Уақытты анықтау сигналдары

Signal used as a reference to determine the time of day

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 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.

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.

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, South Africa, 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, England, at the Castle in Edinburgh, Scotland, and also at Perth in Australia 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, Chile, 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, Afghanistan.

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.

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.

The telegraphic distribution of time signals was made obsolete by the use of AM, FM, shortwave radio, Internet Network Time Protocol servers as well as atomic clocks in satellite navigation systems. Time signals have been transmitted by radio since 1905. There are dedicated radio time signal stations around the world. Time stations operating in the longwave radio band have highly predictable radio propagation characteristics, which gives low uncertainty in the received time signals. Stations operating in the shortwave band can cover wider areas with relatively low power transmitters, but the varying distance that the signal travels increases the uncertainty of the time signal on a scale of milliseconds. Radio time signal stations broadcast the time in both audible and machine readable time code form that can be used as references for radio clocks and radio controlled watches. Typically, they use a national or regional longwave digital signal; for example, station WWVB in the U. S. to delay their analog broadcast by about eight seconds, so it remains in sync with the digital stream. Thus, network generated time signals and service cues will also be delayed by about eight seconds. (Because of the delay, when WBEN AM in Buffalo, New York was broadcasting time markers, and was simulcast on an FM station that broadcast in HD; the FM signal did not carry the time signal. WBEN does not broadcast in HD.) Local signals may also be delayed. The all news radio stations of the CBS Radio Network, of which WCBS is the flagship, air a "bong" (at a frequency of 440 Hz – the international standard for the musical note ) that immediately precedes each top of the hour network newscast. (The same bong could be heard on the CBS Television Network, at the top of the hour immediately before the beginning of any televised program, in the 1960s and 1970s.) An automated "chirp" at one second before the hour signals a switch to the radio network broadcast. 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. KYW AM in Philadelphia broadcasts a time signal at the top of the hour along with its jingle. Bonneville International owned news/talk station KSL (AM FM) in Salt Lake City uses a "clang" that originates from the Nauvoo Bell on Temple Square, in Salt Lake City, which has been a staple on the station since the early 1960s. In Canada, the national English language non commercial CBC Radio One network broadcast the daily National Research Council Time Signal from 5 November 1939 until 9 October 2023. The simulcast would occur daily at 1pm Eastern Time. Its French language counterpart, Radio Canada, broadcasts a similar signal at noon. Vancouver radio station CKNW also broadcasts time signals, using a chime every half hour. Time signals on CBC broadcasts may be delayed up to 3 seconds due to network processing delays between the local radio transmitter and the time signal origin in Ottawa. The CBC's predecessor, the Canadian National Railways Radio network, broadcast the time signal over its Ottawa station, CNRO (originally CKCH), at 9 pm daily and also on its Moncton station, CNRA, beginning in 1923. CNRA closed in 1931 but the broadcasts continued on CNRO when the station was acquired by the Canadian Radio Broadcasting Commission in 1933 and by the CBC in 1936 before going national in 1939. 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. However, the VNG dedicated time signal service has been discontinued. In Cuba, Radio Reloj is a radio station which has a time signal over news. Radio Reloj translates to Clock Radio.

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.