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On December 7, 1972, the Apollo 17 astronauts took a photograph of a gibbous Earth at a distance of eighteen thousand miles from its surface. The photograph showed the planet, partially obscured by swirling clouds, floating all alone like a blue glass marble in the vast, black infinity of outer space. This historic image, dubbed the Blue Marble, became a symbol of the environmental movement and caused a profound shift in the way people conceived of the planet, shrinking it from something incomprehensibly immense and awesome into a fragile, lonely orb that you could cradle in the palm of your hand or crush beneath a careless heel.
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