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Who's turning out the lights? The moon. An eclipse of the sun occurs when the moon passes directly between Earth and the sun. When the moon’s shadow falls on Earth, people within that shadow see the moon block a portion of the sun’s light.
The moon’s shadow has two parts, an umbra and a penumbra. The umbra is the “inner” part of the moon’s shadow. The penumbra is the moon’s faint “outer” shadow.
During a total solar eclipse, the moon appears to cover all of the sun for observers located in the moon’s umbral shadow, also known as the “path of totality.” Those viewing the eclipse from the moon’s penumbral shadow see the moon cover a portion of the sun.
At the moment of totality, when the sun is totally obscured by the moon’s shadow, the sun’s outer atmosphere, called the solar corona, becomes visible. It’s a seldom-seen sight coveted by experienced eclipse watchers and an awe-inspiring vision for first-time viewers. The solar corona extends farther than 620,000 miles from the sun’s visible surface and reaches temperatures up to 2 million degrees.
A solar eclipse is such an extraordinary spectacle that Mark Twain used one in A Conneticut Yankee in King Arthur's Court. Hero Hank Morgan gets whacked on the head and winds up as an inadvertent time traveler. At one key point, he manages to save himself by using his knowledge of an upcoming solar eclipse.
Most of us aren't time-traveling, but we'd still like to see the eclipse. That's where NASA is stepping in with astreaming live webcast of the event from 6 a.m. (EST) until 11 a.m. (EST). The eclipse coverage is part of the Sun-Earth Day celebration, an annual event that strives to share the many ways in which the sun interacts with Earth and the other planets in our solar system.
Here's where you can go to get a front-row seat: The Exploratorium. Because the largest portion of the eclipse will be seen in the area of China's fabled Silk Road, the site includes cultural aspects, such as the Sun-Eating Dragon. In addition to the Exploratorium, Sky and Telescope has information on additional web broadcasts.
A reminder: if you're in the area of the eclipse, do not look directly at it. The intense flares can cause damage to the eyes, or even blindness.
If you're a photographer, Bill Kramer at Eclipse Chasers has tips on how to get good photos. This guide offers special tips for those using digital cameras.
The next solar eclipse will occur on July 22, 2009, with the Western Pacific and China getting front-row seats. If you're like to plan ahead, check out these eclipse-based travel options.


