This new time system is Coordinated Universal Time (UTC). But on Januit became fixed, using predefined leap seconds instead of rate changes. So, the rate of atomic clocks was annually changed or steered to closely match GMT. But as a mean solar time, GMT is defined by the rotation of the Earth, which is not constant in rate.
Originally, time zones based their time on Greenwich Mean Time (GMT, also called UT1), the mean solar time at longitude 0° (the Prime Meridian). The increase in worldwide communication has further increased the need for interacting parties to communicate mutually comprehensible time references to one another. There has been a general trend to push the boundaries of time zones farther west of their designated meridians in order to create a permanent daylight saving time effect. Time zones are thus a compromise, relaxing the complex geographic dependence while still allowing local time to approximate the mean solar time. This problem could be solved by synchronizing the clocks in all localities, but then in many places the local time would differ markedly from the solar time to which people are accustomed. This became increasingly awkward as railways and telecommunications improved, because clocks differed between places by an amount corresponding to the difference in their geographical longitude, which was usually not a convenient number. Its difference from apparent solar time is the equation of time. Mean solar time is the average over a year of apparent solar time. There are variations of the definitions of time zone which generally fall into two meanings: a time zone can represent a region where the local time is some fixed offset from a global reference (usually UTC), or a time zone can represent a region throughout which the local time is always consistent even though the offset may fluctuate seasonally.īefore the adoption of time zones, people used local solar time (originally apparent solar time, as with a sundial and, later, mean solar time). However, political and geographical practicalities can result in irregularly-shaped zones that follow political boundaries or that change their time seasonally (as with daylight saving time), as well as being subject to occasional redefinition as political conditions change. The local time in neighbouring zones is then exactly one hour different. Standard time zones can be defined by geometrically subdividing the Earth's spheroid into 24 lunes (wedge-shaped sections), bordered by meridians each 15° of longitude apart.
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