A NASA image of the moon in shadow. It is a yellow-orange colour - hence the name blood moon.

A lunar eclipse was photographed from the Johnson Space Center in Houston, Texas, during the early morning hours of 31 January 2018. Image: NASA/Robert Markowitz.

The blood Moon: a guide to the lunar eclipse

A lunar eclipse happens when the Moon's orbit takes it into the shadow of the Earth.

What's a blood Moon?

Lunar eclipses also turn the Moon red, a phenomenon which is called a 'blood Moon'.

This happens because the Sun, Earth and Moon are in perfect alignment during a lunar eclipse.

The Earth is in the middle and blocks out most of the sunlight, but a little light will reach the Moon from around the edges of the Earth.

As this light passes through Earth's atmosphere it will tend to refract, or bend, slightly. Blue light bends more than red light, so the blue light is more scattered.

More red light tends to make it to the Moon, and is then reflected off the lunar surface to come to Earth. So to us, the Moon appears red.

The video below records a bright full Moon over North America turned a lovely shade of celestial red during a total lunar eclipse. It was recorded on 4 April 2015.

When does the next lunar eclipse - or blood Moon - start?

The next total lunar eclipse that will be visible from the Northern hemisphere will dazzle viewers on the evening of 21 January 2019.

How can you watch the total lunar eclipse?

It is safe to watch the eclipse without any eye protection or special equipment. For the best view, find a dark spot away from light pollution.

It'll be easier if you're in an open space, away from tall buildings, with a clear line of sight upwards.

When you live in a city, the trick is to find the biggest bit of sky you can, or get up as high as you can above the rooftops. A loft window, a rooftop, the windows of upper-storey flats or a local park (especially on the outskirts of a city) can work well as vantage points.

Ashley King, a planetary scientist based at the Museum, says, 'It's not easy to observe space from cities. The light pollution is usually too much. However, there are a few spots you can go to. The general advice is get as high above buildings as you can and minimise the light in your vicinity.'

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