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original link: http://scienceblogs.com/startswithabang/2011/02/01/why-the-moon-turns-red-during/

本文解释了月全食中红月亮(美国叫blood moon–血月)的物理成因.

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There may be no greater sight for the naked-eye astronomer than a total lunar eclipse. Normally, we get one or two of these a year, and the same wonderful thing happens at each one.

Image credit: Amateur Astronomical Society of Rhode Island.

If you were standing on the Moon, you’d start to see the Earth begin to cover part of the Sun. As the Earth blocks more and more of the Sun, there would be less sunlight landing on the Moon, and one side of it appears to darken slightly, as you can see in the top row of images above.

But then the Earth begins to completely block the sunlight from landing on the Moon, and you start to see the Earth’s shadow land on the Moon. And as more and more of the Moon falls into the Earth’s umbral shadow, less and less of it is visible to us.

But just when you think the Moon is about to disappear completely into darkness…

Image credit: David Lee.

Only the white of the Moon disappears, and gets replaced by a dim, red color. While this red Moon that gets left behind is something like 10,000 times dimmer than the full Moon it left behind, it’s still easily the brightest thing in the night sky.

Well, why does the Moon turn red? why do we have a so called blood moon? why it is not a blue moon?

Image credit: Fred Espenak, Robert Roy Britt, NASA and Space.com.

After all, If the Earth is completely blocking the Sun’s light, you’d expect the Moon to go completely dark. And honestly, if the Earth were more like Mercury, the Moon would go completely dark during a Lunar Eclipse.

But the Earth has something “extra” that makes the Moon light up during an eclipse.

The atmosphere! As photographed by the Space Shuttle Atlantis from the International Space Station, the Earth’s atmosphere glows a pale blue color when the Sun shines directly on it.

But the Moon doesn’t turn blue, it turns red. So what’s the deal?

Image credit: NASA.

Looking at the edge of the Earth before the Sun comes up helps us with a part of the answer. The part of the atmosphere that experiences “day” shines blue, but the part that experiences a sunset or sunrise glows red, as anyone who’s seen one knows.

The Earth’s atmosphere scatters blue light more easily than it scatters red light. During most times of day, when light is plentiful, it means that more blue light goes in all directions, and that’s why the sky looks blue. But at sunrise or sunset, the sunlight needs to pass through much more atmosphere than normal, the blue light gets scattered away, leaving more red light than blue, and turning the sky redder than normal.

Image credit: Stanley David Gedzelman.

So along its journey to the opposite side of the Earth, the sunlight winds up traveling through hundreds upon hundreds of miles of atmosphere during a lunar eclipse, and only the reddest light will make it all the way through! The result?

The Moon does turn red during an eclipse! In fact, the closer you are to the center of the Earth’s shadow, the redder the Moon will appear!

Image credit: Stanley David Gedzelman, again.

The Moon appears whiter as more blue light adds to the red, and more red as less and less blue light gets through. Ever wonder what it would look like if you caught the Moon right at the center of the Earth’s shadow?

Image credit: Martin Pugh, and click for a huge version!

It would be deeply, darkly, and nearly uniformly red! You might, in fact, wonder what this looks like if you were on the Moon. Well, NASA has put together an animation showcasing one of the most spectacular phenomena that you’d see…

Image credit: Larry Koehn.

The “Ring of Fire”, or the sum of all the sunsets and sunrises, simultaneously, from all around the Earth! The result is a great red ring, that would appear bluer/whiter nearest wherever the Sun is closest to the edge. (Not shown in the animation above.) And now you know why the Moon turns red during an eclipse!