Do you remember Y2K? When we all feared that the beginning of 2000 would mean websites would crash, the lights would go out, and planes would fall from the sky? Did you get yourself a Y2K Emergency Kit? Well, if you did, maybe you could dust it off on June 30th, because the world is about to have a leap second. That's right: a leap second. One extra second will be added to the year to allow Earth's atomic clocks to be in sync with the planet's rotation. "The extra second will be tacked on to the final minute of June 30," reports the LA Times. "On that day, the official atomic clocks that keep Universal Coordinated Time will mark the time as 23h 59m 59s, followed by the leap second 23h 59m 60s. July 1 will continue as usual, beginning with 0h 0m 0s."
The planet's rotation can speed up or slow down because of extreme weather, earthquakes, or volcanoes. "Anything that causes a large-scale change in the movement of mass around the planet can change the Earth's rotation," says NASA geophysicist Richard Gross. Atomic time, (or Universal Coordinated Time), however, is constant - it's defined as "exactly 9,192,631,770 oscillations of a cesium-133 atom." I have no idea what that means, but it sounds pretty precise. But it turns out it isn't quite correct. "Back in 1967, the length of a second was determined to be 9,192,631,770 oscillations, based on the time it took to get from one summer solstice to the next at the previous turn of the century. But if the timekeepers had gone with 9,192,631,950 oscillations instead, we would have needed only 10 adjustments over the last 48 years — five to add leap seconds to the official UTC time, and five to take leap seconds away." Instead, this July's adjustment will be the 26th. Although the leap second will effect some websites, tech companies are ready for it. So there's probably no need to worry about your vital tweet you send out at midnight on June 30th about that celebrity nip-slip you just witnessed getting lost in the ether.
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