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A time ball or timeball is an obsolete time-signalling device. It consists of a large, painted wooden or metal ball that is dropped at a predetermined time, principally to enable navigators aboard ships offshore to verify the setting of their marine chronometers. Accurate timekeeping is essential to the determination of longitude at sea. Although the use of time balls has been replaced by electronic time signals, some time balls have remained operational as historical tourist attractions. ==History== Time ball stations set their clocks according to transit observations of the positions of the sun and stars. Originally they either had to be stationed at the observatory itself, or had to keep a very accurate clock at the station which was set manually to observatory time. Following the introduction of the electric telegraph around 1850, time balls could be located at a distance from their source of mean time and operated remotely. The first time ball was erected at Portsmouth, England, in 1829 by its inventor Robert Wauchope, a captain in the Royal Navy. Others followed in the major ports of the United Kingdom (including Liverpool) and around the maritime world.〔 One was installed in 1833 at the Greenwich Observatory by the Astronomer Royal, John Pond, and the time ball has dropped at 1 p.m. every day since then.〔(Greenwich Time Ball ) Retrieved 27 December 2010〕 Wauchope submitted his scheme to American and French ambassadors when they visited England.〔 The US Naval Observatory was established in Washington, D.C., and the first American time ball went into service in 1845.〔 Time balls were usually dropped at 1 p.m. (although in the United States they were dropped at noon). They were raised half way about 5 minutes earlier to alert the ships, then with 2–3 minutes to go they were raised the whole way. The time was recorded when the ball began descending, not when it reached the bottom.〔(【引用サイトリンク】title=Deal Timeball Tower: The Ball Drop )〕 With the commencement of radio time signals (in Britain from 1924), time balls gradually became obsolete and many were demolished in the 1920s. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「time ball」の詳細全文を読む スポンサード リンク
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