Dec. 9, 2011 — -- Look in the western sky Saturday morning before dawn, and if the weather is clear and you're in the right place, you will be rewarded with the last lunar eclipse of 2011.
For just under an hour, the disk of the full moon will almost disappear, turning a dark, rusty red. The catch for Americans is that you'll miss almost everything unless you're west of the Mississippi. Totality -- when the moon is completely consumed by Earth's shadow -- begins at 6:06 a.m. Pacific time Saturday, and ends at 6:57 a.m. Even on the Pacific coast, dawn will start to brighten the sky before the eclipse is over.
Still, if you happen to be up, a lunar eclipse can be a quiet, refreshing experience. Depending on the atmospheric conditions where you are, the moon may turn a rich orange, or it may become hard to pick out in the sky. The reddish hue comes from sunlight that is bent by Earth's atmosphere. As happens during a vivid sunrise or sunset, most colors other than red are absorbed by the air.
From the Rocky Mountain states or the West Coast, the moon may seem larger than usual, since it will loom close to the western horizon, creating a common optical illusion, since you'll have trees or buildings to which you can compare it.
Clearer views will be from places like Hawaii, Alaska and Guam, where it will be the middle of the night, and from eastern Asia and Australia, where (remember, they're on the other side of the International Date Line) it will be Saturday evening. Earth's shadow will start to slide across the moon's face about an hour and 20 minutes before the moon becomes totally covered.
A lunar eclipse takes place when the moon, following its orbit around us, passes directly behind Earth as seen from the sun. It is the opposite of a solar eclipse, when the moon passes between the sun and Earth. Since the moon's orbit is slightly tilted, the bodies do not align perfectly during most months -- but the rules of orbital mechanics are such that in any given year, there will be at least two and no more than seven solar or lunar eclipses.
If you miss Saturday's eclipse, there will be a partial one next June 4. There will not be a total lunar eclipse again until April 15, 2014.
You do not need to be in a special place, or need special equipment, to view a lunar eclipse. All you need is a clear view of the full moon at the right time. Binoculars or a small telescope may be fun. You can experiment and take pictures, though a tripod and a long telephoto lens are essential.
NASA has posted more details on the website of its Goddard Space Flight Center.
Alas, we humans will miss some of the most spectacular views, said Robert Naeye, the editor in chief of Sky & Telescope magazine. The best place to see an eclipse of the moon, he said, is from the moon itself.
"If you were an astronaut standing on the moon and looking up, the whole picture would be clear," he said in an email. "The sun would be covered up by a dark Earth that was ringed all around with a thin, brilliant band of sunset- and sunrise-colored light -- bright enough to dimly illuminate the lunar landscape around you."