If any of you have heard that Earth will soon have 25 hour days, that may sound about right. The idea itself is not wrong: scientists do expect the Earth’s rotation to continue to slow down. What often gets lost in that story is the pace. These changes take place so slowly that they go unnoticed in everyday life. No clock will suddenly be late, and no calendars will have to be readjusted.
It is about subtle shifts that are measured over decades and centuries with instruments that hardly any of us see. The story is not about a dramatic change in the way time works, but about tiny shifts driven by gravity, water and ice, accumulating quietly over time scales far larger than a single human lifetime.
The day seems stable because life is organized around it. However, that is only one way to define a day. If the rotation of the Earth is measured in relation to distant stars, a slightly shorter unit is obtained, the sidereal day. The difference exists because the Earth rotates and moves in its orbit at the same time. In order for the Sun to be in the same place in the sky again, the planet has to turn a little more.
Why are the days on Earth slowly getting longer?
Even a solar day is not perfectly stable. It is slightly lengthening and shortening, but on a very long time scale there is a clear trend: the days are definitely getting longer.
ShutterstockThe moon plays a key role in that slowdown. Its gravity pulls on the oceans, making tidal jumps that don’t perfectly follow the moon’s position, as friction slows the water’s movement. That friction takes away a small part of the Earth’s rotational energy. As the energy is lost, the rotation slows down, and some of the energy is transferred to the Moon, causing it to slowly move away from Earth. The simplest comparison is a chair that spins as the leg touches the floor: the spin continues, but more slowly.
Scientists also track mass changes on Earth’s surface. Data spanning more than 120 years show that melting ice, retreating groundwater, shrinking glaciers and rising sea levels are changing the distribution of the planet’s mass. When large amounts of water move from the land to the oceans, the Earth’s balance changes. This causes a slight movement of the axis of rotation, the so-called polar motion, and lengthens the day by a very small amount of time. Since around 2000, these changes have been accelerating, and researchers associate them with the accelerated melting of Greenland and Antarctica.
In the past, changes in rotation were monitored by observing the apparent motion of the stars. Today, much more precise methods are used, including the measurement of radio signals from distant quasars and laser tracking of satellites. With the help of machine learning methods, scientists have separated the influences of various factors for more than 12 decades. Most of the recurring fluctuations are related to water, ice, glaciers and sea changes, while a smaller part originates from the interior of the planet.
Is man responsible for everything? The answer is complex. Natural climate cycles have always shaped these changes, but recent decades show a strong link between human activity and accelerated loss of ice mass and groundwater. Natural rhythms still exist, but human influence now adds weight to the scale.
When could the Earth actually reach a 25-hour day? This is where perspective is lost. There is no year to mark on the calendar. Based on current understanding of the Earth-Moon system, the transition to a 25-hour day could take about 200 million years. It is so far-fetched that it has no practical significance for people or society. The idea is real, but the time frame is almost unimaginable.
For now, the day length changes at the millisecond value level. Quietly. Almost imperceptibly, writes TToI.