What's a Solar Eclipse?

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Solar eclipses are some of nature's most dramatic celestial performances. Solar eclipses occur when the Earth, moon and sun are aligned in the same plane and the moon passes between the Earth and the sun, partially or completely covering our closest star.

A total solar eclipse typically only lasts for a few minutes, explained NASA solar astronomer Mitzi Adams of the Marshall Space Flight Center in Huntsville, Ala. The longest solar eclipse lasted about seven minutes, she said in a NASA webchat.

At least two, and sometimes up to five, solar eclipses can occur in a given year. Some of these are annular solar eclipses, a term given to eclipses in which the moon covers a large portion (but not all) of the sun. No more than two of those can be total eclipses.

Total eclipses are seen rarely because totality – when the sun appears totally hidden by the moon – only exists along a narrow path on Earth's surface, as opposed to partial eclipses which can be viewed across a much wider region.

But when viewing a solar eclipse, looking at one through a telescope can be dangerous, Adams warned.

"You have to have a proper filter," she said. "The safest way is through a method called projection, where you actually take the eyepiece out and project the image onto a sheet of paper behind the telescope, without looking at the sun. You move the sheet of paper back and forth until you get a focused image."

It is safe to observe a solar eclipse with the naked eye, but only during the stage of complete totality, Adams said. Otherwise, looking at an eclipse is extremely dangerous. Special eye protection is needed to view the sun during partial and annular eclipses.

Regular sunglasses do not make viewing the sun safe, said Adams, as they do not adequately block enough of the infrared and ultraviolet radiation. Instead, only specially-designed "eclipse glasses" that use appropriate filtration should be used for direct viewing in these cases.