Skywatchers wait for eclipse

By BBC News Online science editor Dr David Whitehouse

If the weather co-operates, people in the UK, Africa and much of Asia will see the Moon eclipsed on Tuesday.

Although not of great scientific importance, a lunar eclipse is nonetheless a grand spectacle of nature. It occurs when the Moon enters the shadow cast by the Earth in space.

The mid-point of the eclipse will be at 2020 GMT, by which time the Moon may have turned a deep red or copper colour. The only light reaching the satellite will have been refracted through the Earth's atmosphere.

Photographing the eclipse

Mount your 35-mm camera on a tripod and take scenic views with a red-coloured Moon as part of your composition

Exposure times on ISO 400 film should be 1/250-1/4 second at f/8 for the regressing partial phases, and 1-4 seconds at f/4 for the total phases

But vary the settings, take many pictures and be prepared to dump most of them

And because there have been few volcanic eruptions recently and a consequent lack of dust in the atmosphere, this could be one of the brightest and most colourful lunar eclipses of recent years.

Cosmic clockwork

Eclipses have been described as an example of the "clockwork" of the Universe. They are regular and predictable. The last one visible in Britain was in January 2000.

On Tuesday, the Moon will enter the outer part of the Earth's shadow, the penumbra, at 1744 GMT. Then it will progressively darken until it reaches the Earth's inner shadow, the umbra, at 1842.

The dark curve that is the edge of the umbra will then move over the face of the Moon until it is completely within it by 1950. Mid-eclipse is at 20:20, and the Moon leaves the umbra at 2159. The eclipse will be all over by 2258.

There is no scientific significance in a lunar eclipse - not that this has stopped people trying to find some.

In 413 BC, a lunar eclipse affected the outcome of the Battle of Syracuse.

In the Peloponnesian War between Athens and Sparta, the Athenians had a major force near Syracuse in Sicily. Commander Nicias was in charge. The Greek biographer and essayist Plutarch takes up the story:

"The Athenian soldiers heard the news that the Syracusans had just received more men and supplies from the fickle Sicilians, who now were backing Syracuse again. Nicias realised that the situation was hopeless, and he gave orders to pack up and sail back to Athens. But just as the Athenians were about to hoist anchor, there was an eclipse of the Moon. Nicias called off the departure until the omens were more favourable."

It was a fatal delay for Nicias whose forces were defeated. He was captured and stoned to death in Syracuse. Years later, the Romans used a lunar eclipse to their advantage at Pydna in 168 BC, a battle crucial to their control of Greece.

Byzantines crushed

In April 1453, the Turks laid siege to Constantinople. Despite the heavy Turkish bombardment of the walls, the inhabitants of the city were able to repair the fortifications every night.

The Byzantines were exhausted but took solace in the old legend that Constantinople would never fall while the Moon was waxing.

Then, on the night of 22 May, the Moon rose in eclipse and their morale was crushed. Mohammed knew of the legend and waited a few days before starting a fresh attack.

During the battle a small gate was left open by accident but it was all the Turks needed. The sack of Constantinople lasted three days, as the Moon waned.