Time Zone

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Time zones are areas of the Earth that have adopted the same standard time, usually referred to as the local time. Formerly, people used local solar time, originally apparent and then mean solar time. Their difference is the equation of time. Mean solar time is the average over the course of a year of apparent solar time (sundial time). With the expansion of the railways and as telecommunications improved this became increasingly awkward because clocks in a given town would differ from those in any other by an amount corresponding to their difference in geographical longitude, which was generally not a convenient number. Forcing all localities to run their clocks in synchrony would solve this problem, but under such a scheme, in many places the time shown by clocks at sunrise and sunset would differ too markedly from the solar time values to which people are accustomed. As a compromise, a scheme was devised where the surface of the planet was divided into twenty four "time zones", each separated by 15° of longitude and offset by one hour from its neighbor. Under this scheme, local time is always close to mean solar time, while comparing the time in different places is a simple matter of adding or subtracting whole hours. However, the one hour separation is not universal and, as the map below shows, the shapes of time zones can be quite irregular because they usually follow the boundaries of states, countries or other administrative areas.