The leap second the world's timekeepers will add tomorrow will bring the world's high-tech atomic clocks back into sync with time as defined by Earth's rotation
National Institute of Standards and Technology - Physics Laboratory: Time and Frequency Division
So you'd like a little more free time to kick back during this this holiday season? Enjoy New Year's Eve's leap second. On Dec. 31, at 11:59:59 p.m. Universal Time (6:59:59 p.m. Eastern time), atomic clocks around the world will add one second to the day.
The planned hiccup among the high-tech ticks and tocks comes courtesy of Earth's rotation rate, which is ever so slowly winding down. By some estimates, in 2100 a day will be 2 milliseconds longer than today's day. The leap second the world's timekeepers will add tomorrow will bring the world's high-tech atomic clocks back into sync with time as defined by Earth's rotation.
The idea of a time period that "leaps" at the end of a year dates back to the ancient Egyptians. Back then, it was a leap day added every four years. The Romans picked up on the idea as they adopted the Julian calendar. Later, the Gregorian calendar modified the rule: A leap day is added to any year whose number is evenly divisible by 4, unless the year is evenly divisible by 100. Then it isn't a leap year, unless it's also evenly divisible by 400.
Who needs a leap second anyway?
But the need for a leap second is relatively new. Scientists developed the first atomic clock in 1949. They found themselves using time pieces of beyond-exquisite precision. Today's versions might gain or lose one second after 60 million years. These clocks became the standard for timekeeping, defining the length of one second.
With timing that precise, however, scientists found that Earth's rotation rate varies and is slowing. The moon is the single largest influence on Earth's spin. But other factors can affect the twirl as well -- the bulge in the ocean as the moon's gravity tugs on it, or the amount of ice and snow at the poles. If Earth's rotation rate varies, defined by radiotelescope measurements of distant quasars, so would one second.