Why does this year have an extra second?

-December 14, 2016

Aside from the traditional ballad "Auld Lang Syne," the 10-second countdown might be the most iconic aspect of New Year's Eve. On December 31, 2016, metrologists around the worldespecially those living under Greenwich Mean Time (GMT)—will add a single second to their New Year's Eve countdown. International timekeepers have decreed, after some debate, that the atomic clock is due a leap second.

How we measure time
These scientists depend on the accuracy of our International System (SI) unit for the second based on the alkali metal cesium (Cs) element. The cesium atom has a resonant frequency that's very, very stable over time. When operating in an oscillator—also known as the atomic clock—the atom produces a time measurement that is accurate, give or take a couple nanoseconds. This margin of error comes out to around one second every 1.4 million years.

This reproducible degree of accuracy demonstrates the importance of metrology, and its impact on our day-to-day lives. Take the example of taxis or rideshares that charge based on time and distance. The accuracy of meters and GPS affect how much you pay.

Approximately 200 atomic clocks around the world are averaged together to make the representation of International Atomic Time (TAI). However, when you check current time on your computer, you’re seeing Coordinated Universal Time (UTC). Using a GPS disciplined oscillator that houses rubidium-based atomic clocks that are steered by the cesium clocks in the GPS satellite array, we can continuously synchronize to UTC.

The leap second’s importance
Leap seconds are added to keep international time in sync with solar time. Since 1972, there have been 36 seconds altered between TAI and UTC, all due to Earth’s slowing rotation. We need these UTC adjustments so that scientists, astronomers, and astronauts can stay in sync.

What does this mean for you? If, for example, you live in the United States, the leap second won't be as dramatic, since it will go into effect in the afternoon (at 4 p.m. PST, 7 p.m. EST). If you find yourself celebrating in Greenwich, England, or a select few countries in Europe this New Year’s Eve, it is technically correct to add an extra second to your final countdown.

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