Coming soon to a sky near you: a teachable moment

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.

Total lunar eclipse (Juan Carlos Casado)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.


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


 

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