Blood moon: what it is and where to see it

One of astronomy’s most spectacular sights will appear in Britain at sunset tonight when the longest blood moon of this century will combine with unusually bright sightings of Mars.

The unusual alignment will have “astronomers and amateur stargazers alike looking to the heavens for a show unlikely to be matched for many years”, says The Times.

The main event – the longest lunar eclipse for at least another 82 years – happens when the Moon passes through the Earth’s shadow, glowing red in refracted light.

Subscribe to The Week

Escape your echo chamber. Get the facts behind the news, plus analysis from multiple perspectives.

SUBSCRIBE & SAVE

Sign up for The Week's Free Newsletters

From our morning news briefing to a weekly Good News Newsletter, get the best of The Week delivered directly to your inbox.

From our morning news briefing to a weekly Good News Newsletter, get the best of The Week delivered directly to your inbox.

Sign up

“Weather permitting, it should give the evening a special, exciting edge,” says Sheila Kanani of the Royal Astronomical Society.

“The two red objects in the sky will be a spectacular combination,” says Professor Andrew Coates of University College London’s Mullard Space Science Laboratory.

The eclipse is predicted to last a record-breaking one hour and 43 minutes, says The Daily Telegraph. The paper adds that UK observers will be “slightly late to the party, as the moon will already be in the Earth’s shadow when it appears at around 9pm”.

As if this wasn’t enough, “the night-time spectacle will be topped off by the presence of the International Space Station as it orbits the Earth”, the Telegraph adds.

So what exactly is a blood moon?

A total lunar eclipse happens when the Earth passes between the Sun and the Moon, casting a shadow on the latter.

During totality - the point at which the Moon is entirely in the Earth’s shadow - the only light reaching the Moon is being refracted through the Earth’s atmosphere, and is subjected to a physical phenomenon known as Rayleigh scattering.

Rayleigh scattering, named after British physicist Lord Rayleigh, dictates that smaller-wavelength colours in the electromagnetic spectrum (such as shades of blue, purple and green) are scattered by the edges of Earth’s atmosphere, while longer-wavelength colours (shades of red, orange and yellow) are able to refract or bend around our planet and continue onto the surface of the Moon.

Earth’s atmospheric conditions during the eclipse (particularly dust, humidity and temperature) will then “decide what shade of colour, in the spectrum of copper brown to deep red, the Moon dons”, says news site Tech2.

The Guardian says that some experts are “unimpressed by the biblical moniker for this evening’s event”.

“People keep referring to it as a ‘blood moon’. It’s a lunar eclipse,” Helen Klus of the Royal Astronomical Society told the paper. “It’s something that people have heard of before,” she said.

What time will we be able to see it?

The moon will rise this evening at 8.49pm in London and 9.22pm in Edinburgh. The full eclipse is expected to finish at 10.13pm, from which time a partial eclipse will last until midnight.

Mars will appear about two degrees above the horizon, on the opposite side of the Earth to the sun, “making it appear redder than usual”, says the Times.

The prime time to see the moon’s redness will be immediately after it rises, according to the Royal Astronomical Society. It’s best to position yourself in an area with “low-light pollution, like the countryside, or a high vantage point in built-up areas”, the group says.

There’s 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,” astronomer Tom Kerss of the Royal Observatory Greenwich told The Guardian. “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.”

“This is actually almost as long as a lunar eclipse could be,” Professor Tim O’Brien, an astrophysicist at the University of Manchester, explained.

If you do miss this one, you won’t have to wait too long for another opportunity: the next total lunar eclipse will take place on 21 January 2019 and will be visible across Western Europe and North Africa.