Good Night Moon: Summer Ash Explains the Eclipse
Filed under: The Dagobah System
Beautiful, and understandable.
Summer Ash, astrophysicist to the (radio) stars, sends this primer on tonight's lunar eclipse. She blogs regularly at Newtonianism for the Ladies, and she is so doggone cool.
Special to the BPP from Summer Ash:
Tonight, the full moon will go into shadow for approximately 50 minutes. From 10:01 p.m. to 10:51 p.m Eastern Standard Time, 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. I find I appreciate lunar eclipses best with no 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 catch it, this 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. The conditions for a total lunar eclipse, like you can expect to see Wednesday, require that the moon pass 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.
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 passes 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 its 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, sunlight still reaches the moon indirectly -- 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.
Your next chance to see this could come in 2012.
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 Eastern Time Zone of the U.S., the graphic just above breaks it down for you. For those of you in other time zones, go here to get your customized version. My fellow North Americans had best go out and catch this one, as we won't get another show like it until the winter solstice in 2012.
-- Special to the BPP from Summer Ash
7:14 AM ET | 02-20-2008 | permalink