[Updated to be clear that it is Monday night (the 20th) not Sunday night so not really on the weekend. Whether it starts on the 20th or 21st depends on which time zone your are in.]
There will be a total lunar eclipse this
weekend Monday Night. Starting just before midnight (23:29) on Dec. 20, it will last for several hours.
Lunar eclipses last much longer than solar eclipses because the earth is much larger than the moon and therefore creates a correspondingly larger.
When a lunar eclipse occurs there are two distinct phases. First the moon enters the part of the earths shadow called the penumbra, before entering the darker umbra. Then the process is reversed as the moon moves out of the earths shadow.
The second phase, the umbra is what most people think of as a true shadow. This is when the light from the sun is fully blocked by the earth. The penumbra is a partial shadow. It occurs because the sun is not a point. If you stand at the edge of a building so that you can only see part of the sun, it will be dimmer but not dark. If you look at the ground you will see that the shadow from the building in not a sharp edge but that it gradually changes from light to dark.
In the same way the moon will darken as the light falling on it is partly blocked by the earth, until it enters the umbra where the earth blocks all the light from the sun.
This alignment, with the earth directly between the moon and the sun is a key to the timing of eclipses. Lunar eclipses occur during full moons when from our point of view on the earth, the sun is directly opposite from the moon. The reverse is true of solar eclipses. In which case the new moon is between the earth and the sun and we can only see the dark part facing away from the sun.
More information from Space.com
On the nights of Dec. 20 and Dec. 21, parts of four continents will be treated to a total eclipse of the moon — the only one to occur in 2010.
This NASA lunar eclipse chart shows the visibility of the eclipse from different regions around the world.
The last total lunar eclipse occurred on Feb. 20, 2008. While there are two total lunar eclipses in 2011, North American skywatchers will have to wait until April 2014 for one as potentially spectacular as the eclipse occurring this month. [Amazing Total Lunar Eclipse Photos]
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,