Last Night’s Solstice Lunar Eclipse: The Basic Science Explained

A rare duo of astronomical events, the December 2010 lunar eclipse occurred on the December 21 winter solstice to produce a beautiful light show for those who were lucky (and awake) enough to watch. During a lunar eclipse, the Earth intercepts the Sun’s path to the Moon, casting a shadow on the Moon that appears red, amber, or gray. Lunar eclipses occur at least twice a year, so you will certainly be able to see the event again. The special part about this particular eclipse, however, is its timing: The last time an eclipse occurred on the solstice was on 1638 (372 years ago), and the next one is projected to occur in 2094. On the day of the winter solstice, the tilt of the Earth is at its farthest position from the Sun, giving us winter.  This also makes the day of the solstice the shortest of the year.

The eclipse began on December 21, 2010 at 12:33 AM, and the Moon was completely engulfed in the Earth’s shadow at 1:41 AM (CST). Take a look at this time lapse video for a peek if you missed it:

So when is the Earth’s shadow visible? Well, it’s actually the moment in the video when the Moon turns red. Sunlight is made up of all colors of the spectrum, from red to violet, and then some beyond the visible spectrum. The Earth’s atmosphere scatters blue light (which is why the sky is blue), so the light that makes it to the Moon is red, causing a red shadow to appear on the Moon’s surface.

Why do you see the Moon go black before it becomes red? Shouldn’t it just go from white to red? If you look at a picture of the Sun, Earth and Moon during a lunar eclipse, you can see that sunlight from each edge of the sun crosses each edge of the Earth, creating cone-shaped shadows in space called Penumbra. These shadows appear on the Moon’s surface as darkness before the Moon moves directly into the Earth’s path, or Umbra.

Courtesy of


You can also view this from the Moon’s point of view!

Courtesy of


Speaking of the Moon’s point of view, the temperature of the Moon goes wild during lunar eclipses. Ordinarily, the temperature of the surface of the Moon exposed to the Sun (during a full moon) is 266 degrees Fahrenheit. During a lunar eclipse, the temperature of the Moon drops to -146 degrees Fahrenheit-all in the span of 90 minutes! The lack of an atmosphere on the Moon means that it can’t control heat loss into space during an eclipse. Kind of makes you want to do something nice for the ozone layer, right?

Courtesy of some nice lady on Twitter. Please don't sue me.

Some really gorgeous photos were captured from this event. This one, from a viewer in Manhattan, caught this shot of the Moon from her balcony:

And this one, which showed up on the Associated Press this morning, showing the Moon entering one of the two Penumbra over the top of the Chrysler Building in New York:

Courtesy of the Associated Press


The nice thing about lunar eclipses is that, unlike solar eclipses, in which the Moon blocks the Sun’s path to the Earth, lunar eclipses can be viewed from anywhere in the world at night, and don’t require special eye protection, although visibility is optimal in different parts of the world during different eclipses. There will be two lunar eclipses in 2011: June 15, 2011 and December 10, 2011. Get out there!

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