Sky-High Lunar Eclipse Late Monday Night

Everyone in North and Central America -- weather permitting -- will have ringside seats for a total eclipse of the Moon high in the sky late Monday night, December 20th, and/or early Tuesday morning the 21st.

Earth's shadow will totally engulf the Moon, turning it spooky orange-red, from 2:41 to 3:53 a.m. Eastern Standard Time, or 11:41 p.m. to 12:53 a.m. Pacific Standard Time (see full timetable below). The eclipse will be partial for a little more than an hour before and after.

Unlike a solar eclipse, a lunar eclipse is visible to everyone on the entire Moon-facing side of Earth at once. "We're all looking at this together," says Sky & Telescope
editor in chief Robert Naeye.

Skywatchers in Europe, West Africa, and South America get to see only 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.

Program for the Show

A total lunar eclipse has five stages. It begins when the Moon first enters the *penumbra,* or pale outer fringe, of Earth's shadow. But that event can't be seen; the shading in the outer part of the penumbra is extremely slight. Only when 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. "It's eerie watching this weird, red-brown shadow creeping across the bright Moon," says Sky & Telescope senior editor Alan MacRobert. "Look carefully, and you'll notice that Earth's shadow has a curved edge. This was the first visible proof to the ancients -- at least the ones who knew what they were seeing, like the Greeks -- that the Earth is round."

As more of the Moon slides into the umbra, look around the sky. A second, deeper night is falling -- "night within night," says MacRobert. If you're far from city lights, hundreds of additional stars start appearing in what just a little earlier was a bright, moonlight-washed sky. An hour or so into partial eclipse, only a final bright sliver of the Moon remains outside the umbra -- and the rest of it shows an eerie reddish glow.

The third stage, *total eclipse,* begins when the last bit of the Moon slips into the umbra. For this eclipse, totality lasts a generous 72 minutes, with the Moon looking, says MacRobert, "like a luminous rotten orange."

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 totally blacked out?

"That red light you see on the Moon during a lunar eclipse comes from all the sunrises and sunsets around the Earth at the time," says Naeye. 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," he says, "you'd see the Sun covered up by a dark Earth that was ringed all around with a thin, bright 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-black 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 edge 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-brown, or red-black.

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

Next Eclipses

This is the first total eclipse of the Moon in almost three years (since the night of Feb. 20-21, 2008).

The next eclipse of the Moon is a deep total one on June 15, 2011, but North America misses out; our side of the world will be facing the wrong way.

Skywatchers on the West Coast can catch part of the following lunar eclipse, 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!

It's Geminid Time!

It's Geminid Time!

Mention "meteors," and casual skywatchers usually think of the annual Perseid shower on display every August.

But the Geminid meteor shower of mid-December ties or even surpasses the Perseids as the year's richest and most reliable meteor display. Geminid meteors come from 3200 Phaethon, an asteroid discovered in 1983.

This year the Geminids are predicted to peak on the morning of December 14th around 11h UT, more or less. That's excellent timing for North America, especially out West. The Moon that night is only a day past first quarter and sets around midnight or 1 a.m. local time, depending on where you live. Even before then, on the evening of the 13th, the moonlight isn't bright enough to dampen the shower's visibility too much — and the Geminids, with their radiant near Castor and Pollux, pick up steam as early as 8 or 9 p.m. But the radiant is highest around 2 a.m., so the morning hours are the usually the most productive.

Bundle up as warmly as you possibly can, and lie back in a dark spot with an open sky. You may see as many as two meteors a minute on average if you have a very dark sky and are watching after midnight.


Sky map of Geminid radiant
The radiant (apparent source point) of the Geminid meteor shower is near Castor, the fainter of the Twin Stars in the constellation Gemini. You'll see this view if you face east at about 9 p.m. (The number of meteors is exaggerated.)
Sky & Telescope illustration

If your sky is not too light-polluted, you might try making a careful meteor count and reporting it to the International Meteor Organization. Such counts by amateurs supply much of what we know about meteor showers' behavior. For your count to be useful, you'll need to follow the procedures described on our page or at the IMO's website.

Don't forget that the shower lasts more than one night. Counts are especially needed on nights away from the maximum, because fewer people are watching. In any case, enjoy the show!

A Steamy Super-Earth

A Steamy Super-Earth

In the latest breakthrough for exoplanet science, a team using one of the European Southern Observatory’s 8.2-meter Very Large Telescope reflectors has obtained a crude spectrum for the atmosphere of a super-Earth orbiting a dim red dwarf star 40 light-years away. The planet’s upper atmosphere is apparently dominated by steam or cloudy haze.

The star, 14.7-magnitude GJ (Gliese-Jahreiss) 1214 in Ophiuchus, is 300 times dimmer than the Sun with a spectral type of M4.5. Its planet was discovered in 2009 when the MEarth Project detected the planet's silhouette periodically dimming the star. The planet has 6.5 Earth masses, as determined later by the star’s gravitational wobbles, and it circles the little star very closely in just 38 hours. The transits reveal the planet’s diameter to be 2.6 times Earth’s — making its average density very low, only about a third of Earth’s density.

The astronomers detected a telltale absorption spectrum caused by a tiny fraction of the star’s light filtering through the planet’s atmosphere during each transit. The spectrum was featureless, indicating that the upper atmosphere either consists mostly of water vapor or is dominated by high-altitude clouds or haze.

“This is the first super-Earth to have its atmosphere analyzed. We’ve reached a real milestone on the road toward characterizing these worlds,” said team leader Jacob Bean (Harvard–Smithsonian Center for Astrophysics) in a press release.

Before this observation, astronomers had suggested three possible atmospheres for GJ1214b. The planet could be shrouded by water — which, given its high temperature so close to the star (200ºC; 400ºF), would be in the form of steam. Or it could be a rocky world with an atmosphere of mostly hydrogen obscured by high clouds or hazes. Or it might be a mini-Neptune, with a small rocky core and a deep hydrogen-rich atmosphere, the upper part of which would be clear.

The measurements clearly show no sign of hydrogen and thus rule out the third option. So the atmosphere is either rich in steam or blanketed by clouds or hazes. The planet’s low density, meanwhile, indicates that it's a waterworld.

“Although we can’t yet say exactly what that atmosphere is made of, it is an exciting step forward to be able to narrow down the options for such a distant world to either steamy or hazy,” says Bean. “Followup observations in longer-wavelength infrared light are needed to determine which of these atmospheres exists on GJ 1214b.