The Earth does what it wants: Its real rotation just doesn't take into account what our calendar says. Indeed, the planet is revolving more and more slowly. That's why we have to give in to reality, and add one second.
This New Year's Eve is special: December 31st will be one second longer than usual. The keepers of the time will add a leap second and January 1st will start a little later. The reason for the move: Earth's rotation is actually a tiny bit slower than our valid definition of time. And so we make it to fit our models.
There is a paradox, somewhat reminiscent of the question of what came first - the chicken or the egg? Namely; the length of a second is really defined by the rotation of our planet. A day is defined as what it takes for one full rotation. That's also 24 hours or 1440 Minutes or 86.400 seconds. So how could it be that an additional second is needed all of a sudden?
The definition of a second stems from the 19th century. It was taken, when scientists did not yet know, that the Earth's roation is gradually getting slower. "Indeed, the average day is two milliseconds longer, than was originally believed", Wolfgang Dick says. The scientist works at the Central Bureau of the International Earth Rotation and Reference Systems Service (IERS) in Frankfurt. This is the office, tasked with observing the actual Earth's rotation and alerting authorities worldwide as to when a leap-second is necessary.
'If we do nothing, sunrise would end up at noon.'
The leap second is there to prevent the day from getting longer and longer. "If your alarm clock is running slow, you can simply set to the right time. In the case of Earth it is different. We have to adapt to it's rotation-speed. Otherwise, in about 2000 years, the sunrise would be around noon", Dick says.
Adding the leap second seems to be the only solution - the Earth's rotation just isn't reliable enough
But, what's wrong with Earth, that it keeps getting slower? Is it the weariness of age, considering that it once was faster? Not quite: Our moon is mostly to blame for the phenomenon. It is responsible for the tides and triggers friction between water and the ground - also known as "tidal friction." This friction takes energy away from our planet and slows it down.
No need to change the definition
It's a pity that the researchers, who once defined the length of the second, did not know, that the rotation-speed is constantly changing, Dick regrets. "But they needed to come up with a working definition," the IERS expert defends his scientific predecessors.
And he is convinced, that adapting the definition and making the second a tiny bit longer would not solve the problem either. That's because today's atomic clocks are very precise - more precise than the constantly changing rotation of the Earth will ever be. They measure the time independently of celestial influences. Therefore, the leap second is the best way of bringing the astronomical realities on beat with the clocks.
The first leap second was added in 1972. Then, the time difference between the astronomical time and the atomic clock was already at 10 seconds. This New Year's Eve it will the the 27th time, that a leap second has been added.
Pessimists may grumble that the unpopular year 2016 is going to last a whole second longer. Optimists may say, that there is more time to celebrate New Year's Eve.