The Long History of the Lunar Eclipse

DEBBIE ELLIOTT, host:

This ALL THINGS CONSIDERED from NPR News. I'm Debbie Elliott.

Listeners across the Eastern half of the country had the chance to see a lunar eclipse tonight. From 6:00 to 7:15 Eastern, the Earth's shadow will crept across the moon. People have studied lunar eclipses since ancient times. Predicting them was a science in China as far back as 2300 B.C., and the first people to start to get those predictions right were the Babylonians.

In honor of tonight's sky show, we take a few moments to consider the lunar eclipse in our weekly segment Science Out of the Box.

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ELLIOTT: Fred Espenak is an astrophysicist with the NASA Goddard Space Flight Center. He's known around the world by his nickname.

Mr. FRED ESPENAK (NASA Goddard Space Flight Center): Mr. Eclipse, yeah. My personal Web site is MrEclipse.com.

ELLIOTT: He's Goddard's official eclipse predictor.

Mr. ESPENAK: I've got predictions for lunar and solar eclipses thousands of years in the past and the future. I'm constantly adding to that and refining it, adding more information, more details.

ELLIOTT: These days, Espenak is part of a lonely breed of astronomers. He's the only person studying lunar eclipses at Goddard. But there was a time when lunar eclipses gave scientists important clues about our solar system, because they're visible even measurable without a telescope.

Mr. ESPENAK: Aristotle used a lunar eclipse to determine that the earth was round, because when the Earth's shadow moves across the moon during an eclipse, you can see that it's got a circular shape to it. No matter where on Earth you observe the eclipses, it's still got that circular shape.

ELLIOTT: A hundred years later, another Greek astronomer named Aristarchus came along. He used a lunar eclipse to estimate the relative distance of the moon and sun from the Earth. Eclipses were still useful in the 1960s. They helped astronomers determined whether the astronauts would be stepping on rock, dust or green cheese when they set foot on the moon's surface.

Mr. ESPENAK: It's got different minerals and different materials, and textures are different. There's dust, there's small rocks. There are huge boulders. And the way these materials radiate their energy and their heat content into space differs. So if you can make measurements of the heat radiation of the moon as it cools down in the Earth's shadow during a lunar eclipse, then you can tell something. You can learn something about the surface material on the moon.

ELLIOTT: But in this age of the Mars, rover and the Hubble telescope, is there anything new we can learn from tonight's lunar eclipse? Espenak says there is.

Mr. ESPENAK: By studying how bright or how dark each lunar eclipse is, it tells us something about the amount of dust in the earth's atmosphere.

ELLIOTT: During a lunar eclipse, the moon is not completely blacked out. It has a reddish glow. That's because some rays of the sun pass around the edges of the Earth through our atmosphere and then hit the moon, even while it's in the Earth's shadow. But the exact color of the lunar eclipse changes, depending on what's in our atmosphere.

Mr. ESPENAK: There was a major volcanic eruption in 1991 of a volcano in the Philippines called Pinatubo. Around the Earth people saw bright sunsets and sunrises for a year or more after the eruption because of all the dust that was in the atmosphere. There was also a lunar eclipse about six months after that eruption, and the moon almost vanished during the total phase, because there was so much dust, it blocked almost all of the sunlight reaching the moon.

ELLIOTT: But what about particulate matter that humans have spewed into the atmosphere. We asked Richard Keene, a meteorologist at the University of Colorado, if pollutants have changed the color of the moon we see during the eclipse.

Mr. RICHARD KEENE (University of Colorado): Not really. Not unless they get up into the stratosphere. So we're talking 10 to 20 miles up, and volcanoes are about the only thing that's going to do that. So we're seeing the same thing that they would have seen a thousand years ago. Only big changes would be in the language. Thousand years ago, something like sprinkled with the color of blood, and now we say it's orange.

ELLIOTT: So give or take a few volcanic eruptions, tonight's spectacle is pretty much what early man saw at Stonehenge. Transcript provided by NPR, Copyright NPR.