By BBC News Online Science Editor Dr David Whitehouse

A total eclipse of the Moon on Friday may cause the satellite to turn bright red or even vanish completely.

A total lunar eclipse is not all that rare and unlike a total solar eclipse, when the Earth enters the Moon's shadow, can be seen from an entire hemisphere of our planet.

It might be a regular event, but it has, nevertheless, always captured the imagination.

In ancient Egyptian mythology, the Moon was a boat sailing across the heavens attacked once a month by a huge sow that caused the moon to die only to be born again. But sometimes the sow swallowed the moon entirely and then disgorged it - that was what we now call a total lunar eclipse.

Two and a half thousand years ago, the curved shape of the Earth's shadow as it traversed across the Moon during an eclipse was said to have convinced Pythagoras, the Greek astronomer and mathematician, that the Earth was round.

Lunar tables

Christopher Columbus used tables of the times of lunar eclipses to impress the natives of Jamaica when they refused to provide food.

According to his diary, on 29 February, 1504, Columbus warned that he would cause the moon to disappear because his god was angry. When this duly happened, the terrified inhabitants gave him all the food he needed.

This lunar eclipse will be visible from the UK in the early morning of Friday 21 January. It will also be visible from the whole of north and south America, and from much of Europe and Africa.

The Earth's shadow will first become visible on the Moon when the partial phase of the eclipse begins at 0301 GMT. The total phase of the eclipse occurs between 0405 and 0522 GMT. The eclipse then becomes partial again, finishing completely at 0625 GMT.

When it takes place, the full moon will be in the constellation Cancer, quite low in the western sky (about 30 degrees above the horizon for observers in the UK).

The Moon usually remains visible during a total eclipse, becoming a shade of copper-brown or orange. This is caused by sunlight passing through the Earth's atmosphere being deflected towards the Moon.

Red colouring

If the Earth had no atmosphere, its shadow would be completely black and the Moon would disappear from view. The Moon's colour during an eclipse can vary considerably according to the amount of dust in the Earth's atmosphere at the time.

"We're kind of expecting that this eclipse will be very bright and probably very bright red," said Yale University astronomer Brad Schaefer.

"This same thing happens to us (on Earth) during sunsets," he added. "When the Sun has just set, you look off toward the direction of sunset and the same physics is going on ... and so what you're seeing on the Moon is basically sunset colours."

The colour of the Moon during a lunar eclipse can also be influenced by the amount of dust thrown up into the stratosphere by volcanoes. Big eruptions, like the blast of Mount Pinatubo in the Philippines in 1991, can turn the moon so dark that it is almost impossible to spot during an eclipse.

But since there has not been such an eruption recently, astronomers are forecasting a deep red lunar disk. No special observing equipment should be necessary, but a pair of binoculars might be helpful. Getting away from streetlights is also a good idea.

A lunar eclipse holds no great surprises for scientists but it is a wonder of nature available to all and an intriguing example of the clockwork of the Universe.