page_title: Royal Observatory, Greenwich
section_title: Greenwich Time Ball
hierarchical_section_title: Royal Observatory, Greenwich / History / Greenwich Time Ball
caption_attribution_description: Deutsch: Zeitkugel am Flamsteed House am Royal Greenwich Observatory, Greenwich, London, England, United Kingdom English: Time ball at Flamsteed House at Royal Observatory, Greenwich, London, England, United Kingdom
context_page_description: The Royal Observatory, Greenwich is an observatory situated on a hill in Greenwich Park, overlooking the River Thames. It played a major role in the history of astronomy and navigation, and because the prime meridian passes through it, it gave its name to Greenwich Mean Time. The ROG has the IAU observatory code of 000, the first in the list. ROG, the National Maritime Museum, the Queen's House and Cutty Sark are collectively designated Royal Museums Greenwich.
  The observatory was commissioned in 1675 by King Charles II, with the foundation stone being laid on 10 August. The site was chosen by Sir Christopher Wren. At that time the king also created the position of Astronomer Royal, to serve as the director of the observatory and to "apply himself with the most exact care and diligence to the rectifying of the tables of the motions of the heavens, and the places of the fixed stars, so as to find out the so much desired longitude of places for the perfecting of the art of navigation." He appointed John Flamsteed as the first Astronomer Royal. The building was completed in the summer of 1676. The building was often called "Flamsteed House", in reference to its first occupant.
context_section_description: The red time ball of Greenwich was established in 1833, and is noted as a public time signal. The time ball in modern times is normally in a lowered position, then starting at 12:55 pm, the ball begins to rise, then at 12:58 it reaches the top; at 1 pm the ball drops.
  To help mariners at the port and others in line of sight of the observatory to synchronise their clocks to GMT, Astronomer Royal John Pond installed a very visible time ball that drops precisely at 1 pm (13:00) every day atop the observatory in 1833. Initially it was dropped by an operator; from 1852 it was released automatically via an electric impulse from the Shepherd Master Clock. The ball is still dropped daily at 13:00 (GMT in winter, BST in summer).
  The original time ball system was built by Messrs Maudslay and Field, and cost £180. The five-foot diameter ball was made of wood and leather. In the original ball system, it was hoisted by a rope up from the Octagon room, and there was catch at the top to hold it. This could then be triggered by hand, while observing the time on an astronomical month clock, that was regulated to the mean solar time.
  By dropping the ball, the public, mariners, and clock makers could then get a time signal by viewing it from afar. The ball drop would be repeated at 2 pm also if possible.
  The reason why 12 noon was not chosen was because astronomers at the observatory would record when the Sun crossed the meridian at that time on that day.
  In rare occasions where the ball could get stuck due to icing or snow, and if the wind was too high it would not be dropped. In 1852, it was established to distribute a time signal by the telegraph wires also.
  The time ball was extremely popular with the public, chronometers, railways, mariners, and there was a petition to have another time ball established in Southampton also.
caption: The time ball is the red ball on a post – when it drops a certain time is signalled. This allowed clocks to be set from afar with great accuracy, particularly the chronometers of ships on the River Thames below, prior to sailing. The observatory would first determine the time by stellar observations.
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