Chasing Shadows
A lunar eclipse map of the world.
Credit: SETI Institute.

On Wednesday, February 20, we're in for a celestial treat. There's a total lunar eclipse visible from North and South America as well as Europe and parts of Africa. It's a great excuse to spend time out-of-doors watching the Moon drift into the shadow of the Earth.

Lunar eclipses occur when the Moon and Sun are opposite each other in the sky, and the Earth is in between. The full moon also occurs when the Moon and Sun are opposite each other in the sky. However, most months, the Moon's orbit carries it above or below the Earth-Sun line, the full Moon misses the Earth's shadow, and there is no lunar eclipse. In a typical year, we can see between 0 and 3 total eclipses of the Moon. There are also partial and penumbral eclipses when the Moon passes through just a part of the shadow of the Earth, and dims very little. If you want to plan ahead, there's a comprehensive list of lunar eclipses at NASA's eclipse home page. 

What will happen? As the Moon orbits Earth, it will cross into the Earth's shadow. At first, the Moon enters the penumbra, a partially shaded area that surrounds the darker umbra, or central shadow. The Moon dims only a little in the penumbra. It's hard to see any difference. Once the Moon begins to enter the umbra, the shadowed moon darkens to a dull red or red-brown glow. It doesn't get completely dark because light from the Sun is bent by Earth's atmosphere with the red light bent the most. Thus, when the Moon is deep in the Earth's shadow, reddish light illuminates its surface.

What can you see? Beyond the color change, look at the shape of Earth's shadow on the Moon. Like your own shadow, Earth's shadow reveals its shape. More than two millennia ago, the Greeks reasoned that the Earth was round because they always saw this curve during lunar eclipses. Columbus didn't have to prove that the Earth was round; it was well-understood thousands of years earlier.

Where can you watch the eclipse? There's no need to go to a major observatory to see a lunar eclipse. It's visible from anywhere that you can see the Moon. If you would like to join with other people for an evening to share the experience, many amateur astronomy clubs will host lunar eclipse parties, and they welcome visitors. It's fun to join a group of dedicated amateur astronomers who can answer your questions while you watch the eclipse.

The Night Sky Network is a national network of amateur astronomy clubs. You can locate a club near you, and find out if they are planning a "moon party" on the 20th. Likely, they will have telescopes available for viewing the darkened moon and other celestial treats. They may also be conducting short explanatory demonstrations related to eclipses from "Shadows and Silhouettes," a kit of materials developed by the Astronomical Society of the Pacific in collaboration with SETI Institute and Lawrence Hall of Science. You can learn about lunar and solar eclipses, phases of the moon, and transits via these activities.

"Shadows and Silhouettes" was sponsored by NASA's Kepler Mission which will seek Earth-size planets in orbit about distant stars by looking for transits. A transit occurs when a planet crosses between the Earth and a distant star, blocking a small part of the star's light. Basically, we see the transit because we are in the shadow of that distant planet. This dip in the star's brightness reveals the presence of the planet, and repeated observations of subsequent transits confirm the discovery. The Kepler Mission launches in 2009 to begin observations that may reveal other Earths in our galaxy.

So, simple shadows reveal much about the shape of the Earth, and, possibly, the presence of other Earths around distant stars.

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