Over the last few decades, Earth has been spinning slightly faster around its axis.
So although you probably haven’t noticed it, our days have gradually become the tiniest bit shorter.
But this trend — influenced by everything from tectonic activity to the weather — seems to have suddenly reversed over the last two years.
And scientists don’t really know why.
Measured by atomic clocks and cutting edge astronomical tools, these tiny changes — just a few miliseconds here and there — might not sound like all that much.
But they can have serious consequences for crucial technology like GPS.
Why does the length of a day change?
A day, as you probably know, is one spin of the Earth on its axis. Changes to the planet itself can affect how long this process takes.
Friction associated with tides, for example, has gradually been slowing down Earth’s rotation.
This change has happened incredibly slowly, adding only around 2.3 miliseconds to the length of a day each century, according to University of Tasmania scientists Matt King and Christopher Watson, who have written an article on the phenomena for The Conversation.
As polar ice began melting 20,000 or so years ago, towards the end of the last ice age, pressure on the Earth’s mantle started shifting. Less ice at the poles means less pressure on the surface, allowing the mantle to creep northwards — and southwards — towards them.
As this mass moves closer to the poles, this makes the Earth spin faster, shaving around 0.6 miliseconds off the length of a day each century. This counteracts some of the time gained from tidal friction.
On a much faster scale, variations in fortnightly and monthly tidal cycles can shortern or lengthen a rotation by up to a milisecond. Weather and climate can also impact the length of a day very quickly — as can tectonic activity.
Large earthquakes, like the one that hit the Pacific Ocean off the cost of Japan in 2011, can have teeny tiny impacts on the length of a day. The 8.9 magnitude Great Tōhoku Earthquake is thought to have taken 0.6 microseconds — which are even shorter than milliseconds — off a rotation.
What’s changed recently?
Scientists have come up with a number of explanations for the latest shift, but none of them are completely satisfying, Watson and King explain.
Weather patterns, like the La Niña phenomenon, which could to occur for the third year in a row this year, might contribute to the axal weirdness.
But although it’s unusual for La Niña — which sees cooler-than-normal sea surface temperatures in the equatorial Pacific — to strike frequently, it’s not unheard of.
Although it’s known to impact the length of a rotation, ice sheet melting hasn’t rapidly increased over the last two years.
But it also might mean nothing at all, Watson and Knight wrote. It could just be a weird-looking quirk of the combination of changes our planet is currently experiencing.
What do we do about it?
For now, the days aren’t getting long so fast they’ll impact our technology. But one day, timekeepers might have to add a ‘negative leap second’ to compensate for any dramatic change in pace.
This would be a hassle for software developers, who already have enough trouble dealing with leap seconds designed to compensate for shorter days.
These produce timestamps that programs aren’t expecting and don’t always know how to handle. Back in 2012, a leap second stopped customers buying Qantas flights and stopped users accessing Reddit.
If the world’s timekeepers decide to add a ‘negative leap second’ to clocks, it would bring solar time (based on the Earth’s rotation) back into line with atomic time (which humans measure with atomic clocks).
While taking a second away from the official clocks is unprecedented, there have been 27 ‘leap seconds’ added since the 1970s, to compensate for the Earth’s rotation slowing down at various points.
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