Earth is far from a static, rigid sphere. As water, ice, and air continuously move across the globe in response to the changing seasons, they silently redistribute enough weight to push the planet’s center of mass back and forth by fractions of an inch relative to its geometric center.
Researchers at NASA’s Jet Propulsion Laboratory (JPL) in Southern California have now developed an advanced method designed to measure these seasonal shifts with exceptional precision. Their innovative technique and the resulting findings have been detailed in a new study published in the Geophysical Journal International.
The research sheds new light on how major seasonal changes—including melting snowpack, moving ocean currents, and dense winter air masses—redistribute enormous quantities of mass across Earth’s surface throughout the year. For scientists, tracking this subtle motion is not merely an academic exercise. Earth’s center of mass serves as an essential reference point for satellite navigation systems, global mapping, and precise measurements of surface elevation.
Earth’s Center Is Constantly Moving
For decades, scientists have worked to define and accurately locate Earth’s center of mass using a variety of observations gathered from space. The primary challenge is that this center does not remain in one fixed location.
If Earth behaved like a completely solid, uniform blue marble, its center of mass would align perfectly with its geometric center. Instead, the dynamic planet responds continuously to the shifting weight of water, ice, and the atmosphere. As those materials migrate around the globe, Earth’s center of mass circles around its geometric center by as much as several millimeters.
Determining the exact magnitude of that movement has historically proven difficult. The two most recent international estimates, produced in 2017 and 2023, differ by 0.27 inches (7 millimeters)—an amount roughly equivalent to the height of three nickels stacked together. Notably, that disagreement between scientific models is nearly as large as the physical movement researchers are attempting to measure.
To narrow this persistent uncertainty, JPL geoscientist Donald Argus led the development of a new approach relying on extremely precise satellite tracking. Because gravity causes satellites to orbit around Earth’s center of mass, scientists can detect minute, highly revealing changes in the distances between orbiting spacecraft and tracking stations firmly anchored on the ground whenever that center shifts.
Satellites Help Pinpoint Earth’s Center
The use of artificial satellites to determine the location of Earth’s center of mass has a long history in modern geodesy. In fact, two dense metal satellites launched back in 1976 and 1992 were built specifically for that exact purpose.
Known as the Laser Geodynamics Satellites, or LAGEOS 1 and 2, these objects resemble 900-pound (408-kilogram) disco balls covered entirely in reflective prisms. Ground stations spread across more than 20 countries continuously fire lasers at the orbiting satellites and measure the exact time it takes for the signals to return.
However, a persistent challenge with this traditional method has been that laser tracking stations are not evenly distributed around the globe, leaving gaps in data collection.
The newly developed approach addresses that limitation in two significant ways. First, researchers combine traditional laser measurements with modern GPS tracking and orbital information gathered from several satellites flying in low Earth orbit, giving them a much wider range of objects to follow and analyze. Second, the team accounts for how the changing weight of accumulated water and ice actually bends Earth’s crust, meaning that the ground stations themselves move slightly as the solid surface beneath them shifts under heavy loads.
Argus developed this sophisticated technique in collaboration with 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," Argus said. "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 practical importance extends far beyond the confines of a scientific laboratory. Felix Landerer, a coauthor of the study at JPL, noted that while these movements might appear minuscule, modern society relies heavily on extremely accurate positioning measurements. By unraveling and understanding the physical mechanisms that alter reference systems, scientists can build vastly improved reference systems that ultimately benefit everyday mapping and navigation—ranging from global shipping logistics to precision agriculture.
Snow, Rain, and Oceans Shift Earth’s Balance
To better understand the phenomenon, the researchers tracked the seasonal movement of Earth’s center of mass and successfully traced it to three major environmental sources: the oceans, the atmosphere, and continental water, which encompasses land ice, seasonal snow, lake and river water, soil moisture, and underground groundwater reserves.
The environmental effects fluctuate continuously throughout the year as water and air accumulate in different regions of the world.
For instance, snow accumulation across North America and Eurasia reaches its greatest volume in March. That massive added weight 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 accumulated rainwater, holding an estimated 2,400 gigatons of water. That enormous concentration of mass pulls Earth’s center of mass 2.2 millimeters toward South America.
Another prominent contribution arrives later in the annual cycle. Monsoon rains in Southeast Asia peak at roughly 600 gigatons in November—six months after the Amazon maximum—and add a smaller, yet measurable amount to the planet’s annual center-of-mass movement.
The Pacific Ocean Has an Outsized Influence
Ocean water also plays a major role in driving these seasonal shifts. From August through October, melting ice and seasonal rainfall add fresh mass to the global oceans, collectively pushing Earth’s center of mass toward the South Pacific Ocean.
Because the Pacific Ocean is so vast, changes in its total mass can easily outweigh gains or losses occurring in other ocean basins. Even so, seasonal changes observed in regional bodies of water, including the Mediterranean, Red, North, Baltic, and Barents seas, each make smaller individual contributions to the overall movement of Earth’s center.
The atmosphere adds yet another complex layer to the global cycle. Researchers utilized a sophisticated model created by the European Centre for Medium-Range Weather Forecasts to estimate how seasonal atmospheric changes alter Earth’s center of mass. Their detailed analysis found that cold and dense winter air shifts the planetary 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
Importantly, the mass estimates produced by this new analytical approach strongly agree with independent observations gathered by the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission.
Launched in 2018, GRACE-FO consists of twin satellites that measure monthly changes in Earth’s gravitational field. Much of that changing gravity field is directly caused by water moving above, on, and below the planet’s surface.
The satellites travel together in an extremely precise formation. When the leading spacecraft passes over a geographical region containing extra mass, such as a massive river basin swollen with seasonal water, the slightly stronger gravitational pull changes the distance between the two orbiting satellites by a tiny but clearly detectable amount.
GRACE-FO operates as a joint partnership between NASA and the German Research Centre for Geosciences (GFZ). Looking ahead, scientists are already preparing the next generation of these crucial space measurements. The upcoming GRACE-Continuity (GRACE-C) mission is currently targeting a launch in late 2028, with the core intention of continuing the nearly 25-year unbroken record of tracking how water and mass move dynamically around our planet.
