Believe it or not, the length of each day is not always exactly 24 hours. Though the changes may be virtually undetectable to the regular person, scientists are uniquely aware of the lengthening and shortening of days over time. Over months and years, the length of each day varies by tiny increments of time in agreement with Earth’s shifting systems. These shifts do not significantly affect the total hours of daylight or nighttime in a given season, but instead refer to millisecond shifts in the measurement of a 24-hour day.

Billions of years ago, the average day on Earth was only about 19 hours; the tidal force of the Moon causes tidal friction on Earth, gradually slowing Earth’s rotation and adding about 2.3 milliseconds to the day every century. Seismic activity, glaciation, weather, and ocean circulation also play a role in day length variations on shorter timescales, ranging from seasonal to multidecadal. In the same way that a figure skater gains speed as she brings her hands closer to her body, the rate of Earth’s rotation increases as mass—like ice—moves towards the poles.

As strange as it may seem, Earth’s rotation axis is not perfectly fixed with regard to the solid Earth; rather, the pole of rotation moves across the surface of the Earth in a semi-circular path, moving a few meters each year. This phenomenon is referred to as polar motion, and it governs much of Earth’s rotational speeding and slowing. Scientists have discovered that polar motion is caused by a number of factors, like glacial rebound, mantle convection, and mass loss in Greenland. Length of day variances occur on seasonal, decadal, and millennial timescales. These shifts are by mere milliseconds and their causes are complex and often not completely understood.

Glacial Rebound

During the last ice age, glaciers pressed down on Earth’s surface, causing the land beneath them to sink. As the glaciers melted, the land slowly returned to its original position, causing a shift in Earth’s rotation as the distribution of mass changes. Glacial rebound occurs on a variety of timescales, beginning soon after ice starts to melt. While North America is still responding to the loss of glacial ice at the end of the last ice age about 12,000 years ago, Greenland and Antarctica are already experiencing additional glacial rebound as modern ice sheets shrink.

Mantle Convection

Mantle convection refers to the circulation of rock in the mantle caused by heat from the core. Mantle convection can affect plate motion and regional topography, another catalyst for shifting mass distribution across the surface. In light of recent research, core-mantle coupling is now recognized as a likely source of interdecadal variations in polar motion. This includes complex interactions between the mantle, fluid outer core, and solid inner core exchanging angular momentum. These exchanges may be connected to lurching jumps of Earth’s constantly drifting magnetic field, as well as shifts in the length of day.

Ice Mass Loss

As ice melts in Greenland, sea levels increase and and that mass of water is redistributed across the planet, ultimately causing drift in Earth’s rotation. Since Greenland’s location is farther from the pole, ice loss in Greenland has a far greater effect on drift than ice melting closer to the pole in Antarctica. Ice melting varies on seasonal timescales on top of longer-term trends, so the effect on the pole of rotation can be complex. As ice loss in Greenland continues at an alarming rate, its influence will continue to grow.

Additional Causes

Mass redistribution in the atmosphere, ocean, cryosphere, hydrosphere, and solid Earth can all cause changes between seasons. For example, shifts in zonal wind patterns and atmospheric circulation are responsible for around 90% of seasonal length of day variations. La Niña and El Niño events also play a role in day length variation: El Niño events are associated with longer days whereas La Niña events are associated with shorter days. El Niño and La Niña events are a continual see-saw, with 2-7 years between one El Niño event and the next. Major earthquake events also have the capacity to shift Earth’s rotational speed on short timescales, causing a shift in Earth’s figure axis (the axis Earth balances on) which in turn changes the way the Earth wobbles and rotates.

Anthropogenic Impacts

In addition to natural processes speeding and slowing the rate of rotation, anthropogenic activities have also caused shifts in day lengths over time. Extensive groundwater withdrawals, specifically from regions in the midlatitudes, have been linked to an additional 31.5 inch tilt of the Earth. Increasing tilt occurs when water and ice is redistributed from the equator and midlatitudes to the poles, causing the Earth to spin at a faster rate.

How do we make these calculations?

If all these effects are so incredibly small, how do we even know about them? These precise measurements rely on a complex system of instruments around the world that collectively set a reference frame for the Earth’s position. This includes instruments on the ground and in orbit—Very Long Baseline Interferometry (VLBI) telescopes, Satellite Laser Ranging (SLR) stations, Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) systems, and Global Navigation Satellite System (GNSS) satellites and ground stations. Together, these build a high-precision coordinate system that can be used to calculate the Earth’s position, orientation, or rotation. That includes a Celestial Reference Frame—defining Earth’s position using the stars—and a Terrestrial Reference Frame—defining the position of everything on the surface of the Earth. The International Earth Rotation Service uses these tools to record changes in the exact position of Earth’s pole of rotation and the length of each day. Precise observations allow researchers to study the causes of the small variations in length of day, modeling different processes to understand the effect each can have.

Why does it matter?

Identifying and measuring these tiny differences, though interesting in and of itself, helps scientists to better understand Earth’s interior and exterior systems. Furthermore, this allows scientists to create more accurate oceanic and atmospheric models, improve weather predictions, and measure movements within the Earth’s core. Monitoring and understanding the processes that impact polar motion allows scientists to understand the human impacts on these systems, especially if human impacts begin to cause more significant changes in length of day variations.