Leap year adds a day to February every 4 years, while a the leap second is added every few years in order to synchronize the world’s scientific timekeepers–atomic clocks–with the earth’s rotational cycle. The next leap second is scheduled to be added on June 30, 2018. Atomic clocks, which were created in the 1950s, rely on the precise way that electrons jump around in atoms.
The US is against this, since if it’s not done just right, it could lead to problems with electronic systems that depend on the precise time, such as computer and cell phone networks, air traffic control and financial trading markets. Astronomers worry that if a software-guided telescope is not pointed in exactly the right direction at the right time, it may not capture the right image.
But other countries, including the UK, Canada and China, want to keep the current system, saying that the US is making too much of this problem, in same way fears about Y2K were exaggerated in the year 2000.
In 1967, the UN created a new definition of a second based on atomic clocks. Meanwhile, the earth, which functions like a spinning top, has slowed down over the last century, meaning that the time between sunrise and sunset has grown longer. Atomic clocks that were set in the past now run slightly ahead of what we see in the sky, so since 1972, we have added leap seconds to keep the two sets of clocks synchronized.
The UN appears to be as contentious as our congress: In the January 19th edition of the New York Times, Kenneth Chang reports that "in a poll conducted last year, only 16 nations expressed an opinion. Thirteen would abolish leap seconds. Three wanted to keep them."
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