Time Ball - Royal Observatory, Greenwich (London) | place with historical importance

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Royal Observatory, Greenwich.
Time ball atop the Octagon Room.

A time ball is a large painted wooden or metal ball that drops at a predetermined time, principally to enable sailors to check their marine chronometers. Accurate timekeeping is one way of enabling mariners to determine their longitude at sea.

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. Through the use of the electric telegraph (from around 1850), time balls could be located at a distance from their source of Mean Time and operated remotely.

Time balls are usually dropped at 1pm (although in the USA 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. The time ball was not usually dropped at noon as the observatories would be too busy taking readings.

The first time ball was erected at Portsmouth in 1829 by its inventor Robert Wauchope, a Captain in the Royal Navy. Others followed in the major ports of the UK (including Liverpool) and around the maritime world. One was installed in 1833 at the Greenwich Observatory by Astronomer Royal John Pond, and has dropped at 1pm every day since then.

With the commencement of radio time signals (in Britain from 1924), time-balls gradually became obsolete and many were demolished in the 1920s.