A PRIME-TIME ECLIPSE
Dr. Terry L. Goforth
Professor of Physics, SWOSU
February 2008

Tired of re-runs and reality TV? How about a little reality sky-watching tonight? Step outside this evening at about a quarter to eight and take a look in the eastern sky. If skies are clear, you'll see the Lenten Moon (the full moon that occurs in the season of Lent) sitting at the paws of Leo the Lion (identified by the large, backwards question mark). Pale yellow Saturn will shine just below and north of the moon. But what you will really notice, if you watch for a while, is the increasing sized "bite" being taken out of the moon!

Is this some giant alien nibbling on "green cheese?" No. It's just the beginning of a total lunar eclipse. Between 7:43 p.m. and 9:01 p.m. central standard time, the moon will move into the Earth's umbra (the deepest part of the Earth's shadow). Since the moon does not produce its own light, we can only observe it by the sunlight that reflects off its surface. As the moon enters the shadow of the earth, this sunlight is blocked, and the portion of the moon's surface that is in the umbra goes dark. This phase of the eclipse when the moon is partially light is called the partial eclipse phase.

Note the rounded shape of the boundary between the light and dark portions of the moon. This is the shape of Earth's shadow, showing that the earth is indeed a round object. This detail was noticed by Aristotle and other Greek intellectuals, and it led them to conclude that the earth was indeed round nearly 2000 years before Columbus sailed across the Atlantic Ocean!

Diagram of moon moving through Earth's umbra

Between 9:01 and 9:51 p.m. CST, the moon will be completely within Earth's umbra. This is the total eclipse phase, or "totality" for short. If Earth had no atmosphere, the moon would be completely dark and unseen during this time. But the earth does have an atmosphere, and that thin layer of gas surrounding the planet acts as a lens, focus a pitifully small amount of sunlight into the umbra. Bluer colors of light are preferentially scattered out, allowing the redder colors to be directed into what would otherwise be a completely dark shadow. (This preferential scattering of blue light is also the reason that the sky is blue and sunrises and sunsets are red.) During totality, the moon will glow with a color that ranges from deep red to a coppery orange depending on atmospheric conditions on Earth. The red or orange color is actually present during the partial phase as well, but the light level is so low that it is undetected next to the bright, sunlight portion of the moon, much like trying to read the clock or radio setting in your car when the sunlight is shining directly on the dash.

From 9:51 to 11:09 p.m. CST, the moon will emerge from Earth's umbra. More and more of its surface will enter the sunlight, and after 11:09 p.m., the eclipse is over. The full moon will shine (or more correctly, reflect sunlight) in all its glory high in the eastern sky.

You don't need any special equipment to observe a lunar eclipse–just clear skies and your eyes. (And maybe some warm clothes.) You can watch it start to finish, or just check its progress periodically. And it's completely safe to look at with unprotected eyes. There's no difference between observing a lunar eclipse and just looking at the moon in the sky on any other night.

You don't want to miss this show. There won't be another total lunar eclipse until December 21, 2010.

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Last update:  February 19, 2008