Each year seems to go faster and faster, but astronomers have revealed that Earth’s day has actually been getting longer.

The team found that thanks to the gradual slowing of our planet’s rotation, a day on Earth lengthens by around 1.8 milliseconds every 100 years.

That fractional length of time might not sound like much – it will take around 3.3 million years to gain just 1 minute, and 2 million more centuries for us to add a much-needed extra hour to our day.

But understanding exactly how much Earth’s rotation is slowing down and the factors that influence this is important for scientists to make the most accurate calculations possible about our planet’s future.

In fact, the study showed that the planet has been more unpredictable than expected. Earlier predictions had calculated we’d be adding 2.3 milliseconds a day each century. So the planet is actually slowing down more gradually than expected over long periods of time.

To figure this out, astronomers from the UK looked over nearly 3,000 years of celestial records – all the way from 720BCE to 2015.

The earliest records came from Babylonian clay tablets written in cuneiform – one of the earliest forms of writing. The team also looked at ancient Greek texts, as well as scripts from China, medieval Europe, and the Middle East.

These records all documented the times and places that people had witness solar and lunar eclipses throughout the millennia.

To find out how Earth’s rotation had slowed down over the 2,735-year period, the team compared the historical records with a computer model that predicted where and when these events would have occurred if Earth’s spin hadn’t changed at all.

“Even though the observations are crude, we can see a consistent discrepancy between the calculations and where and when the eclipses were actually seen,” one of the researchers, Leslie Morrison, from Durham University and the UK’s Nautical Almanac Office, told Ian Sample over at The Guardian.

“It means Earth has been varying in its state of rotation.”

So why is Earth’s rotation slowing down in the first place? A large part of it is due to something called tidal braking, which basically means that the Moon’s tidal pull is acting like a drag on our planet.

“The heaping up of water drags on Earth as it spins underneath,” said Morrison. Incidentally, as our spin slows down, the Moon’s orbit is also growing by around 4 cm a year.

But that’s not the only factor involved – the world’s sea levels, as well as electromagnetic forces between Earth’s core and its mantle have an affect on Earth’s spin too, which is why it’s so unpredictable.

One example of this is that ice ages slow the planet’s rotation down by freezing all the water in the poles, causing the planet to squish slightly and spin more languidly, just like an ice skater when she puts her arms wide.

After going back over nearly 3,000 years, the team showed that the planet’s spin hadn’t actually been slowing down as much as previously predicted – only 1.8 milliseconds per century, rather than 2.3 milliseconds.

Which means our days are getting longer, but not as rapidly as we previously thought. And it show that as our climate continues to shift, we can expect the rate of Earth’s spin to vary.

“It’s a very slow process,” Morrison told AFP.

“These estimates are approximate, because the geophysical forces operating on the Earth’s rotation will not necessarily be constant over such a long period of time … Intervening Ice Ages etcetera will disrupt these simple extrapolations.”

Despite the uncertainty, it’s a pretty impressive result, seeing as these changes are incredibly hard for scientists to monitor.

“Geological processes occur on long time scales, which makes direct observation of their evolution extremely difficult on human timescales. This is a particular problem for phenomena such as Earth’s rotation, which don’t leave direct evidence in the geological record,” Jon Mound, a geophysicist at Leeds University, who wasn’t involved in the research, told The Guardian.

“In many ways this is an amazing result that ties together a wide range of investigations at opposite ends of the scale of technological sophistication to determine to high precision an extremely small effect.”