One of Guinness World Records’ more unusual awards was presented at the National Maritime Museum yesterday. After a 100-day trial, the timepiece known as Clock B – which had been sealed in a clear plastic box to prevent tampering – was officially declared, by Guinness, to be the world’s “most accurate mechanical clock with a pendulum swinging in free air”. It was an intriguing enough award. But what is really astonishing is that the clock was designed more than 250 years ago by a man who was derided at the time for “an incoherence and absurdity that was little short of the symptoms of insanity”, and whose plans for the clock lay ignored for two centuries. The derision was poured on John Harrison, the British clockmaker whose marine chronometers had revolutionized seafaring in the 18th century (and who was the subject of Longitude by Dava Sobel). His subsequent claim – that he would go on to make a pendulum timepiece that was accurate to within a second over a 100-day period – triggered widespread ridicule. The task was simply impossible, it was declared. But now the last laugh lies with Harrison. At a conference, Harrison Decoded: Towards a Perfect Pendulum Clock, held at Greenwich yesterday, observatory scientists revealed that a clock that had been built to the clockmaker’s exact specifications had run for 100 days during official tests and had lost only five-eighths of a second in that period.