Time signal
Busy seaports used a visual signal, the dropping of a ball, to allow mariners to check the chronometers used for navigation.
The advent of electrical telegraphs allowed widespread and precise distribution of time signals from central observatories.
A number of accurate audible or visible time signals were established in many seaport cities to enable navigators to set their chronometers.
(This gun was brought to Stanley Park in 1894 by the Department of Fisheries originally to warn fishermen of the 6:00 pm Sunday closing of fishing.)
Until a time gun was installed, the nearby Brockton Point lighthouse keeper detonated a stick of dynamite.
Elsewhere in Canada, a "Noon Gun" is fired daily from the citadels in Halifax and Quebec City and from Signal Hill in St. John's, Newfoundland and Labrador.
In Rome, on the Janiculum, a hill west of the Tiber since 1904 a cannon is fired daily at noon towards the river as a time signal.
In many non-seafaring communities, loud factory whistles served as public time signals before radio made them obsolete.
[5] For example, the University of Iowa's power plant whistle has been reinstated several times by popular demand after numerous attempts to silence it.
[7] One was installed in 1833 on the roof of the Royal Observatory in Greenwich, London, and the time ball has dropped at 1:00 pm every day since then.
[9] Other synchronised time balls were atop the Nelson Monument, Edinburgh; the sailors' home Broomielaw, Glasgow; Liverpool and one at Deal, Kent, installed by the Admiralty.
[9] Telegraph signals were used regularly for time coordination by the United States Naval Observatory starting in 1865.
However, when mobile phones are used, the delays are often more than 100 ms, due to the multiple access methods used to share cell channels.
As radio receivers became more widely available, broadcasters included time information in the form of voice announcements or automated tones to accurately indicate the hour.
[12] In the United States many information-based radio stations (full-service, all-news and news/talk) also broadcast time signals at the beginning of the hour.
As an example, KNX, the CBS Radio Network all-news station in Los Angeles, broadcasts this "bong" sound on the hour.
However, due to buffering of the digital broadcast on some computers, this signal may be delayed as much as 20 seconds from the actual start of the hour (this is presumably the same situation for all CBS Radio stations, as each station's digital stream is produced and distributed in a similar manner), though unlike program content which is on a broadcast delay for content concerns, the time signal airs as-is over-the-air, meaning it can sometimes be talked over during a live news event or sports play-by-play.
[17] In Australia, many information-based radio stations broadcast time signals at the beginning of the hour, and a speaking clock service was also available until October 2019.
Program material, including time signals, that is transmitted digitally (e.g. DAB, Internet radio) can be delayed by tens of seconds due to buffering and error correction, making time signals received on a digital radio unreliable when accuracy is needed.
