The moon appears to take a bite out of the sun over Manila in the Philippines during a partial solar eclipse in January 2009. Tomorrow sky-watchers in parts of Europe and Africa will be treated to a similar sight during the first of four partial solar eclipses slated to happen in 2011.
Solar eclipses occur when Earth, the moon, and the sun are aligned so that—as seen from Earth—the moon appears to cover all or part of the sun's disk, and Earth is cast in shadow. Partial solar eclipses happen when Earth crosses only through the faint outer part of the moon's shadow, known as the penumbra.
On Tuesday the moon's silhouette will start to move across the sun at 6:40 a.m. UT, when the penumbra hits Algeria. The last traces of the moon's outer shadow will cross northwest China at 11 a.m. UT.
A composite picture shows the stages of a partial solar eclipse as it appeared over Spain's Bay of San Sebastian in October 1996.
The amount of the solar disk hidden by the moon during a partial eclipse depends on how far an observer is inside the penumbral shadow. For Tuesday's partial eclipse, many capital cities across Europe and the Middle East will see more than 50 percent of the sun eaten away by the moon. (See pictures of last month's total lunar eclipse.)
"The farther north you are in Europe, the more of the sun you will see covered," said eclipse chaser and astronomer Alan Dyer of the Telus World of Science in Calgary, Canada.
At maximum eclipse—at 8:50 a.m. UT—the moon will blot out 85 percent of the sun, as seen from Scandinavian countries.
A bird sitting on top of a crescent moon and star—the symbol of Islam—is silhouetted against a 2009 partial solar eclipse over Kuala Lumpur, Malaysia. The picture appears dark because it was taken through a special filter.
To safely look directly at the sun requires filter-protected glasses or telescopes. But there are easy alternatives for observing an eclipse. For example, eclipse expert and National Geographic grantee Jay Pasachoff suggests making a simple pinhole camera. (The National Geographic Society owns National Geographic News.)
"Just punch a hole a few millimeters across in a piece of cardboard and hold it up to sunlight, and look at the image that it casts with the sun at your back," Pasachoff said. "You should easily be able to see the shape of the partially eclipsed sun."
Photograph by Bazuki Muhammad, Reuters
People watch the setting sun from Manila Bay in the Philippines during a 2009 partial solar eclipse.
For some regions of Europe, tomorrow's partial eclipse happening so close to sunrise means that a crescent sun may greet sky-watchers at dawn. And a sunset eclipse will be visible farther east, from central Russia to Mongolia. North America will not see any portion of tomorrow's eclipse.
Photograph by Gil Nartea, AFP/Getty Images
An image of the sun resembles a winking eye in the shadow of a pair of bifocals during an annular eclipse over India in January 2010. In addition to partial eclipses, a given year may see two other types of solar eclipses: total and annular.
During a total eclipse, the moon completely blots out the sun, casting its dark central shadow, called the umbra, onto a very narrow strip along Earth's surface. (See pictures of the most recent total solar eclipse, over Easter Island.) An annular eclipse happens when the moon covers only the central part of the sun's disk, leaving a ring of sunlight still visible.
This year will be unusual, because there will be no total or annular eclipses, just partials, said Pasachoff, the Field Memorial Professor of Astronomy at Williams College in Massachusetts.
"That hasn't happened since 1982 and won't happen again until 2029," he said.
Photograph by Aijaz Rahi, AP
A 2009 partial solar eclipse is partly obscured by clouds, as seen from Manila Bay in the Philippines.