This is a fascinating and very readable account of the revolutions in timekeeping, and how the work of Louis Essen overlapped the technical advances in the Second World War leading up to the first atomic clock dedicated to timekeeping. In fact an underlying theme is navigation from the early days of the Greenwich Observatory to where atomic clocks are instrumental in determining our position on the earth to within centimetres. The book has 24 chapters divided into four distinct themes, ranging from early radio to the challenges of linking astronomy with an atomic time scale. But first there is a 'refresher' starting with Charles Ⅱ and the foundation of the Observatory in 1675. Over a hundred years later, John Harrison, a clockmaker who dedicated much of his life to developing a watch that could keep time sufficiently accurately to improve navigation at sea. This was by keeping Greenwich time to set against local noon and thus determine longitude. It is equally well known that Harrison received a huge financial reward. Not to be omitted from the story are the essential Observatory almanacs and sextants. British makers, with some design changes, produced the successful box chronometer so familiar to navigation, a design that endured for over a hundred years.
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