Greenwich Park

This guide to Greenwich Park in London starts with the Royal Observatory, built in 1675 on the orders of Charles II to solve a problem that was killing sailors: finding longitude at sea. It follows John Flamsteed's decades of charting the sky and clockmaker John Harrison's race to build a chronometer accurate enough for a rolling ship. You'll also hear how Astronomer Royal John Pond found a simpler fix in 1833, rigging a red ball above the observatory to signal the exact time to ships on the Thames. Look up at the mast as you climb the hill; the ball still drops at one o'clock.