It will be the 24th leap second since 1972, keeping clocks in sync with 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 holiday season? Enjoy New Year’s Eve’s leap second.
On Wednesday, at 11:59:59 pm Universal Time (6:59:59 p.m. Eastern time), atomic clocks around the world will add one second to the day.
The idea of adding time at the end of the year dates back to the ancient Egyptians, who added a day every four years. But the leap second is a newcomer. This year, the world’s timekeepers will add its 24th leap second since 1972. Wednesday's extra second will bring the world’s high-tech atomic clocks back into sync with time as defined by Earth’s rotation.
Why does the world need leap seconds? Chalk it up to the moon’s braking action on Earth’s rotation and to modern timekeeping that has become so precise it can make your head spin. Indeed, as timekeeping becomes more accurate and portable, it could improve everything from GPS navigation to cellphone reception.
Scientists developed the first atomic clock in 1949. As the clocks improved, researchers found themselves using timepieces of a precision beyond exquisite. Today’s versions might gain or lose one second after 60 million years.
These clocks became the standard for global timekeeping. No swinging pendulums or vibrating wristwatch-scale quartz crystals here. One second is formally defined as 9,192,631,770 cycles, or “ticks and tocks,” of microwave energy emitted or absorbed by cesium-133 atoms under carefully controlled conditions.
Meanwhile, researchers were also taking the measure of Earth’s rotation with greater precision, using a global network of radio telescopes aimed at quasars – the active cores of galaxies in the distant early universe. Quasars are so far away that they constitute the best frame of reference for measuring Earth’s rotation rate.