Time-measuring instruments have marked significant epochs in history for millennia. Six thousand years ago, the shadows cast by obelisks in the Sumerian city of Ur revealed the passage of the sun; two thousand years ago, astronomers in ancient Greece used the Antikythera mechanism to calculate the positions of the sun, moon and planets in the heavens; and in the 18th century, John Harrison’s H4 clock enabled safe and accurate navigation by sea.