A day of total luna-see

August 6, 1999

The two minutes next Wednesday when the sun istotally obscured by the moon will be watched by scientists worldwide. But this could be the first eclipse that teaches us nothing new about the sun. John Davies reports.

From Cornwall to eastern India, via northern France, Bavaria, Austria, Hungary, Romania, Iran and Pakistan, is where the path of totality - the 100km-wide track where the sun will be obscured by the moon - will go next Wednesday. Because of the large land area it will cover, this total eclipse of the sun will be seen by more people than any in history.

But is that all it will be notable for? Some eclipses have been landmarks in the progress of science: Edmund Halley's calculation of the sun's size (1715); the discovery of helium and the sun's chromosphere (1868); and confirmation of the general theory of relativity (1919). But is there anything left to be discovered after Arthur Eddington's momentous calculations?

Probably not. Douglas Gough, professor of theoretical astrophysics at Cambridge, thinks studying the eclipse from the ground is no longer of much use. "Now there are spacecraft such as Soho (the European Space Agency's Solar and Heliospheric Observatory), there is nothing you can do on the ground during an eclipse that is more useful than anything we are doing from space," he says.

Soho's instruments, and those of two other unmanned spacecraft, Yohkoh and Trace, regularly return data about the sun's activity. In the past, observations of the sun's corona, the outer region that is visible as a white halo around the moon's outline during a total eclipse, have been invaluable in revealing the nature of the sun. But on Soho a coronal spectrograph mimics an eclipse by obscuring the sun with a disc, providing data about the corona 24 hours a day.

So will earthbound solar scientists be able to do anything on Wednesday except stand and watch? "There isn't anything obvious or high profile," concedes John Parkinson, Sheffield Hallam professor and chairman of the UK Solar Eclipse Coordinating Group. "But there are what I'd call 'experimental things'."

Chris Davis of the Rutherford Appleton Laboratory is running one such "experimental thing". As head of its Ionospheric Monitoring Group, Davis is interested in how changes in the ionosphere, the upper layer of the earth's atmosphere from about 70km upwards, can be "a very sensitive measure of global warming". The eclipse is an opportunity to study it.

During the day, radio waves are absorbed by the ionosphere as its molecules are ionised by the sun's ultraviolet light. When the sun goes down - and in the shadow of a solar eclipse - this process stops and the waves can be reflected around the globe.

"We are asking the public to help us," says Davis. "There is a Spanish radio station, at La Coru$a on 639kHz medium wave, which you can normally only hear at night in England. By measuring how far the radio signal travels during an eclipse, we can work out how strong the lower layer of the ionosphere is and how much the sun's radiation is absorbed into it."

Members of the public can report in if they can pick up the Spanish signals. Their whereabouts will be plotted, so the strength of the ionosphere can be gauged. "If global warming continues, the ionosphere will weaken, it will shrink," says Davis.

Alan Aylward, head of UCL's upper atmospheric physics group, will be taking a spectrograph to Helston in Cornwall to look at the hydrogen in the corona. "One of the clues to the heat of the hydrogen in the corona is the balance of hydrogen alpha and beta in it. We're setting the spectrograph up with a slit across the sun to look at the various spectral lines from the hydrogen." This will reveal more about how the corona heats up.

And Rutherford Appleton's Ken Phillips and Peter Gallagher, of Queen's University, Belfast, are going to a Bulgarian air force base on the Black Sea that lies under the path of totality. Their Secis (the Solar Eclipse Coronal Imaging System) project is studying the rapid oscillations in the corona's magnetic field, using the fastest digital astronomical camera in the world (taking up to 50 frames a second) linked to a computer.

"The oscillations are too fast to tell from the (Soho) space pictures," says Phillips. "With only two minutes to gather all the data, it is crucial there are no hitches ... When we tried the experiment at the last eclipse (in the Caribbean) we had a power failure."

If there is anything that still qualifies as a mystery during an eclipse, it is the phenomenon of shadow bands, the elusive bands of light and shade that are sometimes seen moving rapidly over the ground just before the sun is obscured. "They've never been properly photographed or filmed," says John Parkinson, "but we are seeing if we can do it for the BBC." The bands are caused by "light from the last bit of the sun coming through the earth's atmosphere, which is behaving like a distorting lens", explains the Open University's Barrie Jones.

"Reports of shadow bands are sporadic," Parkinson says. "If you see them before an eclipse, that implies that you might be able to see them outside the shadow of totality. Building a picture, where they are seen over a wide area, has not been attempted before."

So might something unexpected still come out of eclipse observations? "Yes, possibly," Parkinson says, his enthusiasm undimmed.

For eclipse information: http://www.eclipse.org.uk

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