A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day. Church bells or voices announcing hours of prayer gave way to automatically operated chimes on public clocks; however, audible signals (even signal guns) have limited range. 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. Railways were among the first customers for electrical time signals, which allowed synchronization of their operations over wide geographic areas. Dedicated radio time signal stations transmit a signal that allows automatic synchronization of clocks, and commercial broadcasters still include time signals in their programming. Today, global navigation satellite systems (GNSS) radio signals are used to precisely distribute time signals over much of the world. There are many commercially available radio controlled clocks available to accurately indicate the local time, both for business and residential use. Computers often set their time from an Internet atomic clock source. Where this is not available, a locally connected GNSS receiver can precisely set the time using one of several software applications.
Audible and visible time signals One sort of public time signal is a striking clock. These clocks are only as good as the clockwork that activates them, but they have improved substantially since the first clocks from the 14th century. Until modern times, a public clock such as Big Ben was the only time standard the general public needed. Accurate knowledge of time of day is essential for navigation, and ships carried the most accurate marine chronometers available, although they did not keep perfect time. A number of accurate audible or visible time signals were established in many seaport cities to enable navigators to set their chronometers.
Signal guns
In Vancouver, British Columbia, a "9 O'Clock Gun" is still shot every night at 9 pm. (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.) The 9:00 pm firing was later established as a time signal for the general population. 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 the same manner, a Noon Gun has been fired in Cape Town, since 1806. The gun is fired daily from the Lion Battery at Signal Hill. The Noonday Gun serves a similar purpose in Hong Kong. The tradition, which started in the 1860s under British colonial rule, has become a tourist attraction in recent times. A cannon was fired at one o'clock every weekday at Liverpool (1867-1969), at the Castle in Edinburgh, and also at Perth to establish the time. The Edinburgh "One O'Clock Gun" is still in operation. A cannon located at the top of Santa Lucia Hill, in Santiago, is shot every noon. 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. This was introduced in 1847 by Pope Pius IX to synchronise all the church bells of Rome. It was situated in Castel Sant'Angelo until 1903 when it was moved to Monte Mario for a few months until it was placed in its current position. The cannon was silenced from the start of WWII for about twenty years until 21 April 1959, the 2712th anniversary of Rome's founding, and has been in use since then. For many years an old cannon was fired "about noon" from a mountain near Kabul.
Sirens, whistles, and other audible signals In many Midwestern US cities where tornadoes are a common hazard, the emergency sirens are tested regularly at a specified time (say, noon each Saturday); while not primarily intended to mark the time, local people often check their watches when they hear this signal. In many non-seafaring communities, loud factory whistles served as public time signals before radio made them obsolete. Sometimes, the tradition of a factory whistle becomes so deeply entrenched in a community that the whistle is maintained long after its original function as a time keeper became obsolete. For example, the University of Iowa's power plant whistle has been reinstated several times by popular demand after numerous attempts to silence it.
Visual signals
In 1861 and 1862, the Edinburgh Post Office Directory published time gun maps relating the number of seconds required for the report of the time gun to reach various locations in the city. Because light travels much faster than sound, visible signals enabled greater precision than audible ones, although audible signals could operate better under conditions of reduced visibility. The first time ball was erected at Portsmouth, England in 1829 by its inventor Robert Wauchope. 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. The first American time ball went into service in 1845. In New York City, the ceremonial Times Square Ball drop on New Year's Eve in Times Square is a vestige of a visual time signal.
Electrical time signals
United Kingdom The first telegraph distribution of time signal in the United Kingdom, indeed, in the world, was initiated in 1852 by the Electric Telegraph Company in collaboration with the Astronomer Royal. Greenwich Mean Time was distributed by telegraph from the Greenwich Observatory. This included a system for synchronising the drop of the time ball at Greenwich with other time balls around the country, one of which was atop the Electric's offices in the Strand. 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.
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