Time is vital to the functioning of our everyday lives: from the watches on our wrists to the GPS systems in our phones. Communication systems, power grids, and financial transactions all rely on precision timing. Seconds are the vital units of measurement in timekeeping.

Surprisingly, there is still debate over the definition of the second. But recent advances in the world’s most accurate forms of timekeeping may have just changed the game.

Accurate timekeeping has always been part of humankind’s social evolution. At the Neolithic monument of Newgrange in Ireland, a special opening above an entrance allows sunlight to illuminate the passage and chamber on the shortest days of the year, around December 21st, the winter solstice.

Some 2,300 years ago, Aristotle said that “the revolution of the outermost sphere of the heavens” should be the reference for measuring time. The Greek philosopher believed the cosmos was arranged into concentric spheres, with Earth at the centre.

Water clocks, which appeared around 2,000BC, are among the oldest instruments for measuring time. They do this by regulating the flow of water into or out of a vessel. The mechanical clock was then established in the late 19th century.

Up until 1967, a second was defined as 1/86,400 of a day, with twenty-four hours in a day, sixty minutes in an hour and 60 seconds in a minute (24 x 60 x 60 = 86,400). The International System of Units then changed things, settling for this definition:

The second… is defined by taking the… transition frequency of the caesium-133 atom, to be 9192631770 when........

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