Britain will witness a spectacular and rare celestial spectacle this week. At dusk on Friday, the full moon will rise and reveal itself coloured a deep red. The nation will then experience a blood moon or, as astronomers term it, a total lunar eclipse.

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And this week promises to be a special one, for it will be the longest-lasting total lunar eclipse of the 21st century. After it rises in the south-east – at around 8.50pm in London – the moon’s eclipse will continue until early on Saturday. “Weather permitting, it should give Friday evening a special, exciting edge,” said Sheila Kanani of the Royal Astronomical Society.

Blood moons have only recently been welcomed on Earth. Their deep red colour has usually been seen as an omen of terrible events. The Book of Joel in the Hebrew Bible warns that “the sun will turn into darkness, and the moon into blood, before the great and terrible day of the Lord comes.”

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Today scientists have a more prosaic explanation for the moon’s crimson transformation. It is caused when the moon passes through the shadow of the Earth. However, its disc does not go completely dark because some sunlight – mainly the longer-wavelength, redder end of the spectrum – passes through our atmosphere and is bent around the edge of our planet so that it falls on to the moon’s surface. In effect, it is the light of sunrise and sunset on the Earth that will give the moon its red glow on Friday.

Unlike total solar eclipses, which occur when the moon’s disc passes in front of the sun and completely blots out sunlight for only a few minutes, a blood moon is a fairly leisurely affair. “It will last several hours – when you get a real feeling of the Earth and moon shifting in space,” said astronomer Tom Kerss, of the Royal Observatory Greenwich, which plans to stream live pictures of the event on Friday. “You get a true sense of the solar system moving – and that in itself is a really dramatic experience.”

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For good measure, there is no need to wear goggles or filters to watch a blood moon as is necessary with solar eclipses. “It is safe to watch with the naked eye,” said Kerss. “You could use a telescope but, to be frank, it will be just as dramatic to watch it without aids as the red moon slowly rises in the sky over Britain and the shadow of the Earth passes from its surface.”