Why ocean tides act as a brake on Earth's spin
The Moon pulls harder on the side of Earth facing it than on the far side, and the oceans bulge in response. Because Earth rotates faster than the Moon orbits, those bulges are dragged slightly ahead of the Moon's position. The Moon's gravity pulls back on the leading bulge, creating a torque that opposes Earth's rotation—a tidal brake [4][5]. The same tidal friction transfers angular momentum to the Moon, which is why the Moon recedes at about 3.8 millimeters per year [4].
The effect is small at any instant. One modeling study projects that by the year 2404—roughly 400 years from now—the length of the day will have increased by about 6.46 milliseconds, with a corresponding slight decrease in rotational speed [4]. That is a few thousandths of a second, not a noticeable change in daily life, but it is measurable with precise astronomical techniques.
How scientists separate the tidal signal from weather and other noise
Earth's rotation rate fluctuates on many timescales, and tides are only one contributor. On sub-decadal timescales, changes in the length of day are dominated by zonal tides—tidal components that redistribute mass north-south—along with movements of air and water [2]. To isolate the tidal part, researchers calculate and remove the effects of atmospheric and oceanic mass transport, then compare the residual signal to theoretical tidal models [2].
This correction matters. Below about 32 days, removing both atmospheric and oceanic excitation from length-of-day records tightened the agreement between observed and modeled tidal response by up to 70 percent, and confirmed that the ocean's dynamic response introduces a frequency-dependent effect [2]. In other words, the ocean does not respond to tidal forcing as a simple rigid body; its delayed adjustment leaves a detectable fingerprint in Earth's rotation.
Atmospheric tides add a second, smaller layer of tidal influence
The atmosphere also has tides. Sunlight heats the upper air and the surface in a daily cycle, producing atmospheric tides with 24-hour and 12-hour periodicities that slightly alter Earth's rotation [3]. These are distinct from the ocean tides raised by the Moon and Sun, though they modulate the same rotation signal. Very-long-baseline interferometry (VLBI), a technique that tracks distant quasars to measure Earth's orientation with extreme precision, has detected atmospheric tide effects at the expected level [3].
Atmospheric tides can also matter over geological time. A 2024 modeling study examined whether thermal tides in the atmosphere could, under certain conditions, cancel the braking torque from the oceans and solid Earth and stabilize the length of day—a scenario proposed for the Precambrian. The model suggests that a resonance in atmospheric Lamb waves (large-scale atmospheric wave modes) could amplify the thermotidal torque, but when applied to Earth, the predicted resonance in the Phanerozoic was too weak to achieve rotational equilibrium [1]. So atmospheric tides are real and measurable, but on current evidence they do not overturn the ocean-tide braking that dominates the long-term slowdown.
How large is the total tidal effect over long spans of time?
Over centuries, the tidal slowdown is measured in milliseconds of day length [4]. Over geological time, the cumulative effect is far larger: the day was shorter in the distant past, and tidal friction has been lengthening it for hundreds of millions of years. The 400-year projection of about 6.46 milliseconds is a snapshot of a process that, extended over millions of years, adds up to hours [4].
The exact rate depends on the configuration of continents and oceans, because the shape of ocean basins controls how efficiently tidal friction converts rotational energy into heat. The 2009 study quantified the rate of day-length increase from the axial component of the tidal torque acting on the mantle and ocean [5], and the 2022 admittance study showed that the ocean's dynamic response makes the tidal effect frequency-dependent rather than a single fixed number [2]. The practical answer for a general reader is that tides are a persistent, measurable brake on Earth's rotation—small on human timescales, decisive on geological ones.
Sources used in this answer
Thermal tides in neutrally stratified atmospheres: Revisiting the Earth’s Precambrian rotational equilibrium
A model of thermal tides in neutrally stratified atmospheres predicts a Lamb resonance in Earth's Phanerozoic atmosphere, but with an amplitude insufficient to cancel the opposing ocean and solid-Earth torque and achieve rotational equilibrium.
Admittance of the Earth Rotational Response to Zonal Tide Potential
Removing both atmospheric and oceanic excitation from length-of-day records below 32 days improved agreement between observed and modeled tidal admittance by up to 70 percent and confirmed a frequency-dependent ocean dynamic response.
Atmospheric tides in Earth rotation observed with VLBI
VLBI observations detect atmospheric tide effects on Earth rotation at the expected level, and a previously reported discrepancy in the diurnal S1 polar motion term was reduced from about 10 microarcseconds to 5 microarcseconds.
The Long-Term Effects of Lunar Recession on Earth's Rotation, Solar Eclipses, and Climate: A 400-Year Projection
Tidal friction is causing the Moon to recede at about 3.8 millimeters per year and is projected to lengthen the day by about 6.46 milliseconds by the year 2404, slightly slowing Earth's rotation.
Effect of mantle and ocean tides on the Earth's rotation rate
The axial component of the tidal torque acting on the mantle and ocean produces a measurable rate of increase in the length of day.