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On Dec. 31, 2016, official clocks around the world will add a leap second just before midnight Coordinated Universal Time — which corresponds to 6:59:59 p.m. EST. NASA missions will also have to make the switch, including the Solar Dynamics Observatory, or SDO, which watches the sun 24/7. Clocks do this to keep in sync with Earth's rotation, which gradually slows down over time. When the dinosaurs roamed Earth, for example, our globe took only 23 hours to make a complete rotation. In space, millisecond accuracy is crucial to understanding how satellites orbit. "SDO moves about 1.9 miles…
