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Thursday, 14 June, 2001, 13:00 GMT 14:00 UK
Scientists gear up for Africa's eclipse
Lusaka skyline, AP
Many scientists will be heading for Lusaka
By BBC News Online's Ivan Noble

When the Sun disappears behind the Moon and the Earth falls into darkness, it is not just an amazing spectacle. It is also an opportunity for scientists to study things which are otherwise hidden or hard to see.

Professor Ken Phillips and colleagues from the Rutherford Appleton Laboratory near Oxford, UK, are travelling to see Thursday's total eclipse in Zambia to try to work out what keeps the Sun's atmosphere at its enormous temperature.

He told BBC News Online: "The solar atmosphere has been the source of some mystery for more than half a century.

"It is very hot, much hotter than the surface layers, and the question is: what is heating it?"

Waves and flares

Two possible explanations are that waves of energy from the Sun's surface are heating it, or that tiny flares, known as nanoflares, are responsible.

Prof Phillips and his team took their instruments to Bulgaria during the 1999 solar eclipse.

"We have very fast CCD cameras, a bit like video cameras. We got some really nice results, but we're still poring over the data," he explained.

So far, it seems that the wave heating theory does account for some of the heating of the solar corona, but does not play a major role.

Zambian view

In Zambia Prof Phillips will be looking for evidence to back up his suspicion that nanoflares are the main factor responsible for the extraordinary temperatures above the Sun's surface.

It is possible to use spacecraft to make the same measurements, but it is very difficult to get the volume of data required back to Earth.

If scientists can resolve the question of how the corona is heated, it will go a long way towards increasing understanding of other stars in the universe.

"They often look like common or garden sunlight stars, but we can see from various X-ray photographs that they too have hot atmospheres," Prof Phillips said.

The 2001 eclipse occurs when the solar corona is at its maximum, so conditions will be even better than 1999 for making measurements, he added.

His observations stand in a long tradition of scientific inquiry during eclipses.

Proving relativity

Back at the beginning of the 20th Century, scientists were grappling with a controversial and complex new theory: Einstein's General Theory of Relativity.

It holds that mass can change the shape of space.

Not everyone was convinced by the theory, so, in 1919, British astrophysicist Arthur Stanley Eddington set out to find experimental proof.

He led an expedition to the Portuguese-run island of Principe, off the Atlantic coast of Africa, to observe a solar eclipse.

Bending starlight

The idea was to observe the way that the mass of the Sun bent the path of light travelling from distant stars.

If Einstein was right, then the light would be bent twice as far as conventional Newtonian physics would allow.

Another British team travelled to Brazil to see the same eclipse from the other side of the Atlantic.

Only after months making controlled observations did the two teams finally return home to announce in November 1919 that Einstein was right.

Follow-up measurements during subsequent eclipses came to the same conclusion.

See also:

24 Aug 99 | Total Eclipse
Eclipse shadow unveils scientific mysteries
18 Jun 01 | Africa
Angolan concern ahead of eclipse
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