The planet Earth constantly spins on its axis, defining the daily cycle of light and darkness. The time for one complete rotation relative to the Sun is the Length of Day (LOD), which is approximately 24 hours. Although often considered fixed, the rotation rate is subject to continuous, minuscule fluctuations. Recent scientific measurements show the Earth has been rotating slightly faster, an observation that has drawn attention. This temporary acceleration, measured in milliseconds, deviates from the much slower, predictable trend that governs our planet over vast spans of time.
The Long-Term Trend of Deceleration
Over Earth’s history, the planet’s spin has followed an overarching pattern of gradual slowing down. This secular trend is primarily caused by tidal friction, resulting from the gravitational interplay between the Earth and the Moon. The Moon’s pull creates tidal bulges in the oceans and solid body. Since the Earth rotates faster than the Moon orbits, the tidal bulge is pulled slightly ahead of the direct line between the two bodies.
This misalignment causes the Moon’s gravity to exert a small braking force, or torque, on the Earth’s rotation. The resulting friction dissipates rotational energy, slowing the planet’s spin and lengthening the day. To conserve total angular momentum, the energy lost by the Earth is transferred to the Moon, causing it to spiral slowly outward at about 3.8 centimeters per year.
Geological and astronomical records confirm this deceleration, showing that the average length of a day has increased by about 1.8 to 2.3 milliseconds per century. This slow, predictable increase in the LOD provides the fundamental baseline against which all shorter-term fluctuations are measured.
Short-Term Factors Driving Speed Fluctuations
The recent finding that the Earth is momentarily speeding up is a short-term fluctuation driven by geophysical and atmospheric processes, not a reversal of the long-term trend. Changes in the distribution of mass on and within the planet, such as the shifting of large fluid masses, alter the Earth’s moment of inertia. According to the conservation of angular momentum, if mass moves closer to the rotation axis, the Earth spins faster; if it moves farther away, it slows down.
The movement of the atmosphere, particularly changes in global wind patterns, is a primary contributor to these high-frequency changes. Atmospheric angular momentum is exchanged with the solid Earth, causing predictable seasonal variations in the LOD. Ocean currents also play a role, as major phenomena like El Niño and La Niña redistribute vast amounts of water mass, subtly affecting the rotation rate.
Movements of the Earth’s liquid outer core contribute significantly to decadal variations in rotational speed. The core’s motion is not synchronized with the solid mantle and crust, and interactions at the core-mantle boundary transfer angular momentum to the surface layers. Scientists suggest that a recent, slight slowdown in the core’s rotation could be responsible for the temporary acceleration observed in the solid Earth since 2020. The Chandler Wobble, a small, natural oscillation of the Earth’s axis, is also excited by fluctuating pressure on the ocean floor and in the atmosphere, contributing to minor changes in the LOD.
Measuring the Length of Day
Monitoring the minute fluctuations in Earth’s rotation requires extremely precise measurement techniques. The Length of Day is determined by tracking the difference between two distinct time scales: astronomical time and atomic time. Universal Time 1 (UT1) is the astronomical time scale based on the Earth’s actual, irregular rotation relative to distant quasars.
Coordinated Universal Time (UTC) is the modern civil time standard, based on International Atomic Time (TAI). TAI is a highly stable time scale derived from the weighted average of over 450 atomic clocks worldwide. Since UTC uses the uniform atomic second, its day is exactly 86,400 seconds long. The difference between the irregularly rotating UT1 and the steady UTC is precisely measured to track the LOD.
The primary technique used to measure this difference, known as UT1-UTC, is Very Long Baseline Interferometry (VLBI). VLBI involves a global network of radio telescopes simultaneously observing distant, fixed celestial objects called quasars. By measuring the tiny time delay between the signal’s arrival at different telescopes, scientists determine the Earth’s precise orientation and rotation angle. The International Earth Rotation and Reference Systems Service (IERS) uses this data to predict and publish the Earth Orientation Parameters, which include the LOD.
The Impact on Coordinated Universal Time
The variability in the Earth’s rotation has significant practical implications for global time synchronization, managed through Coordinated Universal Time. To prevent UTC from drifting too far from the solar time represented by UT1, the International Earth Rotation and Reference Systems Service ensures the difference between the two never exceeds 0.9 seconds. This reconciliation mechanism is the leap second, a one-second adjustment added to or, theoretically, subtracted from UTC.
Historically, due to the long-term deceleration trend, all 27 leap seconds inserted since 1972 have been positive, adding a second to the day to allow the slower Earth to catch up with atomic clocks. However, the recent acceleration trend since 2020 has caused the Earth’s rotation to outpace atomic time. This unprecedented situation raised the prospect of requiring the world’s first “negative leap second,” which would involve suppressing one second from the clock to resynchronize it with the faster Earth.
A negative leap second presents a complex technical challenge for modern digital systems, which handle the addition of a second more easily than the subtraction of one. Some researchers predicted a negative leap second would be necessary around 2029, though the exact timing is uncertain. In response to the increasing complexity and potential disruption caused by the unpredictable nature of leap seconds, an international agreement was reached to phase out the current system by 2035. This decision allows UTC to run independently of the Earth’s variable rotation for a longer period, relying on other methods to manage the growing discrepancy in the future.