Days on Earth have gotten mysteriously longer, and scientists don’t (yet) know why


Basics Although Earth reached its shortest day on June 29, 2022, the long-term trajectory appears to have changed from shortening to lengthening since 2020. A mystery for research. Atomic clocks combined with precise astronomical measurements have recently revealed that the length of a day on Earth is suddenly lengthening. This phenomenon has critical implications not only for our measurement of time, but also for things like GPS and other technologies that govern our modern lives. In recent decades, the rotation of the Earth around its axis, which determines the length of a day, has accelerated. This trend has shortened our days; in fact, in June 2022 we hit the record for the shortest day in about half a century. But despite this record, since 2020, this steady acceleration has curiously transformed into a deceleration: the days are getting longer again, and the reason remains unknown at the moment. Although our phone clocks show that a day is exactly 24 hours long, the actual time it takes the Earth to complete a single rotation varies very slightly. These changes occur over periods ranging from millions of years to almost instantaneously; even earthquakes and storms can play a role. So it turns out that a day very rarely corresponds to the magic number of 86,400 seconds. The Constantly Changing Planet For millions of years, Earth’s rotation has slowed due to the frictional effects associated with the tides caused by the Moon. This process adds about 2.3 milliseconds to the length of each day every century. A few billion years ago, a day on Earth was only 19 hours long. Over the past 20,000 years, another process has worked in reverse, speeding up the Earth’s rotation. At the end of the last ice age, the melting of the polar ice caps reduced the pressure on the surface and the Earth’s mantle began to move steadily towards the poles. Just as a ballet dancer spins faster when she brings her arms closer to her body, the axis around which she rotates, so the speed of our planet’s rotation increases as this mantle mass moves closer to the axis of the Earth And this process shortens every day by about 0.6 milliseconds per century. For decades and more, the connection between Earth’s interior and surface also comes into play. Large earthquakes can change the length of the day, although usually by small amounts. For example, the great 2011 Tōhoku earthquake in Japan, with a magnitude of 8.9, is said to have accelerated the Earth’s rotation by a relatively small 1.8 microseconds. Apart from these large-scale changes, over shorter periods of time, weather and climate also have large impacts on the Earth’s rotation, causing variations in both directions. Bimonthly and monthly tidal cycles move mass around the planet, causing changes in day length of up to a millisecond in either direction. We can observe tidal variations in the day length records for 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 precipitation, or groundwater extraction, alters 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. Using radio telescopes to measure Earth’s rotation involves observing radio sources such as quasars. NASA Godard. A comparison between these estimates and an atomic clock revealed an apparently increasingly shorter day length in recent years. But there is a surprising revelation once we remove the fluctuations in rotation rate that we know occur due to tides and seasonal effects. Although Earth reached its shortest day on June 29, 2022, the long-term trajectory appears to have changed from shortening to lengthening since 2020. This change is unprecedented in the past 50 years . The reason for this change is not clear. It could be due to changes in weather systems, such as back-to-back La Niña weather events, although they have happened before. Melting of the ice sheets could be increased, although the ice sheets have not deviated much from their usual melting rate in recent years. Could it be related to the great explosion of the Tonga volcano injecting large amounts of water into the atmosphere? Probably not, since it happened in January 2022. Scientists have speculated that this recent and mysterious change in the planet’s rotation rate is related to a phenomenon called the “Chandler oscillation,” a small deviation of the axis of rotation of the Earth with a period of about 430 days. Radio telescope observations also show that the oscillation has decreased in recent years; the two phenomena could be related.

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A final possibility, which seems plausible to us, is that nothing concrete has changed on or around Earth. They could simply be long-term tidal effects working in parallel with other periodic processes to produce a temporary change in the Earth’s rotation rate. Do we need a “negative leap second”? Accurate knowledge 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 timescales to ensure they are out of sync with our planet. If Earth were to transition to even longer days, we might have to enter a “negative leap second,” which would be unprecedented and could break the Internet. The need for negative leap seconds is considered unlikely at this time. For now, we can rejoice in the news that, at least for a while, we all have a few extra milliseconds each day. This article was written by Matt King, Director of the ARC Australian Center of Excellence in Antarctic Science, University of Tasmania and Christopher Watson, Senior Lecturer in the School of Geography, Planning and Space Sciences, University of tasmania Republished from The Conversation under a Creative Commons license. Read the original article.
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