Millions of people from northern Europe to South Asia are preparing to look skyward to catch one of nature's greatest spectacles, a total eclipse of the Sun.

The event is being anticipated with a mixture of superstition and opportunism.

Thousands of people in India have already gathered at holy sites, where some mystics are predicting that the eclipse will presage war and devastation.

In Britain, Celtic pagan priests have been holding Sun dances to improve the weather for nearly a million people who have travelled to Cornwall to see the total eclipse.

Wednesday will provide the last chance for scientists to study the phenomenon this millennium, although they like everyone else have been warned not to look directly at the sun.

BBC Science Correspondent Sue Nelson explains the stages of a total eclipse

As the Moon moves in front of the Sun, it will send a shadow racing across the surface of the Earth. The temperature will drop, birds will stop singing, and bees will return to their hives.

Day will become night.

Although a total eclipse of the Sun can be seen somewhere on Earth every 18 months or so, its track is often quite short and over areas where few people live.

In this eclipse, however, there will be darkness on at least some part of the planet for over three hours and the path of totality - the band on the Earth's surface thrown into complete darkness - will cross some of the most heavily populated areas.

Europe's opportunity

Europe Today's Catherine Guilyardi visits Perthes in France, one of the best European sites to see the eclipse

"Probably the biggest thing about this eclipse is that it's passing one of the highest population centres in the world - central Europe - so it gives a lot of people an opportunity to see a major eclipse," says Nasa eclipse specialist Fred Espenak.

"The last time an eclipse was visible through central Europe was 1961, so it's been quite a while."

The eclipse begins in the North Atlantic about 300 kilometres south of Nova Scotia where the Moon's shadow first touches down at 0930 GMT.

Not since 1961 has central Europe been in such a good position

From there, it races over and across southwest England and into continental Europe.

The instant of greatest eclipse occurs at 1103 GMT, when the shadow's epicentre will be over the rolling hills of south-central Romania, very near Rīmnicu-Vīlcea.

Spectators on the ground will see the Sun blacked out for a total of two minutes and 23 seconds

From there, the shadow will race on at over 2400 km/h (1500 mph) into Turkey, Iran and Pakistan and ending in the Bay of Bengal off the coast of India at 12.36 GMT.

Uncertain weather

BBC Bombay Correspondent Sanjeev Srivastava reports on how India is preparing for the eclipse

The big unknown, of course, is the weather. Forecasters have predicted cloudy skies for southwest England, the first landfall in the path of totality. And monsoon conditions will most likely spoil the enjoyment of millions at the other end of the eclipse track.

In between, many are sure to get the full experience in cloudless skies, particularly in countries such as Turkey, Iraq and Iran.

National governments have been concerned about eye safety

Many national governments have warned people that looking directly at the Sun is very dangerous because of the damage it can do to the eyes.

But astronomers, whilst advising spectators to take every precaution, have urged everyone to grasp what may be a once-in-a-lifetime experience.

"There is nothing in nature to rival the glory of a total eclipse," said British astronomer Patrick Moore, a veteran of eight eclipses around the world.

"No written desciption, no photograph can do it justice,"