Occasionally, headlines like "The Earth is about to turn 25" appear and quickly attract attention. This is not entirely wrong. Scientists really know that the Earth is slowing down. But that change is happening extremely slowly, so slowly that no one can feel it in real life.
We are familiar with 24 hours because it is the standard for daily life and work schedule. However, in astronomy, "day" is not a fixed concept. If we measure the Earth's rotation time based on distant stars, instead of the Sun, we will have a "star day", slightly shorter than the Sun's day.
This difference stems from the fact that the Earth is both rotating and moving around the Sun. More notably, even 24 hours of sunshine is not completely stable. It fluctuates very little and, on a long scale of millions of years, tends to lengthen.
The main reason why the Earth rotates slowly is the Moon. The gravity of the Moon creates tides, causing the ocean to rise and fall when the Earth rotates.
Due to friction between seawater, ocean floor and Earth's crust, tidal masses are not completely aligned with the Moon. This deviation causes part of Earth's rotation energy to be lost, slowing down the rotation speed. In return, the Moon slowly drifts away from Earth.
It can be imagined that this process is like a rotating chair with its feet gently touching the floor: It still rotates, but gradually slows down over time.
Humans cannot feel a day lengthened by just a few thousandths of a second for decades. However, scientists monitor this with extremely accurate atomic clocks, combined with astronomical observations and historical records of eclipses and lunar eclipses.
Very small deviations between the standard time and the actual rotation speed of the Earth are the reasons why scientists once had to add "profit seconds" to keep the global clock from deviating too far from the movement of the planet.
When will there be 25 hours? This is the point that is often misunderstood the most. There is no specific timeline to mark "from this day onwards" Earth has 25 hours. According to scientific estimates, if the Earth-Moon system continues to evolve as it is now, it will take about 200 million years, the length of 1 day will increase to 25 hours.
That means this change does not affect people, civilization or the calendar system in the near future, even very far future.
So, 25 hours is a scientific reality, but it is the story of hundreds of millions of years later. In the entire imagined future of mankind, every day on Earth will still only be 24 hours.
