A 25-Hour Day Is Coming, Scientists Say: Here’s How and Why

Earth’s familiar 24-hour cycle is slowly stretching, and scientists say a 25-hour day is likely in the distant future. Research suggests this extra hour could arrive in roughly 200 million years, as Earth’s rotation gently slows under the pull of the Moon and other subtle planetary changes.

Experts stress that this shift unfolds over huge timescales, not human lifetimes. The change in spin is measured in tiny fractions of a second each century, yet those minute differences accumulate over hundreds of millions of years, reshaping the length of a day without disrupting present-day life.

How a 25-hour day links with Earth's ancient and modern timekeeping

Modern clocks still follow the same basic structure that tracks Earth’s spin relative to the Sun, known as the solar day. A solar day is slightly longer than a sidereal day, which is measured against distant stars, because Earth is rotating while also travelling around the Sun at the same time.

Scientific calculations suggest a 25-hour day could develop in about 200 million years, although that estimate carries some uncertainty. Researchers note that the rotation rate has never been fixed. Across Earth’s history, there have been periods with both many more days in a year and with very different day lengths.

Why Earth's rotation is slowing towards a 25-hour day

The main reason for the gradual slowdown lies in the interaction between Earth and the Moon. Lunar gravity pulls on the oceans, raising tidal bulges. Because those bulges sit slightly ahead of the Moon’s position, they tug back on Earth’s rotation, creating a small but continuous braking effect that lengthens the day.

NASA explains that the Moon’s gravitational pull "causes the oceans to bulge slightly, and this interaction acts like a brake, slowing Earth's rotation over time". Scientists also track smaller influences from melting glaciers, shifting ocean masses and movements within Earth’s core and mantle, though these play a much weaker role in changing spin.

The slowdown itself is tiny when measured over a human life. Studies show Earth’s rotation lengthens by around 1.7 milliseconds per century. That is far too small to alter clocks in daily use, yet over hundreds of millions or billions of years, these almost invisible shifts extend the day by several hours.

How Earth's 25-hour day future compares with its deep past

Evidence from rocks and fossils reveals that Earth’s days were once very different. As Sarah Millholland, an assistant professor of physics at MIT, told Live Science, "The Earth has experienced days that were both shorter and longer than it is now at different points in history." Geological records help reconstruct these changes.

Millholland added, "About a billion years ago, the length of a day was only about 19 hours," highlighting how much faster Earth rotated in that era. Corals and tidal sediments from hundreds of millions of years ago indicate there were more than 400 days in a year then, with each day lasting just over 21 hours.

Shortly after Earth formed, models suggest a day may have been under 10 hours long. There were also intervals when days stretched beyond 24 hours, though not for extended periods. These swings show that the present 24-hour standard is simply one stage in a much longer story of changing rotation.

To compare those different eras, scientists summarise key stages of Earth’s day length and year length using geological and astronomical evidence.

While the planet’s spin has shifted, human timekeeping follows a scheme rooted in much later history. "The origin of our time system of 24 hours in a day, with each hour subdivided into 60 minutes and then 60 seconds, is complex and interesting," Dr. Nick Lomb, consultant curator of astronomy at the Sydney Observatory, told ABC.

That structure began with the ancient Egyptians, who used sundials and divided daylight into 10 hours, with extra twilight at the start and end, and the night into 12 hours. "Night-time was divided into 12 hours, based on the observations of stars. The Egyptians had a system of 36 star groups called 'decans' — chosen so that on any night one decan rose 40 minutes after the previous one," Lomb added.

"Tables were produced to help people determine the time at night by observing the decans. Amazingly, such tables have been found inside the lids of coffins, presumably so that the dead could also tell the time." Later, Babylonian astronomers adopted and expanded these ideas, using a sexagesimal, or base-60, system.

The Babylonians divided both hours and minutes into 60 equal units, building on earlier Sumerian mathematics. When mechanical clocks appeared in Europe towards the end of the 16th century, this 24-hour and base-60 pattern became fixed in daily life. That historical choice still shapes how smartphones, watches and computers express time.

Despite that apparent stability, Earth’s spin is not perfectly steady from year to year. Small variations mean atomic clocks and planetary rotation can drift apart. Organisations such as the International Earth Rotation and Reference Systems Service sometimes add "leap seconds" when the difference reaches more than 0.9 seconds, keeping official time aligned with Earth’s actual rotation.

Researchers say the slow move towards a 25-hour day should not cause concern. Konstantin Batygin, a professor of planetary science at Caltech, told Live Science, "The change in Earth's spin rate is happening gradually enough that evolutionary processes can adapt to the changes over time." Life on Earth has already adjusted to many different day lengths.

Batygin also noted that "The relative change in orbital speed would not be noticeable in daily life," underlining how gentle the effect is. For humans, any future adjustment would be handled through timekeeping systems, not through noticeable physical changes in daylight. The shift mainly highlights that even timekeeping reflects ongoing cosmic influences rather than a fixed constant.