What is the 'blood moon' and why will it be the longest eclipse of the century?

Britons will be able to view rare sight on 27 July, sometime between sunset and midnight

On the evening of 27 July and the early hours of 28 July, people across the world’s Eastern Hemisphere will be treated to a rare "blood moon".

It will also be the longest total lunar eclipse of this century.

Here's what you need to know about the phenomenon.

What is the blood moon?

A blood moon is the definition used when the moon appears bright, large and reddish in colour, and it only occurs when there is a full moon.

Why does the blood moon happen?

The blood moon is the result of a total lunar eclipse, which is when the moon passes directly behind earth and into its shadow.

For a lunar eclipse to happen, the sun, earth and moon have to be completely aligned or very closely so, with the planet in between the two.

Why does the moon look red?

The moon appears red because of a phenomenon known as ‘Rayleigh scattering’.

Before the sunlight reaches the moon, it is filtered through the earth’s atmosphere, which filters out certain colours.

All the elements of Earth’s atmosphere including pollution, dust from storms and volcanic eruptions can affect the shade of red the Moon takes on during the eclipse.

Dust blocks out higher frequency blue light waves, but the longer wavelength of red light comes through.

During the eclipse, if you were to stand on the moon and look back at the sun, you would see the planet as a large black disk blocking the entirety of the star, with a ring of light around the edges. This glowing ring is the light which falls on the moon during a total eclipse.

Why will it last so long?

Because the moon will pass directly into the darkest region of the earth’s shadow.

It will happen at the same time as the moon’s apogee. This is when the moon is the furthest away from the earth.

It will also be the smallest full moon of the year. This means it will take more time to pass through the dark shadow of the earth, making the eclipse last longer.

When will it be visible?

Europe and Africa will be able to see the eclipse sometime between sunset and midnight on 27 July, whilst most of Asia, Indonesia and Australia will view the eclipse sometime between midnight and sunrise on 28 July.

Sadly, the eclipse will not be visible at any time for those in America.

How long will you be able to see it for?

People in the UK will be able to see the blood moon on 27 July for an estimated average of 103 minutes as the moon moves through the earth’s shadow.

Experts at EarthSky predict the total lunar eclipse will begin at 19:30 and end at 21:13.

Where is the best place to see it?

Those in the south of England will be able to see the total eclipse for the longest period of time, an estimated 1 hour and 23 minutes, whilst those in the Scotland will see it for an average of 40 minutes.