'Negative leap second': Why Earth's changing orbit could mean we lose a second on our clocks

World timekeepers, for the first time in history, might have to consider subtracting a second from our clocks.

Earth takes around 24 hours to complete a full rotation, but the key word to remember there is "around".

In fact, scientists for decades have been adding so-called "leap seconds" to accommodate for the slowing of Earth's orbit.

Now, a new study published in the journal Nature has suggested the potential need for clocks to skip a second by 2029.

But why exactly might we need to lose a second of time? ITV News explains.

What have the study's authors said?

Duncan Agnew, the study's lead author and a geophysicist at the University of California, has described the idea of skipping a second as an "unprecedented situation and a big deal".

He said: "It's not a huge change in the Earth's rotation that's going to lead to some catastrophe or anything, but it is something notable.

"It's yet another indication that we're in a very unusual time."

How do we measure time?

Today, atomic clocks are used worldwide as the official time standard. These were adopted more than 55 years ago.

For example, many smartphones, which contain GPS technology, have their time synced to atomic clocks.

The adoption of atomic clocks established two versions of time: atomic and astronomical - neither of which match with the other because the former does not slow down.

In contrast, astronomical time fell behind atomic time by 2.5 milliseconds every day.

Mr Agnew defined the difference by explaining that to Earth, midnight passes a fraction of a second after the atomic clock has itself recorded midnight.

Those daily fractions of seconds added up to whole seconds every few years.

So, beginning in 1972, international timekeepers decided to add a leap second in June or December for astronomical time to catch up to the atomic time. This later became known as Coordinated Universal Time or UTC.

Between 1972 and 2016, 27 separate leap seconds were added as Earth slowed.

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Why has Earth been slowing?

Earth for thousands of years has generally been slowing down, with the rate varying from time to time.

Dennis McCarthy, a retired director of time for the US Naval Observatory, said the slowing is mostly due to the effect of tides, which themselves are caused by the pull of the moon.

What's happening now?

Scientists have recorded that the rate of Earth slowing has tapered off in recent years.

"In 2016 or 2017 or maybe 2018, the slowdown rate had slowed down to the point that the Earth was actually speeding up," said Judah Levine, a physicist for the time and frequency division of the National Institute of Standards and Technology.

Earth is now rotating slightly faster than it used to because of the unpredictable ways in which its hot liquid core ebbs and flows.

Mr Agnew said the Earth's core has been triggering a speed-up for around the last half a century, but the rapid melting of icecaps in both the North and South Poles since 1990 has masked that effect.

Melting ice shifts Earth's mass from the poles to its centre, which slows the rotation much like a spinning ice skater slows when extending their arms out to their sides, Mr Agnew explained.

Does that mean we need to skip a second?

Despite the masking effect of melting polar icecaps, Mr Agnew believes that by 2029 global timekeepers will need to subtract a second from our clocks.

Indeed, were it not this for this effect, he estimates a "negative leap second" would have been required in 2026.

A negative leap second, for example, would jump from 11.59pm and 58 seconds directly to midnight, skipping 11.59.59pm.

However, not everyone is in agreement that a negative leap second will be needed.

Mr Levine said the overall slowing trend from tides has been around for centuries and continues, but that the shorter trends in Earth's core come and go.

He added: "This is not a process where the past is a good prediction of the future. Anyone who makes a long-term prediction on the future is on very, very shaky ground."

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