The planet’s spinning slowed down after collision with a Mars-sized celestial body around 4.3 billion years ago.

For billions of years, Earth’s daily cycle has been gradually shifting—a process dictated by celestial forces and planetary events. While the planet now makes a full spin in 24 hours, the day’s duration once used to be vastly different.

This process will continue to evolve in the future, adding around 1.35 seconds every 100,000 years, multiple studies show. At this rate, the day on Earth will be 3.75 hours longer in one billion years.

Earth’s rotation was dramatically altered after a massive impact by a Mars-sized object—commonly known as Theia—around 4.5t billion years ago or 60–175 million years after our planet was born. The collision that created the Moon supercharged Earth’s spin, resulting in a day lasting only about four hours. As the Moon drifted away over time, tidal forces caused Earth’s rotation to gradually slow.

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As Earth’s surface cooled and life emerged, the length of a day steadily increased from 4 hours originally. Around 4.1 billion years ago, when the first primitive life forms appeared, a day lasted roughly 10 hours.

By the time photosynthesis began 3.5 billion years ago, the day had extended to 12 hours.

When complex organisms evolved 1.8 billion years ago, Earth’s rotation had slowed enough to create a 21-hour day. Finally, with the rise of modern humans just a few million years ago, the day had nearly reached today’s familiar 24-hour duration.

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The Moon remains a dominant force in regulating Earth’s rotation but other factors also contribute to minor fluctuations in the planet’s spin. Seismic activity, ocean currents, and atmospheric conditions can subtly shift Earth’s rotational speed. Even climate change plays a role, as melting polar ice redistributes mass towards the equator, slightly slowing the planet’s spin, studies show.

While natural disasters and human activities have only negligible effects on Earth’s rotation, significant planetary events can bring subtle changes. The asteroid impact that wiped out the dinosaurs, for instance, altered the Earth’s rotation by only a fraction of a millisecond. Large reservoirs, like China’s Three Gorges Dam, redistribute water and change the length of a day by 0.06 microseconds.

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Looking further into the future, Earth’s days will continue to grow longer. In about a billion years, as the Sun’s heat intensifies and Earth’s water evaporates, mass redistribution could add a fraction of a second to the planet’s daily cycle.

But the most dramatic change will come in 7.6 billion years, when the Sun expands into a red giant, potentially engulfing Earth and the Moon. Before that cataclysm, the Moon will spiral inward and eventually crash into the planet, bringing an end to our planet’s long history of shifting days.

By that time, life on Earth will be wiped out, astrophysical models suggest.

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