Earth’s familiar 24-hour day may not be as permanent as it seems. Scientists say that while a 25-hour day is not happening anytime soon, it could become reality millions of years into the future due to the gradual slowing of Earth’s rotation.
A day on Earth is defined by a solar day — the time it takes for the Sun to return to the same position in the sky. This is slightly longer than a sidereal day, which measures Earth’s rotation relative to distant stars, because the planet is also moving along its orbit around the Sun.
The current 24-hour system has ancient origins. According to Dr Nick Lomb, consultant curator of astronomy at the Sydney Observatory, the structure dates back to the ancient Egyptians, who used sundials and star observations to divide the day and night. Later, the Babylonians introduced the base-60 system for hours and minutes, a format that was cemented by early mechanical clocks in Europe during the 16th century.
However, Earth’s rotation has never been perfectly constant. Scientists explain that the planet is slowly spinning down, mainly due to the gravitational pull of the Moon. This pull creates tidal bulges in Earth’s oceans, which act like a brake on the planet’s rotation.
NASA notes that this process slows Earth’s spin by about 1.7 milliseconds per century. While the change is almost imperceptible on human timescales, over hundreds of millions of years it becomes significant.
Research shows that Earth’s day length has varied dramatically throughout its history. Geological evidence from ancient corals and tidal sediments suggests that around a billion years ago, a day lasted only about 19 hours, with more than 400 days in a year. Shortly after Earth formed, days may have been less than 10 hours long.
Scientists say there have also been periods when days were longer than 24 hours, although these changes occurred gradually. Sarah Millholland, an assistant professor of physics at MIT, has noted that Earth has experienced both shorter and longer days at different points in its geological past.
Experts stress there is no cause for concern. Konstantin Batygin, a professor of planetary science at Caltech, explained that Earth’s slowing rotation happens so gradually that life can adapt without disruption. Even today, small variations in Earth’s spin require scientists to occasionally add “leap seconds” to atomic clocks to keep timekeeping aligned with Earth’s rotation.
In summary, a 25-hour day is not a sign of an impending planetary crisis. Instead, it reflects the slow and natural influence of cosmic forces, reminding us that even time itself evolves over vast spans of history.
