By Professor Todd Brown / Contributor

There are those questions that can really stump someone. Last year, I presented a picture of the eclipse moon from last August and asked for students groups in my natural science class to explain what caused the moon to appear orange-red. Some got it, some missed it and then there was the group whose answer gave me the title to this week’s column. As the impact to their grade was minimal, I didn’t feel guilty about almost falling out of my chair laughing. Sun God or not, this Wednesday, Feb. 20, the moon will be fully eclipsed for the last time until Dec. 21, 2010.

A lunar eclipse is when at least part of the moon passes into the shadow of the earth. A total eclipse occurs when the entire moon is in the deepest part of the shadow, called the umbra. The geometry of a lunar eclipse is easy to realize. In an otherwise dark room, let a light bulb represent the sun, and hold your hand up so that it blocks the light to your eyes. Your hand fills in for the earth and your eyes are the moon. This very low-budget presentation shows one key point: for an eclipse to occur, the earth (your hand) has to be directly between the sun (the light) and the moon (your eyes). Every time the earth and moon line up in this manner, it is a full moon.

So, why isn’t there a lunar eclipse every month? The reason is that the earth travels about the sun in a plane, much like a ball rolling in a circle atop a table. The moon travels around the earth but not exactly in the plane of the table. Its orbit, a small hoop if you will, has a slight tilt (less than 6 degrees) with respect to the table. This tilt is more than enough to insure that eclipses do not happen every month. At full moon, the moon is always near the earth’s shadow but sometimes it misses it by almost a dozen lunar diameters.

Recently, the marksmanship of our nearest neighbor has been really good in that it has passed through the earth’s shadow three times in the last year. With the exception of the Chicago Cubs, life tends to even out and we are about to enter an eclipse drought that won’t end until the 2010 eclipse.

What can you expect to see on Wednesday? Probably nothing as the weather is calling for snow, frigid conditions and, the worse thing of all, clouds. But, if we do get some better weather or at least some nicely timed breaks in the cloud cover then you can expect to see the following. Starting at about 8:43 p.m., the moon will start to enter the umbra.

The once-full moon’s face will start to look like a cookie that someone is taking a bite out of. By 10:01 p.m., the moon will be completely in the earth’s shadow. It will pass through the shadow and start to exit at 10:51 p.m. At the time it is fully immersed, one might think it will simply become invisible. Most things do when sitting in a shadow: I still remember John Campbell’s 8th grade birthday party when Bill Points ran headstrong into the shadow of the barn and straight into a John Deere plow … ahh, memories. But, for the moon completely in the earth’s shadow, the entire face will have a reddish tinge to it. This is caused from the earth’s atmosphere.

I must warn anyone that this is a ‘spoiler alert.’ For those that do not want to understand the science behind the brilliant colors in a sky at sunset, then stop reading now and just go enjoy the eclipse.

As I was about to say, our atmosphere scatters smaller wavelengths of light very effectively. As the sun sets (or rises), the long red wavelengths make it through the air to our eyes the best. Hence the sky is red and orange with the setting (or rising) sun. Now imagine what an observer on the moon sees when they pass into the earth’s shadow. The sun is covered up by the earth but our world is rimmed by a dull layer of red hues. Below this thin layer, which is the atmosphere, every sunrise or sunset is occurring on our planet. The reddish ring is dim but illuminates the moon causing a dim ruddy appearance. Initially, as the moon begins to enter the umbra, the shadow will look black. This is because the red light in the eclipsed area is overwhelmed by the uncovered portion of the moon still in sunlight. It is only when the last portions of the direct sunlight is eliminated from the moon’s surface that we can see the true color of the earth’s shadow.

As an added bonus, stars that normally are overwhelmed by the full moon become visible. The eclipsed moon will be sitting in the constellation Leo. Leo is home to the bright star Regulus. With the moon climbing up the eastern sky, this star will lie to the west of the moon. There will also be an imposter among Leo’s stars: the planet Saturn. The ringed planet’s steady glow will be to the east of the moon.

Now that I have ruined the romantic view of why sunsets are so lovely, I encourage you to look to the moon around 9 p.m. and check out the show. Weather permitting, a lunar eclipse is best observed with your unaided eyes. Nor you or the Sun God will demand a telescope to see it.

What else is out there this week: