The Earth had its shortest ever day this summer, thanks to a wobble in its axis which meant it completed a single spin in a fraction of a second less than 24 hours.

June 29 was 1.59 milliseconds shorter than 86,400 seconds, or exactly 24 hours, according to the website

In recent decades the Earth has been more likely to slow down, giving marginally longer days. But in the last few years, that tendency reversed, and the days have been getting shorter and shorter.

If the Earth continues to speed up, this could lead to the first-ever requirement to subtract a second from atomic clocks.

The Earth is not perfect

It's not uncommon for the Earth to wobble – the spinning which we experience as night and day does not always happen exactly in line with its axis, the line between the North and South Poles.

That's because it is not a precise sphere.

The planet has a bulge at the equator, while the poles are slightly squashed, meaning Earth is slightly elliptical.

Other factors can mess with the spinning too, including ocean tides and gravity from the Moon.

The "Chandler wobble"

Leonid Zotov, a professor of mathematics, believes that the Earth may be spinning faster because of a periodic movement called the "Chandler wobble".

The wobble was first spotted in the late 1880s, when astronomer Seth Carlo Chandler noticed the poles wobbled over a period of 14 months.

This wobble started to slow down in early 2000s, reaching historic minimums since 2017, per The Telegraph.

And between 2017 to 2020, "it disappeared", Zotov told

Zotov is due to present this hypothesis at the Asia Oceania Geosciences Society, per It has not yet been peer-reviewed.

Earth wobbles don't change much in day-to-day life. But they are important to keep track of, so the atomic clock can remain accurate to precisely coordinate GPS and Earth-observing satellites.

This article was originally published by Business Insider.

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