Lucky sky-watchers in Antarctica and southern Africa will get a treat this weekend when the moon’s disk glides across the face of the sun creating a magnificent partial solar eclipse just after sunrise on Sunday, September 13.
The Earth crosses between the moon and the sun every month, but the three celestial bodies don’t always line up. A total solar eclipse occurs only when all three fall into perfect alignment such that the moon casts its dark central shadow, called the umbra, onto a very narrow strip of Earth’s surface.
During a more-common partial eclipse, such as the one this weekend, the Earth passes into only the moon's outer, much wider shadow cone, known as the penumbra, which creates a less dramatic but still impressive sky show for a much larger audience.
This weekend’s event begins at 6:45 a.m. Universal Time (UT), September 13, when the silhouette of the moon starts moving across the sun. The eclipse reaches its deepest point at 7:47 a.m., and by 8:50 a.m. it ends when the last bit of the moon’s disk slides off the sun. By that the time the sun should be about a quarter of the way up the local sky.
The amount of solar disk hidden by the moon in a partial eclipse depends on how far inside the penumbral shadow an observer is. For this event, the best sites will be in Antarctica, where up to 79 percent of the sun will be covered.
The farther north sky-watchers are, the less of the eclipse will be visible. In Cape Town, South Africa, residents will see about 30 percent of the sun eaten away by the moon, while those in Johannesburg will view about 15 percent of the sun covered up. Farther north, Harare, Zimbabwe, will see only 2 percent of the sun covered, but people in Zimbabwe, Namibia, Botswana, and southern Madagascar will get to see at least some of the sky show.
Observers must take precautions so that they do not damage their eyes while observing the partially eclipsed but still blindingly bright sun.
Jay Pasachoff, an eclipse scientist at Williams College in Massachusetts and a National Geographic grantee, says that one should never look directly into the sun without the use of special filter-protected glasses or telescopes. But there are safe and easy alternatives to enjoy the eclipse show. Pasachoff suggests looking for non-round images of the sun cast on the ground under leafy trees or making a simple pinhole camera.
"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," he says. "You should easily be able to see the shape of the partially eclipsed sun."
Space buffs who miss this eclipse won’t have to wait long for the next exciting sky show. On September 27 or 28, depending on where you are, a total lunar eclipse will unfold for western Europe, Africa, and North and South America. Stay tuned for more details.