Earth Spinning Faster, Records Shortest Day Ever. Know Its Impact On Our Clocks

Earth recorded its shortest day ever on June 29, 2022. On that day, our planet rotated about its axis in 1.59 milliseconds less than 24 hours.

Since 2021, Earth has been spinning faster than usual. Universal Time is the time standard based on the average speed of Earth's rotation, and is used by astronomers and timekeepers to express mean solar time.

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Did you know that the Earth does not take exactly 24 hours to complete one full rotation on its axis? Recently, Earth has been spinning on its axis faster than usual, resulting in shorter days.

Our planet recorded its shortest day ever on June 29, 2022. But how exactly do we define a day?

The time taken by our planet to complete one spin about its axis with respect to 'fixed stars' is 23 hours 56 minutes and 4.091 seconds. This day is known as the sidereal day. It is called sidereal because it pertains to the stars. Fixed stars are the stars so far away from us that they appear to remain in the same position with respect to other objects in the night sky. 

Longitudinal lines running around Earth vertically and meeting at the North and South Poles are known as meridians. As the Earth rotates, each meridian takes 23 hours 56 minutes and 4.091 seconds to return to the same position, with respect to the fixed stars. 

What Is A Solar Day? 

However, our clocks and calendars are not based on the sidereal day. This is because we have a bigger star nearer to us than other stars, and which significantly impacts our lives — the Sun. When we say there are 24 hours in a day, we are referring to a solar day, which is the time taken by Earth to complete one rotation so that the Sun appears in the same position in the sky. However, in reality, a solar day is not exactly 24 hours.

The Earth rotates counterclockwise on its axis as well as around the Sun. In other words, the revolution of Earth around the Sun is counterclockwise. After a sidereal day, the Earth has moved a bit along its orbit because of which some more rotation is required for a particular meridian to point back towards the Sun. Modern clocks and calendars are based on this longer definition of one rotation. However, the time required by Earth to complete a solar day changes day to day. In reality, each solar day could either be more or less than 24 hours. 

Clocks are based on the average of all the solar days in a year. Thus, on an average, the Earth takes 24 hours to complete one rotation about its axis.  

How Do Atomic Clocks Work?

However, atomic clocks give the most precise measurement of time. Designed to measure the precise length of a second, atomic clocks have an expected error of only one second in about 100 million years. 

According to the International System of Units (SI), a second is the time taken by a caesium-133 atom in a precisely defined state to oscillate exactly nine billion, 192 million, 631 thousand, 770 times. Thus, an atomic clock measures the time required by a caesium-133 atom to oscillate 9,192,631,770 times. 

Super-accurate atomic clocks were first developed in the 1950s and 1960s, and since then, measurements of Earth's rotation have been made using atomic clocks.

Average Solar Day Lengths Over The Years

The length of a solar day is within a few milliseconds of 86,400 seconds, which is equivalent to 24 hours.

According to timeanddate.com, the average solar day length in 2022 is 24 hours minus 0.32 milliseconds.  Earth rotated about its axis in 1.59 milliseconds less than 24 hours on June 29. In other words, the solar day on June 29 was 1.59 milliseconds short of 24 hours. June 29 is the shortest day ever recorded on Earth. 

On July 26, the solar day was 1.5 milliseconds less than 24 hours. 

May 14 is the longest day of 2022 so far, as it was 0.68 milliseconds longer than 24 hours. 

What Are Leap Seconds?

Though a few milliseconds do not seem to make any difference, they can add up over the years and cause our clocks to go out of sync with Earth's spin. This is where leap seconds come to the rescue. A leap second can be added or subtracted to modern clocks to synchronise them with Earth's rotation. 

The system of leap seconds was introduced in 1972, and so far, there have been 27 leap seconds. A positive leap second adds a second to our clocks, while a negative leap second subtracts a second from our clocks. So far, only positive leap seconds have been used.

The last time a leap second was added to our clocks was in 2016. 

The reason why only positive leap seconds have been added so far is that the Earth was rotating slower than usual till 2019. On March 22, 2019, the solar day was 1.68 milliseconds more than 24 hours. This was the longest day of 2019. 

To compensate for the fact Earth was spinning slower than usual, positive leap seconds were used.

In 2020, the average solar day length was 24 hours, according to timeanddate.com. July 19, 2020 was the shortest day of that year. The day was 1.47 milliseconds short of 24 hours. 

Since 2021, Earth has been spinning faster than usual. The average solar day length in 2021 was 0.18 milliseconds short of 24 hours.

Since Earth is spinning faster than usual, timekeepers may have to use negative leap seconds for the first time. In other words, leap seconds might need to be subtracted from our clocks to synchronise them with Earth's rotation.

How Do Timekeepers Decide The Time?

Universal Time is the time standard based on the average speed of Earth's rotation, and is used by astronomers and timekeepers to express mean solar time. International Atomic Time is the super-precise time scale calculated by a network of 400 ultra-precise atomic clocks around the world, including in Australia. Universal Time is compared with International Atomic Time.

The magnitude of the difference between the International Atomic Time and Universal Time is added or subtracted to 24 hours. If the International Atomic Time is greater than the Universal Time on a particular day, the magnitude of the difference is added to 24 hours. If the International Atomic Time is less than the Universal Time on a particular day, then the magnitude of the difference is subtracted from 24 hours. This practice is necessary because the length of the solar day or astronomical day varies by a tiny amount each day. 

The Coordinated Universal Time (UTC), set to our time zone, governs the time we see on our computers. The UTC is based on International Atomic Time. 

According to an article published by ABC News, Fred Watson, an Australian astronomer, said that Earth is not just a solid ball that is spinning, but has liquid in its core, on the surface, and is surrounded by an atmosphere. 

Due to the tug of the Moon, Earth has been slowing down by about three milliseconds per day, per century, the article said. 

Should A Negative Leap Second Be Used?

Watson said that if nothing is done about this, the seasons will gradually get out of step with the calendar. 

He explained that leap seconds are introduced to keep the atomic clocks and the astronomical clocks together. However, if a negative leap second is introduced, it would mean that our clocks skip one second, which could create problems for IT systems. 

In a recent blog post, Meta wrote that the impact of a negative leap second has never been tested on a large scale, and that it could have a devastating effect in the software relying on timers or schedulers. The company also said that in any case, every leap second is a "major source of pain" for people who manage hardware infrastructures.

Meta stated that they believe that the current level of 27 leap seconds will be enough to get us through the next millennium.