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UK Clock Changes and the Anatomy of a Second

GMT, BST, UTC - plus EU legislation

Summer Period - The Full Story

It sounds like a brief note on a diary page, but ‘summer-time arrangements’ is actually a proper phrase in a very official EU document.

The Directive 200/84 of 19 January 2001 defines what ‘summer-time period’ means and when it starts.

As it is all about time, definitions are based more on what has been agreed rather than on astronomical facts.

So, summer time starts when clocks are put forward by an hour, and ceases when clocks are put back to where they were before.

The starting day is day is the last Sunday in March and the end day is the last Sunday in October.

Finally, the time itself, as mentioned by the EU Directive , is 1.00 a.m. Greenwich Mean Time.

As far as the ‘forward-moving’ action is concerned, the EU document does not actually mention the word ‘hour’, but its equivalent in the smaller unites of time, ‘sixty minutes’.

Someone could ask, why is this move, this ‘spring forward’ expressed in minutes?

The National Physics Laboratory in the UK makes a most necessary distinction between the moment before the clocks change and the moment after that.

The hour and the minute make way for the second as a unit of time. One second before and we are still in Standard Time, one second after and it is all about Daylight Saving and Summer Time.

One could say in the blink of an eye we find ourselves in a completely new territory.

Not quite. It may be a blink and a quarter ! The current time standard takes into account the Earth’s rotation, not the mean solar time at the Prime Meridian in Greenwich.

Coordinated Universal Time (UTC) is the formal international successor of Greenwich Mean Time (GMT).

But GMT continues to be used widely and interchangeably with UTC in a variety of areas, including air and sea travel.

There is a minuscule difference between the two, a fraction of a second. With UTC, leap seconds get added every now and then to bring the solar day in line with the very accurate atomic clocks.

In the UK, GMT is Standard Time during winter months and when the clocks change, it becomes BST or British Standard Time.

Only one second separates GMT from BST. Or, to reflect what happens in visual terms, a second and an hour when looking at the clocks as the change happens. 1.00 am GMT becomes 2.00 am BST. The hour hand moves to 2. On digital clocks, 1 becomes 2.

Somewhere in that interval, at what could be called the atomic level, or in the depths of Earth and Sun movements, there is a tiny morsel of time.

Read the entire NPL site post here


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