Hundreds of tourists will travel to the Sahara this spring to witness a four-minute natural phenomenon. Mark Mackenzie finds out why

The total solar eclipse will last for around four minutes, the path or "track" of its shadow first striking the Earth in South America. Of the 2,500 Britons predicted to be in Libya, the optimum viewing location, most will pay around £1,700 for the privilege.

Worth the expense? Well, if you're serious about catching one of the more spectacular astronomical phenomena visible from Earth, yes. Eclipse tours may be a highly specialised offering but they are, apparently, a growing business.

"When a total eclipse happens there is only a small band of the Earth from which you can see it," explains Robert Massey, senior astronomer at the Royal Observatory, Greenwich. "The March eclipse may be partially visible from Britain but it won't begin to compare with what's visible from the centre of the track." Mr Massey is referring to phenomena such as Baily's Beads, a bulging oval of light on the Moon's rim like a diamond ring, which occurs when sunlight passes between the mountains on the Moon's outer edges. Or the corona, the Sun's outer layer of white cloud-like plasma, which is only visible when the intense light of the star's surface is dimmed.

Britain's largest eclipse tour company is Explorers. In March it will welcome 800 passengers aboard a chartered ship for an eight-night cruise, culminating in a sortie into the Libyan

Sahara. "Solar eclipses occur every 16-18 months and vary in quality depending on the distance of the Moon from the Sun," explains Brian McGhee, Explorers' managing director. He has been organising tours for 25 years, so has interest grown in that time? "Absolutely. Now we're only limited by the volume of flights and hotels available." For the solar eclipse of 1999, in Cornwall, for example, he chartered a train for 1,600 people.

Mr McGhee estimates that 70 per cent of his customers are repeat business, so-called "eclipse chasers". Unlike those operators who, according to him, "simply take advantage of the fact that their ordinary tours coincide with an eclipse track", Mr McGhee is realistic about the probability of seeing an eclipse. For all the pyrotechnics of the solar system, nothing dampens proceedings so quickly as low cloud. To avoid falling victim to the weather, Mr McGhee uses sophisticated data sources, including an insider at Nasa, but stresses that most of the information is available on the internet. "Taking a group with a 25 per cent chance of success is not something I want to be involved in," he says. "In the Sahara in March, the chance is around 80 per cent."

And, according to Mr Massey, knowing what you're doing has a more important dimension. "During the partial phases of the eclipse, the eye is fooled into thinking that the Sun is safe to look at," he explains. "Only a tenth of the surface may be visible but the intensity of light is the same." Mr McGhee agrees: "We brief clients fully on what's safe and what isn't. The light level of an eclipsed Sun drops considerably but the possibility of permanently damaging your eyes from infrared light remains high."

Explorers provides clients with eclipse viewers - glasses coated in a protective film. "The joy of the eclipse is that during the totality, when the disc of the Sun is completely covered and the Sun's atmosphere is visible, you need just a small pair of binoculars to get the best view of it," he adds.

For all the precautions, Mr Massey believes that everybody should experience a solar eclipse at least once in their lives. "It is a truly spectacular and overwhelming experience," he says. "The fact is that if you see one, you'll probably mortgage your house to see another one."