UPDATE: Cloudy? Watch a webcast here, here, or here.

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It's been almost three years since those of us in North America saw a total lunar eclipse.

Now the whole continent is in for another on Monday night and Tuesday morning, December 20-21. Earth's shadow will totally engulf the Moon from 2:41 to 3:53 a.m. Eastern Standard Time, or 11:41 p.m. to 12:53 a.m. Pacific Standard Time, as shown in the timetable below. The partial phases of the eclipse will last for a little more than an hour beforehand and afterward.

Unlike a solar eclipse, each stage of a lunar eclipse is visible to everyone on the Moon-facing side of Earth. Observers in Europe, West Africa, and South America will see part of the eclipse before it's interrupted by moonset and sunrise on the morning of the 21st. In East Asia, Australia, and New Zealand, the eclipse is already in progress at sunset and moonrise on 21st local date.

The Moon will be at the northernmost part of the ecliptic, north of Orion between the feet of Gemini and the horns of Taurus. An eclipsed Moon is always full, so if the eclipse happens in the middle of the night for your location the eclipsed Moon will be about as close as it can ever be to straight overhead.

Total Eclipse of the Moon, December 20–21, 2010
Eclipse event	EST	CST	MST	PST
Penumbra first seen?	12:55 am	11:55 pm	10:55 pm	9:55 pm
Partial eclipse begins	1:33 am	12:33 am	11:33 pm	10:33 pm
Total eclipse begins	2:41 am	1:41 am	12:41 am	11:41 pm
Mid-eclipse	3:17 am	2:17 am	1:17 am	12:17 am
Total eclipse ends	3:53 am	2:53 am	1:53 am	12:53 am
Partial eclipse ends	5:01 am	4:01 am	3:01 am	2:01 am
Penumbra last seen?	5:35 am	4:35 am	3:35 am	2:35 am

A total lunar eclipse has five distinct stages. It begins when the Moon first enters the penumbra, or pale outer fringe, of Earth's shadow. But this event is unobservable; the shading in the outer part of the penumbra is extremely slight. Not until the Moon's leading edge is about halfway across the penumbra does the first slight dimming become detectable to the eye.

The second stage, partial eclipse, starts when the Moon's edge reaches the umbra, or Earth's inner shadow. Few sights in astronomy are more eerie and impressive than watching this black-red shadow creeping, minute by minute, across the bright lunar landscape. You'll soon notice that Earth's shadow has a curved edge — visible proof that the world we live on is round.

As more of the Moon slides into the umbra, look around the sky. You'll notice that a second, deeper night is falling around you — night within night. In fact, if you're far from city lights, hundreds of additional stars start appearing in what earlier was a bright, moonlight-washed sky. An hour or so into partial eclipse, only a final bright sliver of Moon remains outside the umbra — and the rest of it shows an eerie reddish glow.

The third stage, totality, begins when the last bit of Moon slips into the umbra. For this eclipse, totality lasts a generous 72 minutes.

Then, as the Moon continues moving eastward along its orbit, events unwind in reverse order. Totality ends when the Moon's leading edge reemerges into sunlight, returning once again to a partial eclipse (stage four). Then, after all of the Moon escapes the umbra, the dusky penumbral shading (stage five) gradually fades away, leaving the full Moon shining as brightly as if nothing had happened.

Red in the Darkness

The umbra is the part of Earth's shadow where the Sun is blocked from the Moon completely. So why does the Moon here glow deep orange or red, rather than being completely blacked out?

That red light you see on the Moon during a lunar eclipse comes from all the sunrises and sunsets that ring the Earth at the time. Our atmosphere scatters and refracts (bends) the sunlight that grazes the rim of our globe, sending it into Earth's shadow. If you were an astronaut on the Moon, the situation would be obvious. You would see the Sun covered up by a dark Earth that was ringed all around with a thin, brilliant band of sunset- and sunrise-colored light.

On rare occasions the eclipsed Moon does go black. Other times it appears as bright and coppery orange as a fresh penny. And sometimes it turns brown like chocolate, or as dark red as dried blood. Two factors affect an eclipse's color and brightness. The first is simply how deeply the Moon goes into the umbra. The center of the umbra is much darker than its edges. This time the Moon will pass fairly deep through the umbra, and at mid-eclipse the Moon's southern limb almost reaches the umbra's center. The other factor is the state of Earth's atmosphere along the sunrise-sunset line. If the air is very clear, the eclipse is bright. But if a major volcanic eruption has polluted the stratosphere with thin haze, the eclipse will be dark red, ashen gray, or blood-black.

In addition, blue light refracted by Earth's clear, ozone-rich upper atmosphere can also add to the scene, especially near the umbra's edge, creating a subtle mix of changing colors. Such variable shading can give the eclipsed Moon a very three-dimensional appearance.

The next eclipse of the Moon is a deep total one on June 15, 2011, but North America misses out completely. Skywatchers on the West Coast can catch part of the following one, on the morning of December 10, 2011, until it's interrupted by moonset and sunrise. The next total lunar eclipse for the whole continent doesn't come until April 14-15, 2014 — an unusually long wait. So hope for good weather this time.

Extra-credit projects

• In a telescope, can you spot any faint stars close to the Moon during totality? Any star just to the Moon's east will soon be occulted (covered). Normally the full Moon is so bright that even in a telescope you can see only the brightest stars as they approach its edge. But during totality, even the smallest telescope may reveal faint stars coming right up to the colorful lunar disk and suddenly winking out as it covers them up.
  
  
• Roger Sinnott continues to collect amateurs' telescopic timings of when the edge of the umbra crosses lunar craters, as part of a decades-long project tracking slight unpredictability in the umbra's diameter. For crater maps, instructions, and where to report your timings, click here.
  
  
• John Westfall continues to collect timings of the four contacts, when the partial phase begins and ends before and after totality, made with the unaided eye. This information will help calibrate the historical timings made by mariners and others when this was one of the few ways to determine longitude at sea. For more information, click here.