A total lunar eclipse will take place on December 20/21, 2010. It will be visible after midnight Eastern Standard Time on December 21 in North and South America. It will be the first total lunar eclipse to occur on the day of the Winter Solstice since Dec. 21, 1638.
A lunar eclipse occurs when the moon passes behind the earth so the earth blocks the sun’s rays from striking the moon. This can occur only when the Sun, Earth, and Moon are aligned exactly, or very closely so, with the Earth in the middle. Hence, there is always a full moon on the night of a lunar eclipse.
Some interesting and informative sites on the lunar eclipse include Wikipedia, NASA, Lunar Eclipses for Beginners, a page of lunar eclipse videos which can help explain what’s going to happen, and a helpful page on how to photograph a lunar eclipse.