The Earth is spinning faster than ever, here's why

The Earth spins on its axis once every 24 hours, right? Actually, no.

The length of each day varies widely by a few milliseconds. According to the International Earth Rotation and Reference Systems Service (IERS) which is responsible for maintaining global time and reference frame standards, today was  23 hours, 59 minutes, 59.9992442 seconds long, or 0.7558 milliseconds shorter than the 24 hours we expect.

Since the introduction of the atomic clock in the mid 1950s, only a single day (July 19, 2008) has been observed to be exactly 24 hours long.

The IERS measures each day's length to the ten thousandth of a second, issuing a weekly report with predictions for the next 365 days. That look forward is critical to the other service the IERS provides, leap days. More on that below.

Earth is slowing down, days are getting longer

Scientists have long known that Earth's spin is gradually slowing. This causes days to grow by an average of about 2 milliseconds each century, which really adds up as you go further back.

620 million years ago, days were 2 hours shorter than today. Another 1.8 billion years further back, around the time life on Earth began moving from single to multicellular organisms, days were just 17 hours long.

This was discovered in part through fossil records. Study of Babylonian and Chinese astronomical observations dating back to 720 BC helped scientists better understand these changes. Descriptions of eclipses just weren't lining up with the math until you took into account the Earth's different rotational period of the time.

Earth's rotation is slowing over the millennia in large part because the Moon is moving away from the Earth at about 1.6 inches each year, or about the speed your fingernails are growing. Like a figure skater that spins slower as they extend their arms, the Earth's rotation slows as the Moon gets further away.

But Earth has been speeding up lately, days are getting shorter

But recent history has been much different.

Records for the shortest day have been falling each summer since 2019. The new shortest day was recorded on June 29, 1.59 milliseconds less than 24 hours. To look at it another way, the Earth rotatesa bit more than 29 inches at the equator during that time.

So what changed?

The cause isn't yet known, but scientists have some theories:

• Melting glaciers and polar ice caps reduced mass at the poles
• Seismic activity such as the 9.0 earthquake which hit Japan in 2011 shifted Earth's axis by 6.7 inches speeding up rotation by about 1.8 microseconds
• The "Chandler wobble", named for Seth Carlo Chandler, Jr., an American businessman turned astronomer who first described the it in 1891. As the geographic north and south poles move across the surface of the globe, it causes the Earth to wobble slightly, possibly slowing the Earth's rotation.  Think of what happens to a spinning top as it begins to wobble.

Why do we care?

Accurate time is critical to today's highly networked technology.

While the length of a day varies wildly when measured by Earth's rotation, it is very consistent when measured by an atomic clocks. Leap seconds are periodically added to account for that difference.

As the IERS finds Earth's rotation drifting more an 0.9 seconds from 24 hours, they give six months notice that a leap second to be added either to the last day of December or last day of June. This has happened 27 times since the system was introduced in 1972, but not since 2017.

Atomic clocks around look something like this at the end of that day:

The leap second added in 2012 caused outages at Reddit, LinkedIn, FourSquare and Yelp. The Amadeus airline reservation system was also disrupted that year when its systems were unable to handle the change smoothly. This caused hundreds of flight delays and forced gate agents at Qantas Airways to check passengers in manually.

The recent trend toward shorter days has generated concern in tech circles about the need to take a way a second from UTC sometime in the future.

Engineers at Facebook support spreading the addition (or removal) of a second over a longer period of time rather than setting clocks to tick from 23:59:59 to 23:59:60 to 00:00:00, as in the video above.

The technique, known as leap-smearing, distributes that extra second across a full 24 hours, beginning at noon UTC. Google has been doing this with their time servers, distribute the extra seconds added in December of 2008 and 2016, and June of 2012 and 2015.

For now there are not plans to add (or subtract) a leap second, smeared or otherwise. The IERS's predictions though this time next year don't suggest that will change either. But if it does, we'll find out when the organization issues its next announcement on the state of leap seconds in the first week of January 2023.