Lunar eclipse: The view from history's perspective

There's more to a lunar eclipse than mere spectacle and natural wonder.

Aristotle, it turns out, used lunar eclipses to prove that the Earth was not flat - but spherical - back in 350 B.C., said Bradley Schaefer, a professor of astronomy at Louisiana State University.

So when the shadow of the Earth creeps over the face of the moon about 10 o'clocktonight, you can retrace this critical juncture in the evolution of Western thought, when observation and reason started to eclipse superstition and myth.

And, said Schaefer, you don't need a telescope or a Ph.D. in astronomy. Aristotle didn't have either.

Armed with nothing but reason and vision, the Greek philosopher pointed out that the shadow of our planet looks like a circle or a portion of one, Schaefer said. That doesn't prove by itself that the Earth isn't a circular flat disk, he said. But if we did, in fact, live on a planet shaped like a dinner plate, our shadow should only look circular when the moon is straight overhead. If it's anywhere else, the shadow would appear elongated.

Aristotle reasoned that the only object whose shadow always looks like a circle from any angle is a sphere. But what about Columbus? Why does just about every schoolchild think that he discovered that the Earth was round more than a millennium later?

Historians say the joke's on American schoolkids. By Columbus' time, "the spherical shape of the Earth was universally known among educated people," said Owen Gingerich, a professor emeritus of astronomy and of the history of science at Harvard University.

Aristotle, Gingerich said, used several other observations to argue his case. The philosopher noticed, for example, that if you travel north you see the stars in different positions in the sky, the northern ones moving higher and southern ones disappearing over the horizon - as if you were moving around on the surface of a sphere.

The idea that Columbus battled a flat-earth establishment was concocted by writer Washington Irving in the 1820s, Schaefer said. "He was trying to create a fun story for a new nation - trying to create heroes," he said. "It's a great story to tell, except that it's totally wrong."

So we've been taught to believe a myth about a myth.

But don't be too hard on teachers who fail to set the record straight, since even Aristotle was wrong on many things. Although the less influential philosopher Democritus argued that the stars were other suns, Aristotle insisted stars must be much smaller entities - otherwise they would have to exist unimaginably far away.

Eclipses themselves, while used to bust a myth, also inspired dozens. Many people around the world regarded them as omens that pointed to the collapse of empires or the end of the world.

A popular myth in medieval times was that a dragon devoured the moon during an eclipse, said Steve McCluskey, a now-retired historian from West Virginia University.

Some of these fearful ideas stem from the seemingly sporadic nature of eclipses, he said. Once astronomers learned to predict them, eclipses went from a fearful breach of nature's harmony to a reassuring proof of it.

Astronomers from different cultures noticed that eclipses occur about every six months, McCluskey said, when the moon passes through the same plane in which the Earth rotates around the sun.

But sometimes this happens when it's daytime here, so observers in any given place see an eclipse only about half the time.

For those in the right place to see an eclipse, the moon doesn't completely disappear, but instead usually changes color.

That's because some sunlight skirts around Earth, bent by our atmosphere toward the moon and then reflected back. Oxygen, ozone, and other molecules in the atmosphere distort the light - absorbing and reemitting blue light in a process called scattering. That leaves more of the red wavelengths to pass through, thus making eclipses and sunsets appear orange or red.

During an eclipse, this phenomenon can transform the moon to blood red, dusky pink, pumpkin orange or dull gray, depending on volcanic ash and other atmospheric conditions.

Clouds are in the forecast, but if they happen to part between 10:00 and 10:52 p.m., we could get a good show, as well as a reminder of the power of observation and a warning not to believe everything we're told.

Eclipse details, and the view from the Franklin Institute: http://go.philly.com/scienceEndText