New Year’s Eve will be a bit longer than expected. An extra second will be added to keep our timekeeping accurate. This “leap second” is needed to bring our standard time measurements, based on atomic clocks, in line with the rotation of the Earth.

Atomic clocks are based on the highly consistent oscillations of a caesium133 atom, and are so precise that they are accurate to one second in 1.4 million years. The problem is that the rotation of the Earth is far less accurate. If we did not add on extra seconds every now and again, eventually in thousands of years’ time our clocks would read midnight when the sun was high in the sky at what should be midday.

One reason for this discrepancy is that over the long term the Earth is slowing down. Each day, the pull of the oceans’ tides slows the Earth’s rotation, like applying a brake on a moving wheel. This is why more than 200 million years ago the dinosaurs may have witnessed 375 days in a year.

The rotation is also changed by earthquakes. The Indonesian earthquake that set off the Indian Ocean tsunami of 2004 decreased the length of day by 2.68 microseconds. A large earthquake changes the distribution of the Earth’s mass and so changes the speed of its rotation.

But the largest variation is seasonal, when the Earth slows down a fraction in January and February in the northern hemisphere. This is when westerly winds are at their strongest and the days get longer by a few thousandths of a second.