Unsung Science

Leap Seconds, Smear Seconds, and the Slowing of the Earth

Dec. 17, 2021

The earth’s spinning is slowing down. Any clocks pegged to the earth’s rotation are therefore drifting out of alignment with our far more precise atomic clocks—only by a thousandth of a second every 50 years, but that’s still a problem for the computers that run the internet, cellphones, and financial systems.

In 1972, scientists began re-aligning atomic clocks with earth-rotation time by inserting a leap second every December 31, or as needed. It seemed like a good idea at the time—until computers started crashing at Google, Reddit, and major airlines. Google engineers proposed, instead, a leap smear: fractionally lengthening every second on December 31, so that that day contains the same total number of seconds. But really: If computer time drifts so infinitesimally from earth-rotation time, does anybody really care what time it is?

Guests: Theo Gray, scientist and author. Geoff Chester, public affairs officer for the for the Naval Observatory. Peter Hochschild, principal engineer, Google.

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