Goodnight Moon

Tomorrow night the full moon will go into shadow...for approximately 50 minutes. From 10:01 p.m. to 10:51 p.m EST, the Moon will be fully eclipsed by the Earth when its orbit places it directly opposite the Sun in our skies.

The great thing about lunar eclipses, unlike solar eclipses, is that the Moon is completely harmless to stare at directly, with the naked eye or otherwise, because it "shines" due to reflected, and not direct, sunlight. In fact, I find I appreciate lunar eclipses best without any fancy equipment at all. The Moon moves fairly fast across the sky and the more you magnify it, the faster it will move out of your view. However you look at it though, it will be a beautiful sight to behold.

The alignment of the Earth between the Sun and the Moon is not a rare thing. However, the inclination of the Moon's orbit ensures that not every alignment produces an eclipse. While Earth essentially orbits the Sun in the plane of the ecliptic, the Moon orbits earth at an inclination of five degrees. This means that the Moon is only in the plane of the ecliptic twice for every completed orbit around the Earth. When you take this into account with the lunar cycle, lunar eclipses can occur approximately every six months. This doesn't mean that everyone should be seeing such a dramatic event this frequently. The conditions for a total lunar eclipse, like you can expect to see tomorrow night, require that the Moon passes through the Earth's umbral shadow. Yep, the Earth has more than one type of shadow; two to be precise, an umbra and a penumbra, as the diagram below illustrates.


Within the umbral shadow region, the Earth blocks all direct light from the Sun. In the penumbral shadow zone, the Earth's blocks most, but not all sunlight. When the Moon moves into the Earth's penumbra, only those with very keen eyes are likely to notice any dip in the moonlight. However, as the Moon passed into the umbra, the effects can be dramatic. The colors and brightnesses of the Moon can vary considerably both over the duration of an eclipse and from one eclipse to another, but most often the Moon transitions from it's normal steel gray to a vivid red or orange hue and then back again. While the Earth's shadow does indeed block all direct light from the Sun, indirectly sunlight still reaches the Moon after being refracted through Earth's atmosphere. As the light passes through our protective layers of gas and dust, the longer (redder) wavelengths are bent more. The less refracted shorter (bluer) wavelengths don't hit the Moon, but the red ones do, producing the brilliant color changes that we can observe. The dustier our atmosphere at the time of the eclipse the more sunlight is blocked completely, rather than just refracted, and the darker the eclipsed Moon appears during totality. 

The image at the start of this post is roughly what you can expect to see tomorrow night. If like me, you are in the Easter Time Zone of the U.S., the following graphic breaks it down for you. For those of you in other time zones, go here to get your customized version. My fellow North Americans best go out and catch this one as we won't get another show like this till the winter solstice in 2012.



(Image credits: Fred Espenak, NASA's GSFC, University of Nebraska - ClassAction)