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Railroads and long-distance communication led to the development of time zones, with Greenwich established as the prime meridian—the longitude of 0 degrees—due to British advancements in star mapping and navigation.
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Additional insights we found via StarTalk
Before time zones, locations on Earth set their clocks based on high noon, when the sun was at its highest point in the sky, but this introduced confusion for railroad timetables as train companies tried to keep track of train times across vast distances.
The British had mapped when every star in the sky passed over Greenwich as the Earth rotated, which allowed sailors to determine their longitude by comparing when stars were observed overhead to clocks set to Greenwich time.
Because the Earth orbits the sun as it spins, the time it takes for the sun to return to its highest point in the sky—a solar day—differs from the time it takes Earth to complete a rotation—a sidereal day—and, when time zones were defined based on a prime meridian at Greenwich, these differences were averaged to establish Greenwich Mean Time, or GMT.
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