This has a critical impact not only on our timekeeping, but also on things like GPS and other technologies that govern our modern lives. In recent decades, the rotation of the Earth on its axis – which determines the length of the day – has accelerated. This trend has made our days shorter. In fact, in June 2022 we set a record for the shortest day in about half a century. But despite this record, since 2020 this steady acceleration has curiously turned into a slowdown – the days are getting longer again, and the reason is so far a mystery. While the clocks on our phones indicate that there are exactly 24 hours in a day, the actual time it takes for the Earth to complete one rotation varies slightly. These changes occur over periods of millions of years to almost instantaneously – even earthquakes and storm phenomena can play a role. It turns out that a day is very rarely exactly the magic number of 86,400 seconds.
The ever-changing planet
For millions of years, the Earth’s rotation has been slowed by the frictional effects associated with the tides driven by the moon. This process adds about 2.3 milliseconds to the length of each day every century. A few billion years ago an Earth day was only about 19 hours. For the past 20,000 years, another process has been working in the opposite direction, speeding up the Earth’s rotation. When the last ice age ended, the melting of the polar ice sheets reduced the surface pressure and the Earth’s mantle began to move steadily towards the poles. Just as a ballet dancer spins faster as he brings his arms toward his body—the axis around which they spin—so our planet’s rotation rate increases as this mantle mass approaches Earth’s axis. And this process decreases every day by about 0.6 milliseconds every century. For decades and more, the connection between Earth’s interior and surface also comes into play. Large earthquakes can change the length of a day, although usually by small amounts. For example, the great 2011 Tōhoku earthquake in Japan, with a magnitude of 8.9, is believed to have sped up the Earth’s rotation by a relatively tiny 1.8 microseconds. In addition to these large-scale changes, over shorter periods weather and climate also have significant effects on the Earth’s rotation, causing fluctuations in both directions. The fortnightly and monthly tidal cycle move mass around the planet, causing changes in the length of the day of up to a millisecond in each direction. We can see tidal fluctuations in the day length records over periods of up to 18.6 years. The movement of our atmosphere has a particularly strong effect, and ocean currents also play a role. Seasonal snow cover and rainfall, or groundwater extraction, change things further.
Why is the Earth suddenly slowing down?
Since the 1960s, when radio telescope operators around the world began devising techniques to simultaneously observe cosmic objects such as quasars, we have had very precise estimates of the Earth’s rotation rate. A comparison between these estimates and an atomic clock revealed an apparently shorter day length in recent years. But there is a surprising revelation once we remove the variations in rotation speed that we know occur due to tidal and seasonal effects. Although Earth reached its shortest day on June 29, 2022, the long-term trajectory appears to have shifted from shortening to lengthening as of 2020. This change is unprecedented in the last 50 years. The reason for this change is not clear. It could be due to changes in weather systems, with back-to-back La Niña events, although these have happened in the past. It could be the increased melting of the ice sheets, although they have not deviated significantly from their steady melting rate in recent years. Could it be related to the volcanic eruption in Tonga that released huge amounts of water into the atmosphere? Probably not, since it happened in January 2022. Scientists have hypothesized that this recent, mysterious change in the planet’s rotation speed is related to a phenomenon called the “Chandler oscillation”—a small deviation in Earth’s spin axis with a period of about 430 days. Observations from radio telescopes also show that the wobble has decreased in recent years. the two may be related. A final possibility, which we think is plausible, is that nothing in particular has changed in or around Earth. It could simply be long-term tidal effects working in tandem with other periodic processes to cause a temporary change in the Earth’s rotation rate.
Do we need a “negative leap second”?
An accurate understanding of the Earth’s rotation rate is crucial for a number of applications – navigation systems such as GPS would not work without it. Also, every few years timekeepers insert leap seconds into our official timelines to make sure they don’t become out of sync with our planet. If the Earth shifted to even longer days, we might have to incorporate a “negative leap second”—that would be unprecedented and could break the internet. The need for negative leap seconds is considered unlikely at this time. For now, we can welcome the news that—at least for a while—we all have a few extra milliseconds in each day. This article is republished from The Conversation under a Creative Commons license. Read the original article. Image credit: qimono / 504 images
