Earth’s shortest day sets new record

Timeanddate.com originally published this article by Graham Jones and Konstantin Bikos on July 27, 2022. Reprinted here with permission. Edits by EarthSky.

Shortest day in the atomic clock era

Earth has clocked its shortest day since scientists began using atomic clocks to measure its rotational speed. On June 29, 2022, Earth completed one spin in 1.59 milliseconds less than 24 hours. This is the latest in a series of speed records for Earth since 2020.

In general, Earth’s spin is slowing down, although only ever-so-gradually. That’s why we occasionally have to add a leap second to our calendars. The last leap second added was on December 31, 2016. But over the last few years, Earth’s spin is slightly faster. Why?

At the 19th annual meeting of the Asia Oceania Geosciences Society this week, some scientists will present their ideas for why Earth may be speeding up.

Earth’s spin fluctuates

Generally speaking, Earth spins once every 24 hours. That’s why the sun appears to rise and set every day.

Also, in general, over long periods, Earth’s spin is slowing. Every century, Earth takes a couple of milliseconds or so longer to complete one rotation (where 1 millisecond equals 0.001 seconds).

Within this general pattern, however, the speed of Earth’s spin fluctuates. From one day to the next, the time Earth takes to complete one rotation goes up or down by a fraction of a millisecond.

Length of day

Scientists who study Earth’s rotation use the term length of day to talk about how slow or fast Earth is spinning. The length of day is the difference between the time it takes Earth to rotate once on its axis, or 86,400 seconds (which is 24 hours).

When the length of day is rising, Earth is spinning more slowly. When it is decreasing and becomes a negative number, Earth is spinning more quickly.

Earth in a hurry

In recent years, Earth has been speeding up. In 2020, timeanddate reported that Earth had achieved its 28 shortest days since accurate daily measurements using atomic clocks began in the 1960s. The shortest day of all in 2020 was -1.47 milliseconds on July 19.

Earth continued to spin quickly in 2021, although the shortest day of the year in 2021 was fractionally longer than in 2020. Now, in 2022, things have speeded up again. On June 29, Earth set a new record for the shortest day of the atomic-clock era: -1.59 milliseconds.

Earth nearly beat its record again the following month, posting a length of day of -1.50 milliseconds on July 26.

New record shortest day

The following chart shows how the length of day has fluctuated in recent years. The data is provided by the International Earth Rotation and Reference Systems Service (IERS).

The narrow, jagged spikes in the chart are a result of the moon’s monthly orbit around Earth. The longer, smoother waves – with the shortest days coming in or around July each year – are related to movements in Earth’s atmosphere.

Why did we have a new shortest day?

What is causing the current downward trend in the length of the shortest day?

It could be related to processes in Earth’s inner or outer layers, oceans, tides or even climate. Scientists are not sure, and they struggle to make predictions about the length of day more than a year ahead. But there are tentative ideas.

At this week’s annual meeting of the Asia Oceania Geosciences Society, Leonid Zotov – together with his colleagues Christian Bizouard and Nikolay Sidorenkov – will suggest the current decrease in the length of day could have some relation to the Chandler wobble.

Chandler wobble is the name given to a small, irregular movement of Earth’s geographical poles across the surface of the globe.

Zotov told timeanddate:

The normal amplitude of the Chandler wobble is about three to four meters at Earth’s surface, but from 2017 to 2020 it disappeared.

Will we need a negative leap second?

If Earth’s fast rotation continues, it could lead to the introduction of the first-ever negative leap second.

This would be required to keep civil time – which is based on the super-steady beat of atomic clocks – in step with solar time, which is based on the movement of the sun across the sky. A negative leap second would mean that our clocks skip one second, which could potentially create problems for IT systems.

Will Earth keep speeding up?

Will the length of day continue to decrease, or have we already reached the minimum? Nobody knows for sure, but we asked Dr. Zotov for his best guess. He said:

I think there’s a 70 percent chance we’re at the minimum, and we won’t need a negative leap second.

Bottom line: Earth has been speeding up the past few years, resulting in our shortest day ever as measured by atomic clocks. Will we need to subtract a leap second?

For a super-interesting work of fiction about a natural slow-down in Earth’s spin, try the book “Age of Miracles,” by Karen Walker