By Bob Berman
The oddest celestial event? This year there's an easy winner. It's the millennium's first total lunar eclipse completely visible from all of North America and Hawaii. And whoa, beat the drums, it happens right on the solstice.
This was exactly the kind of spectacle that inspired the fun-loving Mayans to push their most annoying relatives off pyramids. (Does any scholar actually know how they selected people for sacrifices? "Annoying" seems logical.) These days, our own citizenry is much too overweight to engage in such energetic rituals. But if you're tempted, be sure to first check local ordinances before you perform even a single goat sacrifice.
At midnight December 20/21, we'll have the highest Full Moon until 2020. From the West Coast, that Moon will be in total eclipse at midnight — how cool is that? Observers in Key West will see the magical Moon straight up, an imperceptible ½° from the zenith. Count on crowds blowing conches at Sunset Pier at that overhead moment of 12:17 a.m. But like all Eastern time-zoners, they must wait 'til 1:32 a.m. for the eclipse's umbral beginning.
We'll also get a rare chance to gauge the Full Moon's brightness. During the night's first half, it defiles the sky with a creamy glow that obliterates everything fainter than 3rd magnitude. Then, if you live in the country away from streetlights and other light pollution, behold the metamorphosis. Drink in the glory of the winter Milky Way after the Moon's been reduced to a coppery phantom, with the visible star count boosted from 120 to 2,600.
Earth's shadow usually appears red. So the Moon becomes copper when fully immersed. Yet the initial "bite" as the event begins always looks inky black. Culprit: our retinas. When the sunlit portion is still fat and brilliant, our eyes underexpose the shadowed part. As you watch the eclipse progress, it's fun to note when the ebony starts becoming reddish, about halfway through the event.
The eclipsed Moon doesn't have to be red. It can be pale pink, beige, or even black so that it vanishes altogether. It all depends on clouds and dust around Earth's edge, or limb. This is the only occasion when a celestial body gives us a report card about our planet. Coppery red means everything is normal. Black means dust from the Iceland volcano and perhaps China's industrial pollution has dirtied our air big time. Green means you didn't set the alarm and are dreaming the whole thing.
Now for the ancient debate over equipment. People often fail to appreciate things that are free, so maybe they'll find this event enhanced through their most expensive telescope and largest 14-pound eyepiece. In my opinion, however, a lunar eclipse looks best with either the naked eye or else image-stabilized binoculars.
The Moon will occupy a very special place in the heavens that night. Our sky is a carnival of intersecting planes. There is the flat plane of our solar system seen edgewise, defined by the ecliptic. Then there is our galaxy's disk, the Milky Way. These eternal planes meet and cross at the Taurus-Gemini border. Astoundingly, a third plane called the solstitial colure — the sky-circling meridian running through the celestial poles in the precise direction of Earth's tilt — is right there, too.
It's an implausible three-way intersection. And yet that is where — within a few degrees — the Full Moon will stand during the eclipse.
So the Moon will be eclipsed while aligned with the galaxy's plane, Earth's tilt axis, and the centerline of our solar system all at once. Are you kidding? The Mayan priests would have tossed extra people.
The three sky-lines intersect in a teensy triangle that manages to surround NGC 2129, an open cluster 7,000 light-years away. Use your go-to telescope to locate those baby stars near the eclipsed Moon. Because the heavens will be nice and dark, deep-sky lovers take note: At mid-eclipse (3:17 a.m. EST), the ruddy Moon sits exactly halfway between Gemini's Rolls-Royce cluster M35 and Taurus' freakish Crab Nebula (M1).
Can you imagine the uproar from the paranoid and gullible if this notable display had coincided with the Mayan calendar flip? Then they'd be sure it's Armageddon-time. Happily, no eclipse or conjunction will occur 2 years from now on December 21, 2012. To show your gratitude, that non-event deserves an additional goat sacrifice.
One more thing. The Full Moon's hot surface aims infrared radiation our way like a bathroom heater. This makes Earth's lower atmosphere 4/100° warmer whenever the Moon is full. Will we lose this warmth during the eclipse? Let's bring along our high-tech lab thermometers that night and really puzzle onlookers.
Wait a minute. Onlookers? After midnight in December? No, you and I might just be alone for this one. Unless we move to Key West.
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