The total lunar eclipse will shroud the full moon in a deep red glow on the night of Friday, July 27.
During the eclipse the moon is expected to clock in for one hour and 43 minutes – the single longest total eclipse for at least another 82 years.
Royal Observatory Greenwich Astronomer Dhara Patel told Express.co.uk there are a number of explanations for why this happens.
The space expert said: “There are actually multiple reasons why this happens. When we think about the shadow behind the Earth caused by the sun’s light, that shadow is what the moon passes through.
“We can imagine it to be like a circular region in the sky and sometimes when the moon doesn’t pass directly through the central part, it will move towards the edges of that circular shadow.
“This time the moon is passing through the central part of that shadow rather than skimming it at the bottom or the top.
“So that’s one of there reasons why the eclipse will be longer and another one is the lunar apogee.”
The moon’s orbit around the planet is pretty irregular and more elliptic than perfectly round.
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Because of this irregular orbit the moon’s average distance from Earth lengthens and shortens depending on its potion in the sky.
During the July eclipse, the moon will be at its farthest position from the Earth – the so-called lunar apogee.
Ms Patel said: “So what we’ll see is an apparently smaller moon and because it’s further from Earth it will be travelling a bit slower.
“Those two things combined mean we will see a longer eclipse since it takes longer for the moon to pass the Earth’s shadow.
“One final thing is that the Earth’s orbit around the sun isn’t perfect either, it’s an elliptic orbit, and during the summer the Earth is at it’s farthest orbit from the sun – it’s aphelion.
“During this time, the shadow behind the Earth is actually a little bit longer and wider due to the angles between the sun and the Earth when the Earth is a bit further.
“This means the moon will take a bit longer to the pass through the shadow as well.”
In the UK, the initial phase of the lunar eclipse will have already begun by the time the moon peaks over the horizon.
This means the moon should already look red or just about start to turn red at moonrise.
The partial eclipse of the Blood Moon will begin below the horizon at around 7.24pm BST, followed by the so-called total eclipse phase between 8.30pm and 9pm BST.
Eclipse 2018: The Blood Moon turn red because of scattered sunlight in the atmosphere
The Blood Moon will hit maximum eclipse, or greatest eclipse, at 9.20pm.
This time the moon is passing through the central part of Earth’s shadow
But why exactly will the full moon turn blood-red during the total lunar eclipse?
Ms Patel explained: “The moon orbits around the Earth and when the moon passes directly behind the Earth in the planet’s shadow, on the opposite side of the Earth compared to the sun, we see a lunar eclipse.
“Normally when the moon is in that position you would expect no sunlight to be reaching it, so you would expect it to be completely dark.
“The reason why it turns that reddish hue is because the Earth has an atmosphere and when the sun’s light reaches the atmosphere, gases in the air actually scatter or bend the light, and it’s red light that is scattered by just the right amount that it ends up being directed onto the surface of the moon.
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“So during a lunar eclipse when the moon is in an Earth’s shadow the atmosphere causes the red light to be bent towards the moon and that is why we see that reddish colour and people call it a Blood Moon.”
The Blood Moon eclipse will be visible from the whole of the UK from moonrise until about 10.13pm BST when total eclipse ends.
Moonrise times all differ by a few minutes from location to location so check your local times ahead of the eclipse.
By about midnight the full moon should return to its normal white glow high up above the horizon.