Asked by: Bilge Sutcliffe, Newquay

The appearance of a huge, perfectly circular halo round the sun in Mexico earlier this year sparked rumours of the impending end of the world. In reality, it was the product of sunlight being refracted through hexagonal ice crystals high in the atmosphere. Water droplets can do the same, producing a smaller but more colourful halo.

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Robert is a science writer and visiting professor of science at Aston University.