The clocks are nowhere near exotic. They are regular electronic boxes with little digital displays, stacked together in a sanitised chamber—you must leave your shoes outside—where the temperature and humidity are clinically constant at around 23 degrees and 55 per cent respectively. “Back in the mid-1970s, when we got our first atomic clock, people flocked to NPL to take a look, but went home disappointed by its boxy appearance,” Dr Banerjee says. But these clocks are the keepers of time, smoothening out the kinks in our measured knowledge of this most dynamic of parameters. While Dr Banerjee trusts NPL-7—the current master clock that was installed three or four years back—the most, he has tasked research scholars working under him with developing algorithms to calculate an average of the five clocks that would have a higher degree of accuracy than individual measures.

Even a cesium atomic clock—cesium is preferred because it is a stable element that is also easy to handle—cannot be 100 per cent correct, since it “probes” the cesium atom and disturbs it. “Let me give you an example. To a scientist, a thermometer tucked under the armpit can never reveal the exact temperature of the body, since it absorbs some of the body heat while measuring it,” Dr Banerjee says. For all practical purposes, though, this discrepancy is negligible. Just as the micro-inaccuracies of atomic clocks may be.

What is not negligible is the difference between the time taken by the earth to rotate on its own axis with respect to the sun—which accounts for what is known as a mean solar day—and the time kept by the exactest of clocks. “Man has traditionally identified time with the movement of heavenly bodies. Twenty-four hours as measured by our atomic clocks must never be too different from the time the earth actually takes to complete one rotation around its own axis,” says Dr Banerjee. To the meticulous mavericks of scientific persuasion, “too different” is a variation of over 0.9 seconds, stemming from slight anomalies in the earth’s rotation due to geological fluctuations. And to take them into account, a ‘leap second’ is introduced in atomic clocks around the world. (In keeping with the exigencies of accuracy, a second is defined as “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 cesium 133 atom”.) The International Earth Rotation and Reference Systems Service (IERS), which monitors the earth’s rotation through observatories across continents, adjusts Coordinated Universal Time (UTC)—determined by the Bureau International des Poids et Mesures (BIPM) in Paris—accordingly whenever necessary.

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