Science
Why is there a leap second added to clocks now and then?
- The Earth's spin is uneven and slowly slowing
- Atomic clocks drift
- Time zones change
The answer is The Earth's spin is uneven and slowly slowing. Atomic time is steadier than the planet, so clocks wait for it.
Because the second stopped being defined by the Earth. Since 1967 it has been a fixed number of oscillations of a caesium atom, and atomic clocks keep time far more steadily than the planet does - the Earth's rotation is slightly irregular and, on average, slowing, so the two drift apart.
A leap second is inserted to keep clock time within 0.9 seconds of the Sun's position, which means an occasional minute of 61 seconds. Twenty-seven have been added since 1972, at irregular intervals, announced only about six months ahead - and that unpredictability is the problem. Software has repeatedly failed on them, taking down airline booking systems and large websites, because a minute with an extra second in it is not something most code expects.
In 2022 the world's metrology bodies voted to stop by 2035 and let the difference accumulate instead. The Earth has also unhelpfully sped up recently - 2020 produced the shortest days on record - which raises the prospect of a negative leap second, a minute with 59 seconds, that no system has ever been tested against.