Earth’s Rotation Is Slowing, and Scientists Are Now Explaining Why on Every Timescale

Earth’s day is getting longer, and in the past six months scientists have published research narrowing down why — pointing to forces ranging from the pull of the moon over hundreds of millions of years to a gravitational tug-of-war between the planet’s inner core and mantle playing out over decades.

Averaged over the long term, the length of a day is increasing by roughly 1.7 to 1.8 milliseconds per century, primarily because of tidal friction from the moon, according to geophysicists. Geological evidence — tidal rhythmite deposits in South Australia and growth bands in fossil corals — indicates a day lasted about 21.9 hours some 620 million years ago and close to 21 hours at the start of the Cambrian Period, roughly 600 million years ago. Some models put the length of a day at under 10 hours shortly after the moon formed, about 4.5 billion years ago.

That long-term slowdown is the dominant, oldest-understood driver of the change in day length. But two further layers of research published this year describe additional effects operating on much shorter timescales.

A newer, faster driver: melting ice

In March, researchers from the University of Vienna and ETH Zurich, writing in the Journal of Geophysical Research: Solid Earth, reported that day length is currently increasing by about 1.33 milliseconds per century — a rate they said was unmatched in the past 3.6 million years. The team, led by Mostafa Kiani Shahvandi, reconstructed ancient sea-level and day-length fluctuations using fossilized single-celled marine organisms called benthic foraminifera, then compared that record with recent satellite measurements.

The researchers attributed the acceleration to rising sea levels driven by melting glaciers and ice sheets, which shift mass from the poles toward the equator. Study co-author Benedikt Soja, a professor of space geodesy at ETH Zurich, compared the effect to a figure skater slowing their spin by extending their arms. Soja said the rate of modern climate change appeared unprecedented going back to the late Pliocene, 3.6 million years ago, and predicted that by the end of the century its effect on day length could exceed that of the moon.

The team said the change, though measured in milliseconds, carries practical consequences for systems such as satellite navigation and space missions, which depend on precise knowledge of Earth’s rotation to synchronize atomic clocks with the planet’s actual spin.

A newly explained wobble: the inner core

Separately, a study published in Nature on Sept. 23 by Huifeng Zhang and Mathieu Dumberry, both at the University of Alberta, proposed a mechanism for a different, shorter-term pattern: multidecadal swings of a few milliseconds in day length that have been observed for about 30 years but not fully explained.

The researchers modeled the exchange of angular momentum between Earth’s solid mantle and its liquid outer core, work built around the finding that when the core’s rotation speeds up, the mantle — and the surface with it — slows down to compensate, and vice versa. Combining seismic measurements of the inner core’s rotation with flow models derived from changes in Earth’s magnetic field, Zhang and Dumberry concluded that a gravitational torque between the solid inner core and dense structures deep in the mantle is the primary driver of these decades-long cycles, running on a period of roughly 60 to 70 years. Electromagnetic and mechanical forces at the core-mantle boundary act mainly to resist that torque rather than drive it, the study found.

Dumberry said the inner core “wants to be aligned” with denser regions of the mantle, but is repeatedly pulled out of alignment by flows in the surrounding liquid outer core before gravity slowly pulls it back. The authors said the work offers researchers a way to probe the deep interior of the planet — including the shape and stiffness of the inner core — using nothing more than tiny variations in day length measured at the surface.

Three timescales, three mechanisms

Scientists caution that these effects operate independently and on different clocks, which is why Earth has in recent years also recorded some of the shortest days since precise measurement began, even as the long-term trend points toward longer days. According to predictions from the International Earth Rotation and Reference Systems Service, the shortest day of 2026 was expected around late July, continuing a run of unusually fast rotation observed since 2020 whose cause researchers say is not yet fully understood.

Taken together, the current published research describes at least three layers acting on Earth’s spin: lunar tidal friction slowing the planet over hundreds of millions of years; a climate-linked, sea-level-driven effect measured in decades; and the newly modeled inner core-mantle torque cycling over 60 to 70 years. How these layers interact from year to year, and how much each will contribute going forward, remains an active area of research.

What remains unconfirmed

The precise long-term trajectory of climate-driven day-length change beyond the 21st century has not been independently verified and rests on projections rather than direct observation. The exact cause of the unusually short days recorded since 2020 has not been established. This account is based on published, peer-reviewed research and statements from the study authors; it does not draw on unpublished or preliminary findings.

Sources

Figures on long-term tidal slowing and prehistoric day lengths are drawn from geophysical background reporting and cited scientific literature; the climate-driven rate and the inner core torque mechanism are drawn from the peer-reviewed studies and their institutional and journal coverage below.

Wire services and international press

• Scientific American, “Earth’s Days Are Getting Longer at an Unprecedented Rate. Climate Change Is to Blame.” https://www.scientificamerican.com/article/earths-days-are-getting-longer-climate-change-is-to-blame/
• Forbes, “Earth’s Rotation Is Slowing Faster Than In 3.6 Million Years.” https://www.forbes.com/sites/jamiecartereurope/2026/06/07/earths-rotation-is-slowing-faster-than-in-36-million-years/
• BBC Science Focus, “Climate change will soon make Earth’s days longer.” https://www.sciencefocus.com/planet-earth/climate-change-is-making-days-longer

Peer-reviewed research and journal coverage

• Kiani Shahvandi, M. & Soja, B., “Climate-Induced Length of Day Variations Since the Late Pliocene,” Journal of Geophysical Research: Solid Earth (2026). DOI: 10.1029/2025jb032161
• Zhang, H. & Dumberry, M., “Gravitational torque drives multidecadal variations in length of day,” Nature (2026). DOI: 10.1038/s41586-026-10999-2
• Nature News, “Gravitational tug-of-war inside Earth is changing the length of our days.” https://www.nature.com/articles/d41586-026-03018-x
• Phys.org, “Earth’s deep secret: Why your day isn’t quite 24 hours.” https://phys.org/news/2026-09-earth-deep-secret-day-isnt.html

Institutional statements

• ETH Zurich / University of Vienna joint release, “Climate change slows Earth’s spin: Day lengthening unprecedented in million years.” https://baug.ethz.ch/en/news-and-events/news/2026/03/climate-change-slows-earths-spin-day-lengthening-unprecedented-in-3-point-6-million-years.html

Background on prehistoric day length and real-time monitoring

• timeanddate.com, “After Fast Spins, Earth Is Slowing Down Again,” citing International Earth Rotation and Reference Systems Service data. https://www.timeanddate.com/news/astronomy/earth-rotation-2026
• EarthDate, University of Texas Institute for Geophysics, background on tidal rhythmites and Precambrian day length. https://www.earthdate.org/files/000/002/134/EarthDate_168_C.pdf

Editor’s note on sourcing The claim that a day lasted “21 hours” 600 million years ago, as raised in the original query, is consistent with multiple independent sources citing tidal rhythmite and coral growth-band evidence, though estimates vary slightly (21 to 21.9 hours) depending on the specific dataset and period cited (620 million years ago vs. the start of the Cambrian, roughly 538–600 million years ago). That range is reflected here rather than a single precise figure. Claims about the exact future trajectory of climate-driven slowing beyond 2100, and the cause of the 2020–2026 run of unusually fast rotation, rest on a single research group’s projections or remain actively debated among scientists, and are flagged in the text as unconfirmed rather than stated as settled fact.