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Before clocks were invented, people kept time using different instruments to observe the Sun’s meridian passing at noon.The earliest time measuring devices we know of are sundials and water clocks.
Many nations today use standard time zones, but some places use 30 or 45 minute deviations from standard time.
GMT was the universal reference standard – all other times being stated as so many hours ahead or behind it – but the French continued to treat Paris as the prime meridian until 1911.
Even so, the French defined their civil time as Paris Mean Time minus 9 minutes and 21 seconds. In 1972, Coordinated Universal Time (UTC) replaced GMT as the world's time standard.
Various meridians were used for longitudinal references among different countries before the late 1800s, and the Greenwich Meridian was the most popular of these.
The Greenwich Observatory's reputation for the reliability and accuracy in publications of its navigational data was one factor that contributed to the Greenwich Meridian’s popularity.
Four standard time zones for the continental United States were introduced on November 18, 1883.