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Today is the longest day of 2015. Not the longest interval between sunrise and sunset — that came a few days ago, on the summer solstice. Instead, an extra second will be added to the world’s official timekeeping services, so today will last exactly 24 hours and one second.
The extra second is needed because Earth’s rotation is slowing down. Today, it takes our planet between one and two milliseconds longer to make one full turn than it did a couple of hundred years ago.
In an era of ultra-precise timekeeping, that’s a problem. Atomic clocks keep almost perfect time. But as Earth slows down, these clocks drift away from solar time — the time measured by the passage of the Sun across the sky — by between one and two milliseconds per day.
That difference adds up. So every once in a while, the world’s timekeepers add a “leap second” to bring the atomic clocks back in line with solar time. This one will be added between 6:59:59 and 7 p.m. Central Daylight Time.
Earth’s changing rotation rate isn’t smooth and predictable, though, so leap seconds can’t be predicted very far in the future. So they may be added in back-to-back years, or it may be several years between them.
Adding a leap second can disrupt computers, though, so some have suggested eliminating the leap second. Scientists will vote on that proposal later this year — perhaps deciding to let atomic clocks drift away from the time kept by our home planet.
Script by Damond Benningfield, Copyright 2015