Thursday, March 01, 2007

I see a red moon rising

Those of you in the Eastern U.S. will, weather permitting, see the moon rise on Saturday night while in the middle of a total eclipse. Instead of a bright yellow or white moon hanging just above the eastern horizon, you will see a coppery red moon. If you live in Europe, you get to see the entire eclipse, while those west of the Mississippi will only get to see the end of the eclipse (a partially-eclipsed moon) when the moon rises. If you are in the far west (Idaho, Utah, Arizona, or further west), you will miss out this time.

If you want to look for the eclipse, look east shortly after sunset. Remember, a total eclipse of the moon can only happen at the full moon, when the moon is exactly opposite the sun in the sky. So, as soon as the sun sets, the moon will rise on the opposite side of the sky. Want more information about the eclipse? Look at this article.

On a side note, why does the eclipsed moon look red? After all, an eclipse of the moon happens when the moon passes through Earth's shadow, so shouldn't it be dark?

If the Earth had no atmosphere, then this is true. The moon would be almost completely dark (though a small amount of light from the sun's outermost atmosphere would still shine, though weakly). But Earth's atmosphere bends sunlight around the edge of the Earth. Further, the atmosphere tends to filter out the blue light (this is why the sky is blue!), so just the red light passes through. (This is also why sunsets are often red or orange!) So, if you were on the moon during a lunar eclipse, you would see every sunset in the world all at once -- the Earth would be a dark sphere surrounded by a brilliant reddish orange ring!

The same sort of effect was visible from Earth when the planet Venus transited the sun in 2004. If you look at this picture of the transit. In the picture, a bright arc of light extends around the part of Venus that is beyond the sun's limb, AND you can see darkening and reddening of the sunlight caused by Venus's atmosphere around the part of Venus still covering the sun. Cool!

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