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Solar eclipse a rare Yuletide gift

By David J. Craig

The cause of a solar eclipse is no mystery to scientists today. That doesn't mean they're not filled with wonder, though, when the moon totally occults the sun to produce a few moments of eerie midday twilight, or even when it partially obscures sunlight, as it will for most of the Northern Hemisphere this Christmas Day.

A solar eclipse, illuminating the sun's gaseous atmosphere, or corona. Photo: Bettman/Corbis

"I still get excited by an eclipse," says Michael Mendillo, a CAS astronomy professor. "It's one of nature's greatest treats. I watched a total solar eclipse in 1999 just outside of Paris, and people were screaming and yelling in the streets when the sun came back out."

There was a time when solar eclipses were important in investigating and answering scientific mysteries. Early in the 20th century, according to Mendillo, solar eclipses were used to verify Albert Einstein's general theory of relativity, as light from a star beyond the sun could be observed bending as it passed the sun, in essence deflected by the sun's gravity.

And until the 1950s, he says, the only time scientists could observe the sun's atmosphere, or corona, was during a solar eclipse. At other times, the sun's bright center completely obscured the gases hovering around it. Today, scientists constantly monitor the sun's atmosphere with a coronagraph, a camera with a round disk suspended before its lens to mimic a natural eclipse.

	Harlan Spence, a CAS associate professor of astronomy (left), and Frank F. Sienkiewicz view the sun and a large sunspot on a video monitor attached to their telescope. Photo by Kalman Zabarsky

"Solar eclipses taught us how dynamic the sun is," says Mendillo. "Its corona actually changes from year to year and from decade to decade. Sometimes there are long ejections being sent out from the sun, called coronal streamers, and at other times the corona is very calm." The corona is usually largest during periods of great solar activity, such as when sunspots are visible.

The moon also has a corona, Mendillo says, which was discovered in 1988 and is made of sodium gas that evaporates from the moon's dusty soil. It is so faint that scientists often use a coronagraph during a natural eclipse to observe it. Mendillo was part of a team of researchers that observed a full lunar eclipse in Australia this past July, and determined that it was between 10 and 20 times the diameter of the moon in size.

According to Frank Sienkiewicz, curator of BU's Judson Boardman Coit Memorial Observatory, because the solar cycle is in its most active stage, solar maximum, sunspots might be visible during the Christmas Day eclipse. A safe way to view the eclipse, he says, is by projecting the sun's image onto a white piece of paper through a pinhole in a piece of cardboard.

The eclipse will be visible from 10:27 a.m. to 2:43 p.m., Sienkiewicz says, with the largest proportion of the sun - about 65 percent - covered by the moon at 12:35 p.m.

Frank F. Sienkiewicz, curator of BU's Judson Boardman Coit Memorial Observatory, looks through a solar filter that attaches to a telescope and allows astronomers to safely view the sun. Photo by Kalman Zabarsky

Besides the fact that the earth gets dark for a few minutes and you must not look at the sun, the only other practical consequence of a total solar eclipse, according to Mendillo, is that low-frequency radio transmissions are sometimes disturbed. That's because radio waves bounce off the earth's outermost atmosphere, the ionosphere, which is composed of ions and electrons that receive their electrical charge from sunlight.

"When the sun turns off, the ionosphere begins to disappear and you might notice an AM radio station fading in and out a little bit," says Mendillo. "But no wars are going to be won or lost because of it."

Visit www.earthview.com/observation/pinhole.htm for tips about how to safely view a solar eclipse. Of course, never look at an eclipse without a viewing device made for that purpose.

15 December 2000
Boston University
Office of University Relations