Before the invention of radio and satellites, accurate timekeeping on steam and sailing ships was a challenge, but essential for marine navigation. Ship navigators therefore used highly accurate timepieces called marine chronometers, but these needed checking before leaving port. The task of maintaining a reference time signal fell to observatories such as those at Greenwich in London, on Calton Hill in Edinburgh and at the Port of Liverpool. This was done through the use of highly sophisticated clocks – too large and expensive for ship use – and astronomical observations. Calibrating Continue Reading