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Techly Explains

Techly Explains: Who decided how long one second is?

We all know a second is simply one-sixtieth of a minute – or roughly “one Mississippi” – but how was it decided how long it should be?

Actually, the strict definition of what makes a second is “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.”

But that obviously raises more questions than it answers, so we’ll come back to that.

The idea of splitting time into units based on the number 60 came from the Babylonians, and it was more or less a simple case of logic.

In a nutshell, the number 60 is highly divisible, with 1, 2, 3, 4, 5, 6, 10, 12, 15, 20 and 30 all going into 60 when multiplied by a whole number.

However, time was a relative number for millennia of history, with units such as sundials and water clocks a good way to give people a bit of an idea of the time, but hardly a great way to – for example – measure Usain Bolt sprinting 100 metres in 9.58 seconds.

Rather, it was the advent of the pendulum clock in the 17th century that led to measurement of seconds, and this smaller, more accurate measure grew in popularity in the ensuing centuries.

However, the problem with all these measurements of time – and, by extension, the second – was that they were based around an unreliable base: the Earth itself.

The concept of measuring time by basing it on the planet upon which it is taking place makes sense in theory, however the Earth has an irregular rotation, meaning simply calling a second 1⁄86400 (that’s one-eighty-six-thousand, four-hundredths) of a day would see clocks found massively incorrect within a few years.

So it was in 1955, at Britain’s National Physics Library (NPL), that Louis Essen and Jack Parry created the world’s first caesium atomic clock.

This has seen humans cease measuring time using the Earth and instead begin to rely on caesium 133 – specifically, how long it takes one of these atoms to convert from one state to another.

It’s a measure so accurate that, according to the NPL, “the caesium fountain atomic clock at NPL can measure time to an accuracy of one second in 158 million years.

“The next generation of atomic clocks at NPL, using laser-cooled trapped ions or atoms, should achieve accuracies around 100 times better than the current best atomic clocks – equivalent to gaining or losing no more than one second in the age of the universe.”

As a result, in 1967, the International Committee of Weights and Measures adopted the definition of the second mentioned above (that thing about “9,192,631,770 periods”).

About the author

Joe was Junior Vice-President at Compu-Global-Hyper-Mega-Net until it was bought out by Bill Gates. He now subedits for Conversant Media and considers it a step up.

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