Supermoon: why is the moon bigger?

At 15:35 (GMT) today, Saturday 5 May, the moon will appear bigger than usual in our skies. Much bigger, in fact, providing the sky is clear of course. Depending on atmospheric conditions it will appear as much as 14 per cent bigger and 16 per cent brighter in our sky. And because of this, it will have as much as 42 per cent more effect on the sea, so we can expect some big tides. But why is this happening?

Incredibly, it’s because the moon will be at its closest approach to the Earth in the cycle of its orbit. This perigree means it will be 356,955 kilometres (221,802 miles) from the Earth, as opposed to its apogee when it’s around 405,500 kilometres (252,000 miles) away, which will happen on 28 November this year.

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The moon’s proximity to the Earth changes because its orbit isn’t a perfect circle. It moves around the Earth in an ellipse and, what’s more, the Earth is nearer one end of the moon’s elliptical orbit than the other by around 50,000 kilometres (30,000 miles). When the moon’s perigree coincides with a full moon, it’s considered to be a ‘supermoon’.