After 240 years – and 100 days – a clock at the Royal Observatory Greenwich has proved the predicted accuracy of pendulum clocks, and scooped a Guinness World Record in the process.

In a unique historical project, a team of horologists set out to test the theories of John Harrison, the genius clockmaker who solved the longitude problem in the 18th century with a series of ground-breaking chronometers H1-5, all held at the Observatory.

In 1775 towards the end of his life Harrison had made the claim that his own precision pendulum clocks could keep time to within a second in 100 days. The claim was never taken seriously.

One reviewer wrote of the book: “Indeed we are sorry to say that every page bears the marks of incoherence and absurdity, little short of the symptoms of insanity.”

Two centuries after the claim, in 1975, Martin Burgess with the aid of a group that became known as the Harrison Research Group attempted to put these theories to the test. The result was what has become known as the Burgess “Clock B” based on Harrison’s designs and concept.

Burgess "Clock B", in its case, is tested for accuracy

Royal Observatory horology specialist Jonathan Betts said: “As soon as we set the clock running it was clear that it was performing incredibly well, so then we got the case sealed because nobody was going to believe how well the clock was running.”

After over three decades of work, the clock was finally finished and trialled at the Observatory.

The clock was set running in March 2014, in its Perspex case, which was made tamper-proof by the application of wax seals, overseen by the Worshipful Company of Clockmakers and the National Physical Laboratory. The official 100-day trial started on 6 January and ended last week, coinciding with the Harrison Decoded conference.

The Royal Observatory Greenwich was presented with a Guinness World Records for the “most accurate mechanical clock with a pendulum swinging in free air” to mark the success of the experiment.

Curator of horology Rory McEvoy said the design was “something approaching the perfect clock” and added: “The results of this experiment are truly remarkable and add a new chapter to the history of precision timekeeping.”

The test proved successful, earning a place in the Guinness Book of Records