Parts of four continents will be treated to a view of a total eclipse of the moon during the overnight hours of Dec. 20 to Dec. 21. This spectacle of celestial shadows will be the best of its kind residents of North America will see until the year 2014.
December's total lunar eclipse is the only total eclipse of the moon of this year. For the Western Hemisphere, the eclipse will "officially? begin on Dec. 21 at 12:29 a.m. EST (9:29 p.m. PST on Dec. 20) as the moon begins to enter Earth's outer, or penumbral, shadow.
But even in clear weather, skywatchers will not notice any changes in the moon's appearance until about 45 minutes later when a slight "smudge" or shading begins to become evident on the upper left portion of the moon's disk.
This NASA lunar eclipse chart shows the visibility of the eclipse from different regions around the world.
While it can be seen in its entirety from North and Central America, parts of the eclipse can also be seen from Europe, northwestern Africa and parts of Australia. It will not be visible from southern and eastern Africa and India.
Unlike an eclipse of the sun, an eclipse of the moon presents no hazards to the observer. No precautions to protect the eyes are needed. [Amazing Total Lunar Eclipse Photos]
Moon's holiday treat
Lunar eclipses occur when the moon passes through a point in its orbit in which the Earth is directly between it and the sun. When the moon enters the shadow of Earth, it creates a lunar eclipse.
A total lunar eclipse is when the entire moon is completely inside the Earth's shadow. Since the sun's rays are bent by Earth's atmosphere so that some still reach the moon, the moon is still visible in an eclipse.
For December's lunar eclipse, the first definitive change in the moon's appearance will come on the moon's upper left edge. At 1:33 a.m. EST (10:33 p.m. PST), the partial phase of the eclipse will begin as the Earth's dark shadow ? called the umbra ? starts to slowly creep over the face of the full moon.
At 2:41 a.m. EST (11:41 p.m. PST) the eclipse will reach totality, but sunlight bent by our atmosphere around the curvature of the Earth should produce a coppery glow on the moon.
At this time, the moon, if viewed with binoculars or a small telescope, will present the illusion of seemingly glowing from within by its own light.
At 3:18 a.m. EST (12:18 a.m. PST), the sun, Earth and moon will be almost exactly in line and the light of the moon ? assuming clear skies ? will appear at its dimmest.
Totality ends at 3:53 a.m. EST (12:53 a.m. PST) and the moon will completely emerge from the umbra at 5:01 a.m. EST (2:01 a.m. PST). About 15 or 20 minutes later, the last vestige of the fainter penumbral shadow will disappear from the moon's upper right edge and it will return to its normal brilliance.
A sight for the billions
The entire 72 minutes of the total lunar eclipse will be visible from all of North and South America, the northern and western part of Europe, and a small part of northeast Asia including Korea and much of Japan. Totality will also be visible in its entirety from the North Island of New Zealand and Hawaii.
In all, an estimated 1.5 billion people will have an opportunity to enjoy the best part of this lunar show.
In other parts of the world, either only the partial stages of the eclipse will be visible or the eclipse will occur when it's daytime and the moon is not above their local horizon.
Portions of western Africa and central Europe can catch the opening stages of the eclipse before the moon sets below their horizon during the morning hours of Dec. 21, while the eastern third of Asia and central and eastern Australia can catch the closing stages just after moonrise on the evening of Dec. 21.
Generally speaking, about half of the world's population ? about 3 billion people ? will be able to view at least a part of this eclipse.
For any one location, total lunar eclipses occur at an average frequency of four or five times per decade. The last total eclipse of the moon occurred on Feb. 20, 2008.
There will be two total lunar eclipses in 2011. The first will occur on June 15 and will be visible from most of the Eastern Hemisphere. Then on Dec. 10, those living in the western half of North America will be able to catch totality just before moonset.
Not until April 14, 2014 will a total lunar eclipse again be visible in its entirety from North America.
Shadowy tale of eclipsed moon's color
Although astronomers do not expect to gain new astronomical insights from the eclipse, lunar eclipses vividly reflect the overall state of the Earth's atmosphere.
Under normal weather and atmospheric conditions, as the moon slides into the shadow of the Earth, its normal yellow-white color changes into a still-visible but dull coppery-red at the height of the eclipse.
However, because of the recent eruptions of the Mount Eyjafjallaj?kull volcano in Iceland last spring and the Mount Merapi volcano in Indonesia in October, one and possibly two clouds of ash and dust might be currently floating high above the Earth. As a result, the moon may appear darker than usual during this eclipse; during totality, parts of the moon might even become black and invisible.
There's also another possibility for the upcoming lunar eclipse.
The moon might wear its normal eclipse cloak of a deep red or a coppery-hue or take on still other colors (orange, chocolate brown or gray). Color possibilities are unpredictable and that it is impossible to tell exactly how much light the earth's atmosphere will refract as its shadow creeps across the moon.
Cloud cover and other atmospheric conditions may also affect the visibility and coloration of the moon.
In short: we'll all just have to wait for eclipse night and see what actually happens.
SPACE.com will provide a more detailed outlook on the eclipse during a Dec. 17 skywatching alert, with an Eclipse Observer's Guide coming on Dec. 20. So you'll be covered for all your total lunar eclipse needs this holiday season.
- Gallery: Photos of the Feb. 2008 Total Lunar Eclipse
- Top 10 Lunar Eclipse Facts
- Best Meteor Shower of 2010 Arrives in December
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.