What Is Greenwich Mean Time?

What Is Greenwich Mean Time?
What is Greenwich Mean Time (GMT), and why does it matter? Here is a short overview of GMT and some of the history behind it.

What Is Greenwich Mean Time?

Greenwich Mean Time, or GMT, is exactly what its name describes – a mean (or average) time. It is described as the “yearly average of the time each day when the sun crosses the Prime Meridian.”

Its designation as “Greenwich” Mean Times comes from the fact that this average is observed at the Royal Observatory in Greenwich, England (which lies on the Prime Meridian). From 1884 until 1972, GMT was used internationally as the standard of civil time.

A Brief History of GMT

Mean time is what we also refer to as clock time. This is in contrast to solar time, which varies throughout the year depending on the location of the sun and where you are in the world. Recognizing that this differentiation in time across the world would be a problem, John Flamsteed set out in the 1650s to reconcile the difference and to craft a formula for converting solar time to mean time so that everyone in the world could have a universal time reference. The invention of the pendulum clock allowed Flamsteed to publish a set of conversion tables in the early 1670s, and they set the stage for establishing GMT, which to this day is used by the Met Office, the Royal Navy, and BBC World Service.

His work wasn’t done however. While there was now a way to calculate time according to the sun, as per Flamsteed’s recommended conversions, this didn’t mean that everyone kept the same time. Indeed, all over the world, almost every town had its own local time based on the action of the sun. With railway and communication expanding however, there needed to be an international time standard. In response, British railway companies started using GMT as their point of reference for train timetables, making it the official “Railway Time.”

In 1884, at the International Meridian Conference in Washington, D.C., 22 countries voted to adopt the Greenwich meridian as the Prime Meridian of the world. Greenwich Mean Time was established as the world’s time standard. All time zones in the world refer back to GMT on the Prime Meridian.

Do People Still Use GMT?

The daily rotation of the Earth is irregular, and is constantly slowing, presenting some issues with the accuracy of GMT.

On January 1st, 1972, Greenwich Mean Time was replaced as the international civil time standard by Coordinated Universal Time (UTC), which is a more stable and accurate timekeeping method than GMT. UTC is measured using advanced atomic clocks found throughout the world, with leap seconds added to compensate for any of Earth’s irregularities.

Today, UTC is often used interchangeably with GMT, even though this is not accurate.

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