The current length of a day is approximately 24 hours. This measurement is not fixed, as the planet’s rotation speed is gradually slowing down over immense spans of time. This deceleration is a fundamental cosmic process driven by the gravitational relationship between Earth and its Moon. Precise measurements demonstrate that this change is still happening today, leading to the eventual reality of a 25-hour day.
The Engine of Change: Tidal Friction
The primary force responsible for the Earth’s slowing rotation is the gravitational interaction with the Moon, a phenomenon known as tidal friction. The Moon’s gravity pulls on Earth’s oceans, creating bulges of water on both the near and far sides of the planet. Because Earth spins much faster than the Moon orbits, the rotation attempts to drag these water bulges along with it, ahead of the Moon’s direct line of sight.
The gravitational pull of the Moon then acts on these leading bulges, exerting a slight backward force, or torque, on the Earth. This constant drag, where ocean water rubs against the seafloor in shallow seas, dissipates Earth’s rotational energy as heat. This process slows the planet’s spin, which in turn lengthens the day. The angular momentum lost by Earth’s rotation is simultaneously transferred to the Moon’s orbit, causing the Moon to recede from Earth by about 3.8 centimeters each year.
This exchange of angular momentum has been ongoing since the formation of the Earth-Moon system. The drag created by tidal friction is powerful enough to brake the rotation of an entire planet, though the effect is only measurable with highly sensitive instruments.
The Slow March of Time: Calculating the 25-Hour Day
The rate at which Earth’s rotation is slowing is extremely gradual, averaging an increase in the length of the day by about 1.7 to 2.3 milliseconds per century. This minute change is nearly imperceptible in human lifetimes but accumulates significantly over geological timescales. Projecting this rate into the future allows scientists to estimate when the 25-hour day will finally arrive.
Based on current models, the length of the day is projected to reach 25 hours in roughly 200 million years. This prediction places the event in the deep future. Non-tidal influences, such as the redistribution of mass within the planet from mantle convection and post-glacial rebound, are minor factors compared to the long-term tidal interaction.
The exact prediction remains complicated because the rate of slowing has not been constant throughout history, often occurring in distinct stages rather than a steady line. Furthermore, the Sun is expected to enter its red giant phase in approximately five billion years, a stellar event that will dramatically alter conditions on Earth, likely before the full 25-hour day is achieved.
Days Past: Evidence from Deep History
The evidence that Earth’s rotation has been decelerating for eons is preserved in the geological and paleontological records. Scientists use the growth patterns in ancient organisms to reconstruct the planet’s rotational history. Specifically, certain fossil corals and bivalves exhibit fine growth lines that represent daily increments, similar to tree rings.
By counting the number of these daily growth lines within an annual growth band, researchers can determine how many days were in a year in the past. For example, fossil evidence from the Devonian Period, about 400 million years ago, indicates that a year contained approximately 400 days. Similarly, during the age of the dinosaurs, roughly 180 million years ago, a year had about 377 days.
Another source of evidence comes from sedimentary rock formations known as tidal rhythmites. These layered rocks, which date back hundreds of millions to billions of years, preserve the rhythmic patterns of ancient tidal cycles. Analysis of these rhythmites, such as those from the Late Proterozoic, confirms that the Earth’s spin rate was substantially faster in the distant past. These geological records validate the physical models of tidal friction.