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There are 100 days to go until the next eclipse… On January 31st 2018 a total lunar eclipse will produce a red full moon for people in parts of Asia, Australia and the Pacific, and North America.

Image: Juan Carlos Casado

Why does the moon turn red? The earth – a rocky planet about 13,000 km across – moves between the sun and the moon. This blocks the sun’s rays from reaching the moon directly.

But the earth has an atmosphere – a layer of air about 100 km thick. The atmosphere bends some of the sun’s rays around the earth (this is called refraction; it’s why a straw looks bent when you see it in a glass of water). The atmosphere also filters out the blue colour from the sun’s rays (this is called scattering; it’s why the sky looks blue).

So the sun’s rays that eventually reach the moon – after having travelled around the earth, through the atmosphere – are red. This is what gives the moon its awe-inspiring change of colour.

If you miss the red moon in January – because of weather, geography or some other reason – you might get another chance six months later. On July 27th/28th 2018 there will be a total lunar eclipse across Africa, Europe, Asia and Australia. And six months after that – on January 20th/21st 2019 – there will be another total lunar eclipse across North America, South America, Europe and parts of Africa.

• Find eclipses in your city on timeanddate.com

Thank you to the brilliant Science Learning Hub for the pages on refraction and scattering.