As no meridian divides the globe into east and west in the same way as the equator divides the world into north and south, countries were free to pick their own. In 1675, Britain built the Royal Observatory and chose the Greenwich meridian. In 1880, after over 200 years of pick and mix, it was agreed Greenwich would be the universal Prime Meridian. In 1928 Universal time, shortened to UT1, became a synonym for GMT. Then in January 1972, Universal Coordinated Time, (UTC), using a worldwide selection of atomic clocks, was introduced.

GPS uses atomic time, which until January 1980 was the same as UTC. Since then leap seconds have been added to UTC (atomic time is more accurate and does not need these) resulting in nearly 15 seconds difference between GPS time and UTC. However, GPS makes its own corrections and the times it displays are within one second of UTC.

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