A Guinness World Record has just been awarded to Harrison's Clock B, 250 years after it was designed. It is the world’s “most accurate mechanical clock with a pendulum swinging in free air”.

The clock was built to Harrison's specification and lost lost only five-eighths of a second in 100 days. The tests were carried out at The Royal Observatory in Greenwich, and were overseen by the Worshipful Company of Clockmakers and the National Physical Laboratory.

Harrison had said it would be accurate to 1 second, and they said he was mad.