Find this orientation by looking through the crystal at the sky at a time when you can see the sun, mark the sun's direction on the crystal, and your mark will always point towards the sun when you match the beams from even a tiny patch of blue in an otherwise overcast sky.

ECONOMIST: How Norsemen found their way round in cloudy weather

To mark the switchover the Crystal Palace transmitter, in south London, will be lit up on Wednesday night with 200, 000 watts of energy efficient lighting at a ceremony attended by Sir David Attenborough and DJ Chris Evans.

BBC: London analogue TV signal switched off

But if anybody claims to have a crystal ball out there, that person is lying, says Mark Burnett, better known as the creator of Survivor.

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