Earth’s center of mass does not stay perfectly still. As water, ice, and air move around the planet with the changing seasons, they redistribute enough weight to push the center of mass back and forth by fractions of an inch relative to Earth’s geometric center. This subtle yet constant motion is carefully tracked by scientists at NASA, because Earth’s center of mass serves as an essential reference point for satellite navigation, global mapping, and accurate measurements of surface elevation.

Researchers led by NASA’s Jet Propulsion Laboratory in Southern California have now developed a method designed to measure these seasonal shifts with exceptional precision. Their technique and results are described in a new study published in Geophysical Journal International. The research illustrates how major seasonal changes, including melting snow, moving ocean water, and dense winter air, redistribute enormous amounts of mass across Earth’s surface throughout the year.

Earth’s Center Is Constantly Moving

Scientists have spent decades developing methods to define and locate Earth’s center of mass using observations from space. The core challenge is that the center does not remain in one fixed location. If Earth behaved like a completely solid blue marble, its center of mass would line up neatly with its geometric center. Instead, the planet responds dynamically to the shifting weight of water, ice, and the atmosphere. As those materials move across the globe, Earth’s center of mass circles around its geometric center by as much as several millimeters.

Determining the exact size of that movement has historically proven difficult. The two most recent international estimates, produced in 2017 and 2023, differ by 0.27 inches, or 7 millimeters, which is roughly equivalent to the height of three nickels stacked together. That disagreement among scientific models is nearly as large as the actual movement scientists are attempting to measure, creating a frustrating margin of uncertainty for researchers relying on precise geodetic data.

To narrow that uncertainty, JPL geoscientist Donald Argus led the development of a new approach that relies on extremely precise satellite tracking. Gravity causes satellites to orbit directly around Earth’s center of mass. As that center shifts due to environmental changes on the surface, scientists can detect minute changes in the distances between orbiting satellites and tracking stations anchored to the ground.

Satellites Help Pinpoint Earth’s Center

Scientists have long used satellites to determine the precise location of Earth’s center of mass. Two dense metal satellites launched in decades past—specifically in 1976 and 1992—were built entirely for that purpose. The Laser Geodynamics Satellites, known as LAGEOS 1 and 2, resemble 900-pound, or 408-kilogram, disco balls covered with reflective prisms. Ground stations spread across more than 20 countries fire high-powered lasers at the satellites and precisely measure the returning signals to calculate orbital trajectories with incredible accuracy.

One ongoing challenge with this traditional method is that the laser tracking stations are not evenly distributed around the globe, leaving gaps in observational coverage. The new approach developed by Argus and his team addresses that limitation in two key ways. Researchers combine the traditional laser measurements with advanced GPS tracking and orbital information from several satellites operating in low Earth orbit, giving them a much wider range of celestial objects to follow. They also account for how the changing weight of surface water and ice actually bends Earth’s crust, meaning the ground stations themselves move slightly as the solid surface beneath them shifts under environmental loads.

Argus developed the technique alongside scientists from JPL’s satellite orbit determination team, the University of Nevada, the University of Montana, and the Helmholtz Centre for Geosciences in Germany.

"We’re now estimating the size of the movement of Earth’s mass center back and forth each year to be about half of what we believed it to be eight years ago," said Argus. "Our findings suggest that the mass of Earth’s water and air moving between the hemispheres is smaller than previously thought."

Although the distances involved are tiny, their importance extends far beyond the laboratory. Felix Landerer, one of the study’s coauthors at JPL, noted that while these movements might appear minuscule, the modern world relies heavily on extremely precise positioning measurements. By unraveling and understanding the mechanisms that change reference systems, researchers can build better reference systems that ultimately benefit mapping and navigation—ranging from global shipping logistics to precision agriculture.

Snow, Rain, and Oceans Shift Earth’s Balance

The researchers followed the seasonal movement of Earth’s center of mass and successfully traced it to three major sources: the oceans, the atmosphere, and continental water, which is made up of land ice, snow, lake and river water, soil moisture, and deep groundwater. The effects change continuously throughout the year as water and air accumulate in different parts of the world.

Snow across North America and Eurasia reaches its greatest accumulation in March. That massive weight of frozen precipitation shifts Earth’s center of mass about 3 millimeters toward the North Pole.

Just a month later, in April, the Amazon River basin reaches its seasonal maximum for rainwater, holding an estimated 2,400 gigatons of water. That enormous concentration of liquid mass pulls Earth’s center of mass 2.2 millimeters toward South America.

Another contribution arrives later in the year from a different hemisphere. Monsoon water in Southeast Asia peaks at 600 gigatons in November, occurring six months after the Amazon maximum, and adds a smaller yet measurable amount to the annual movement.

The Pacific Ocean Has an Outsized Influence

Ocean water also plays a major role in the overall seasonal shift. From August through October, seasonal meltwater and heavy rainfall add mass to the global oceans, pushing Earth’s center of mass toward the South Pacific Ocean.

Because the Pacific is so vast, changes in its mass can outweigh gains or losses occurring in other ocean basins. Even so, seasonal changes in smaller bodies of water, including the Mediterranean, Red, North, Baltic, and Barents seas, each make smaller, distinct contributions to the continuous movement of Earth’s center.

The atmosphere adds yet another layer to the complex global cycle. Researchers used an advanced model created by the European Centre for Medium-Range Weather Forecasts to estimate how seasonal atmospheric changes alter Earth’s center of mass. Their analysis found that cold and dense winter air shifts the balance toward Arabia, Asia, and northern Africa around December 21 each year. Conversely, around June 21, the corresponding atmospheric effect occurs over South America and South Africa.

GRACE Satellites Confirm the Mass Changes

The mass estimates produced by the new analysis strongly agree with independent observations from the Gravity Recovery and Climate Experiment Follow-On mission, widely known as GRACE-FO. Launched in 2018, GRACE-FO consists of twin satellites that measure monthly changes in Earth’s gravitational field. Much of that changing gravity is driven by water moving above, on, and below the planet’s surface.

The satellites travel together in an extremely precise formation along the same orbital path. When the leading spacecraft passes over a geographic region containing extra mass, such as a river basin swollen with seasonal water, the slightly stronger gravitational pull alters the distance between the two satellites by a tiny but detectable amount. GRACE-FO is operated as a partnership between NASA and the German Research Centre for Geosciences.

Scientists are already preparing the next generation of these vital measurements. The upcoming GRACE-Continuity mission is targeting a launch in late 2028 and is intended to seamlessly continue the nearly 25-year record established by the GRACE series in tracking how water and mass move around our dynamic planet.