Time ball
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A time ball is a large sphere, visible to shipping that drops at a set time to allow the ships to set their Chronometer, so that they are able to find their longitude.
Time Balls were used by shipping before the days of radio and in particular the timing pips and announcements that made time balls obsolete.
Time ball stations used to set their clocks from observatories, who would be tracking the position of the sun and stars to work out the time. At first they either had to be stationed at the observatory itself, or they had to keep a very accurate clock at the station itself, and set it against ether another clock set at an observatory, or in latter years from signals carried by the telegraph system.
Time balls are normally dropped at 1.00 pm (although in the USA they were dropped at 12.00pm). They were raised halfway about 5 minutes earlier to alert the ships, then with 2-3 minutes to go they were raised the whole way, and on the hour exactly they were dropped. The time ball was not dropped at noon (in most places), as observatories (where they tended to be placed) were themselves too busy taking readings from the sun at this time.
The first time ball was put up in Manchester, at their observatory. Others followed across the major ports in the UK and across the maritime world. One was installed in 1833 at the Greenwich Observatory, and has dropped at 1 PM every day since then. They gradually became obsolete and many were demolished in the 1920's.
Today
Today in the world there are over 60 time balls left standing. These include the time balls at the Greenwich Observatory, Edinburgh, Point Gellibrand, USA, Deal, Kent and others. The modern version of a time ball is dropped at the stroke of midnight on December 31, notably in Times Square, New York City.
See also
References
Time Ball Tower Museum (http://dealtimeball.tripod.com/)