The Royal Observatory, Greenwich, London, mid 19th century.
© Science Museum / Science & Society Picture Library
Photograph showing a view acros the courtyard towards Flamsteed House at the Royal Observatory, Greenwich in the mid nineteenth century. This is oldest part of the site and forms the nucleus of the buildings erected in 1675 when the observatory was founded. The view clearly shows the time ball on Flamsteed House, a device that was used to help sailors reset their chronometers from ships in the nearby London docks. Five minutes in advance the ball was raised to the top of its pole and was dropped at the exact moment of 1 pm. The practice is still continued to this day despite the observatory now forming part of the National Maritime Museum.