Science: Eclipse of a blazing nuclear inferno: When the Moon puts light from the Sun's giant hydrogen bomb in the shade, looking is still dangerous, say Heather Couper and Nigel Henbest

Early in the evening on 10 May, the Sun will be eclipsed by the Moon. Nowhere in the world will the eclipse be total: the Moon is at its farthest from the Earth, and so its disc will be too small to overlap that of the Sun completely.

People watching from parts of America, the Atlantic Ocean and Morocco will see an 'annular' eclipse, with the Sun shining around the Moon's edge. Those observing from Britain will see half the Sun blotted out by the Moon.

A partial eclipse is dangerous to watch - the Sun is still dazzling. Even squinting at the Sun for an instant can damage sensitive parts of your eyes, and the heat radiation does additional harm. Using exposed photographic film as a filter can be tricky, too: a colour film, in particular, does not block out all the damaging radiation.

There are only two safe ways to watch the eclipse. One is by acquiring an eclipse viewer (available for pounds 1.50 from Broadhurst Clarkson & Fuller, Telescope House, 63 Farringdon Road, London EC1M 3JB, 071-405 2156). The other is to use a telescope or binoculars 'in reverse' to project the Sun's image on to a piece of white card.

Even at a distance of 150 million kilometres, the Sun still has the power to inflict harm through its brilliance. What is the source of its immense power?

Astronomers began to come up with ideas over a century ago. One theory was that the Sun is gradually shrinking. The resulting compression of its gas (like the squeezed air in a bicycle pump) produced the heat that kept the Sun shining. But that didn't stand up to scrutiny: the Sun would shrink to nothing in about 30 billion years, and it was obvious - from the age of the rocks of the Earth - that the Sun and planets had been around a lot longer than that.

Other ideas - including a Sun powered by the in-fall of countless meteorites - came and went before the nature of its power was discovered. In the Twenties, calculations by the pioneering British astronomer Sir Arthur Eddington revealed that temperatures at the centre of the Sun must run into millions of degrees. This knowledge, coupled with advances in physics, led Eddington to propose that the Sun is nuclear- powered.

Made almost entirely of hydrogen gas, our local star is a giant, slow-running hydrogen bomb. At the Sun's centre, hydrogen gas is so compressed that it reaches temperatures of 15 million degrees. Under the heat and pressure, the centres of hydrogen atoms (hydrogen nuclei) fuse to make up nuclei of helium. Because the Sun is so big, the resulting energy adds up - in fact, the Sun converts four million tonnes of matter into energy every second.

Until recently, there was no way of getting direct access to the Sun's core. But about 20 years ago, the American physicist Raymond Davis constructed a highly unconventional telescope. It was a tank containing 100,000 gallons of cleaning fluid, located in a goldmine under the Black Hills of Dakota to shield it from extraneous radiations. Davis's aim was to catch neutrinos - highly elusive particles that wing their way from the Sun's core at the speed of light. The number of neutrinos produced act as a 'barometer' of the pace of the Sun's nuclear reactions.

Because they have no mass and no charge, neutrinos hardly notice solid matter - 100,000 million million passed through the tank every second. But about every three days, one interacted with a chlorine atom in the cleaning fluid and changed it into an atom of radioactive argon. By measuring the amount of argon that accumulated, Davis was able to measure the neutrino rate - and hence the rate of the Sun's fusion reactions.

The rates Davis measured, however, were only a third of what had been expected. The reason may be quite unconnected with the Sun - if neutrinos are allowed to have a tiny mass. 'Massive' neutrinos can change their identity. Ordinary 'electron neutrinos' can change into two other types: muon neutrinos and tau neutrinos. What starts off as a beam of normal neutrinos at the centre of the Sun could have split into a mixture of all three kinds in the 8.3 minutes it takes to reach the Earth. Davis's tank, however, detects only the normal neutrinos - and therefore sees just one-third of the total produced.

Groups in Japan, Russia and Italy are tracking down other kinds of neutrino and searching for neutrinos from other objects - such as exploding stars. If neutrinos are found to have mass, the implications will be profound.

The Planets

Mercury puts in an evening appearance this month in the north-west evening sky. During the last two weeks of May, it sets almost two hours after the Sun. In the same part of the sky is Venus, which looks like a steadily shining lantern in the twilight. By the end of the month, it is setting close to midnight, offering a chance to see the dazzling planet against a dark sky.

Mighty Jupiter is visible nearly all night long, close to the bright star Spica in Virgo. And both Mars and Saturn are morning planets: Mars rather lost in the dawn twilight, and Saturn rising at 2am.

Eclipses

On 10 May there will be a partial eclipse of the Sun visible from all over Britain. At maximum, about 50 per cent of the Sun will be covered by the Moon. The eclipse begins at 6.36pm and ends at 8.31pm. In the early morning of 25 May the Moon will be partially eclipsed - 25 per cent of the Moon will move into the Earth's shadow. In Britain the eclipse begins at 3.37am and ends at 5.23am. However, except as seen from the extreme south-west of Britain, the Moon will set before the end of the eclipse, at 5.06am.

The Stars

Holding centre stage this month is the second-largest constellation in the sky: Virgo. Apart from Spica, the stars making it up are rather faint - but Jupiter is acting as an excellent locator for this Y-shaped pattern of stars. Virgo is a graphic demonstration of the incredible depth of space. Currently passing in front of the stars of the constellation, Jupiter lies just over 600 million kilometres away. If you sweep the 'bowl' of Virgo's Y-shape with binoculars or a small telescope, you'll spot a few of the galaxies making up the giant Virgo cluster - at about 60 million light-years away, the nearest large cluster to our Milky Way.

Diary (all times BST)

2 May 3.33pm Moon at last quarter

10 6.07pm New moon; partial eclipse of Sun (6.36pm-8.31pm)

18 1.50pm Moon at first quarter

25 4.40am Full Moon; partial eclipse of Moon (3.37am-5.23am)

30 Mercury at greatest eastern elongation

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