patient. You may see a meteor zipping into the upper atmosphere every few minutes on average.

Click here for Sky & Telescope's guide to the year's major meteor showers.

This might just be the busiest time in living memory for people who love to observe Jupiter. First the plane of its moons pointed edge-on to Earth, allowing remarkable "mutual events" when the moons eclipse and occult each other.

Then, on July 19th, Australian amateur astronomer Anthony Wesley discovered that a comet or asteroid had struck the giant planet, leaving a dark marking that's still clearly visible. This is just the second time in the 400-year history of telescopic observing that such an event has been clearly documented.

And on the night of August 3-4, Jupiter will cover a 6th-magnitude star — a once-in-a-lifetime occurrence for most locations on Earth. The event occurs roughly from 22:53 Universal Time on August 3rd to 1:00 UT August 4th, varying slightly depending on your location. It happens at prime time for stargazers in Europe and Africa, and is also readily visible in the Middle East and Brazil.

In North America, the occultation can only be seen from New England and the Maritime Provinces — and barely so. In those locations, Jupiter will rise while it's hiding 45 Cap, and will still be extremely low in the sky when the star reappears.

But by amazing good luck, 45 Cap will be masquerading as a fifth moon during a particularly eventful period for Jupiter's Galilean moons. So weather permitting, every telescope owner on Earth will have a chance to see many fascinating events during the days before and after the occultation.

All of these events should be visible through small telescopes if the atmosphere is very steady, but extra aperture and high magnification will improve the views greatly. Try our Javascript utility or our PDF table for details of the moons' interactions with Jupiter. And download a PDF article from the July 2009 issue of Sky & Telescope to see the details of the "mutual events" between the moons.

Aug. 1-2, 10:19 p.m. to 1:48 a.m. PDT, 1:19 to 4:48 a.m. EDT: The first sequence is visible across the Americas. It starts with just three moons visible (not counting 45 Cap), because Ganymede is behind Jupiter. During this period Europa and its shadow pass over Jupiter, Ganymede reappears, and Io disappers.

Aug. 2-3, 9:49 p.m. to 12:25 a.m. PDT, 12:49 to 3:25 a.m. EDT: Again visible across the Americas, a normal transit of Io and its shadow across Jupiter — fairly common but spectacular nonetheless.

Aug. 2, 15:55 to 16:32 UT: Best in eastern Asia and Australia. Io casts its shadow on Europa from 15:55 to 16:02 UT, dimming it 50%. Then Io passes in front of Europa from 16:26 to 16:36 UT, blocking 74% of its surface. Meanwhile, 45 Cap is just 30" to their south.

Aug. 3-4, 22:53 to 4:39 UT, Jupiter rise to 12:39 a.m. EDT: This is the big event. The occultation of 45 Cap is best observed from Europe and Africa, but as you can see in the diagram above, Europa disappears into Jupiter's shadow while the star is hidden, and Io disappears shortly after the star reappears. By 4:39 UT (12:39 a.m. EDT or 9:39 PDT), all three have reappeared from behind Jupiter, but they're still spectacularly close to Jupiter and each other.

Aug. 4, 21:47 to 22:57 UT: In most of Europe and Africa, and much of Asia, Ganymede casts its shadow on Europa, diminishing its light 94%, from 21:47 to 21:59 UT. Then Ganymede clips the edge of Europa from 22:48 to 22:57 UT.

Aug. 4-5, 23:18 to 1:51 UT: In Europe and Africa, a normal transit of Io and its shadow.

Aug. 5, 14:38 to 23:05 UT: A very long sequence of events, best viewed in the Middle East and eastern Africa, but with parts visible across Eurasia and Oceania. A transit of Ganymede and its shadow overlapping a transit of Europa and its shadow, with Io disappearing and reappearing toward the end of the sequence.

Mission controllers and scientists for NASA's newest lunar explorers are breathing easier these days. The Lunar Reconnaissance Orbiter (LRO) is settling into its polar orbit nicely, readying its instruments to begin a complete survey of the Moon.

So is the companion Lunar Crater Observation and Sensing Satellite — whose acronym, LCROSS, is so much easier to remember. LCROSS skirted 1,988 miles (3,183 km) from the Moon on June 23rd, and in doing so got a gravity assist that put the craft in a looping 37-day orbit. Impress your friends by remembering this mouthful: Lunar Gravity Assist Lunar Return Orbit (LGALRO).

In any case, late last week project officials released the "short list" of candidate sites where, on October 9th at 11:30 Universal Time, LCROSS and the Centaur rocket that helped boost it will slam into the lunar landscape. These spots are lying in permanent shadow near the Moon's south pole. In the table below, Sun mask is each spot's depth (in kilometers) below the surrounding sight lines to the Sun.

LCROSS Candidate Impact Sites
Designation	Crater name	Sun mask	Earth mask	Latitude	Longitude
SP_A	Faustini	2.3	0.9	–87.2°	89°E
SP_B	Shoemaker	3.5	–0.6	–88.5°	50°E
SP_C	Cabeus	3.0	8.5	–85.0°	35.5°W
SP_CB	Cabeus B	0.9	–0.5	–81.7°	54.5°W
SP_CC	(none)	2.5	0.1	–83.5°	16°W
SP_D	Haworth	2.7	0.8	–87.4°	5°W
SP_F	(none)	1.8	0.5	–82.3°	12°E
SP_G	(none)	2.4	0.4	–84.3°	1°E

Likewise, Earth mask denotes how far each spot lies below a direct sight line from Earth. On the Big Day, the lunar south pole will be tipped about 2½° toward Earth, a favorable libration that allows terrestrial observers to peek directly at the targets in Shoemaker and Cabeus B. Project officials won't select the final target until 30 days before impact. That will permit LRO to photograph the candidate sites thoroughly.

Brian Day, who's coordinating amateur observations for the LCROSS project at NASA's Ames Research Center, told me that ground-based observers probably won't see the momentary flash created when the 5,200-pound (2,360-kg) Centaur slams into the surface. However, seconds afterward a rising plume of debris might become as bright as a 5th-magnitude star along the limb. He thinks the plume will spread into a thin band a few arcseconds tall and perhaps 25 arcseconds wide before it fades from view.

Although many professional telescopes will be turned Moonward that night, Day hopes amateurs will join the hunt. Chances are you'll need an aperture of at least 10 or 12 inches to see anything visually. Sky & Telescope is preparing a complete guide for watching the LCROSS impacts, but for now you can check out the project's website.

"If you live in Asia and Australia you have a rare opportunity for an observing adventure," writes Sky & Telescope's Moon columnist, Chuck Wood, on LPOD, Lunar Photo of the Day:

On June 10th at 18:25 Universal Time, the Japanese lunar orbiter Kaguya (formerly named Selene) will end its two years of science with a final impact experiment. The location of the impact is very near the southeast limb, close to 80.4º E, 65.5º S This time and location are according to an update early on June 10th by the Japanese aerospace agency JAXA.

Wood writes, "This area will be in shadow as the Moon has just passed full, but easy to find just beyond the terminator south of [the crater] Janssen. Visual observations and video monitoring may be rewarded with a bright flash or (possibly) a cloud of ejecta that rises into sunlight as the large spacecraft rams into the surface at 6000 km/hr. . . .

"There is a possibility that Kaguya might impact 1 or 2 orbits (2 or 4 hours) earlier — it is very close to the surface, and both unexpected topographic highs and gravity anomalies may hasten its demise. . . ."

Here is Wood's entire article, with images showing where to watch.

Here's a Kaguya image gallery.

Japanese TV has been broadcasting high-res video from Kaguya as it skims over the lunar landscape. I hope they can keep the camera going right to the final moment.

A Warmup for the LCROSS Impact!

Observers in western North America and in Hawaii should be better positioned to watch NASA's own Moon-impact experiment — which has been delayed and will probably now happen between October 7th and 11th. This event is the subject of a feature article in the June Sky & Telescope. Two craft in the LCROSS mission will hit inside a permanently shadowed polar crater four minutes apart. Many instruments in orbit and on Earth will watch for any signs of water vapor in the ejecta.

Countless amateurs will also be watching; NASA mission specialists say signs of the LCROSS impacts may be detectable in 10-inch or larger telescopes.

The date, time, and place of the LCROSS impacts have yet to be finalized. Here's the NASA site for observations, and here's the LCROSS_Observation Google Group for planning and discussing observations and the latest mission news.

The Moon will be only about 16 hours from full when, on Saturday evening June 6th in the Americas, it will cross the 1st-magnitude red supergiant star Antares. The occultation will be visible across much of the United States and Canada, all of Central America and the Caribbean, and northern South America. Surrounding areas get a still-spectacular near miss.

In a telescope, you’ll see the round Moon’s most shadow-marked rim creep up to fire-colored Antares. The star will blink out behind the invisible dark limb just before it reaches the brightly sunlit lunar mountains and plains. Antares has such a large angular size for a star (40 milliarcseconds) that, seen from locations where it grazes the Moon’s edge, it may appear to fade down for a second or less rather than snapping out instantly.

In North America, Antares disappears some time between 9:40 and 11:20 p.m. EDT, depending on your location. See the IOTA website for a timetable with local details.

Antares will reappear up to an hour or more later from behind the sharper bright limb, with the Moon now higher in the southeastern sky.

Antares and Aldebaran are the only strongly reddish stars that are near enough to the ecliptic, and bright enough, ever to be seen well on the Moon’s bright limb. For these rare few seconds, to me they look like a fire on the Moon.