Seasonal water, ice, and air movements shift Earth’s center of mass by fractions of an inch, according to a study published in Geophysical Journal International. Researchers at NASA’s Jet Propulsion Laboratory developed a hybrid satellite tracking technique showing this annual movement is about half of prior estimates.
If Earth were a solid, flawless marble, its center of mass would map neatly onto its geometric center. Instead, our planet sloshes and sags under the immense, shifting weight of global water, moving ice sheets, and dense atmospheric air. This constant redistribution causes Earth’s true mass center to swivel around its geometric midpoint by as much as several millimeters every year.
Tracking these seasonal oscillations is not just an academic exercise. Satellites orbiting the globe rely on the planet’s center of mass as an invisible navigational benchmark. Until recently, however, measuring the exact size of this planetary swivel proved surprisingly difficult.
Resolving a Seven-Millimeter Discrepancy in Global Geodesy
For decades, geophysicists have tracked mass movements using satellite laser ranging, relying heavily on specialized metal satellites. Launched in 1976 and 1992, the 900-pound (408-kilogram) Laser Geodynamics Satellites—LAGEOS 1 and 2—resemble metallic disco balls studded with reflective prisms. Ground stations scattered across more than 20 countries track these targets with extreme laser precision.
A major limitation has plagued this approach: the uneven distribution of ground stations around the globe, with roughly half located in North America and Europe. This geographic imbalance causes the geodetic network center to diverge from a genuine global average.
The uncertainty in past calculations is stark. The two most recent global estimates, published in 2017 and 2023, differed by 0.27 inches (7 millimeters). That discrepancy is about the height of three stacked nickels—a margin of error nearly as large as the seasonal motion itself.
A New Hybrid Tracking Method Combining GPS and Low-Earth Satellites
To clear up this ambiguity, a research team led by JPL geoscientist Donald Argus developed an advanced technique. The method blends traditional satellite laser ranging with GPS tracking and orbital data from multiple low-Earth-orbit satellites, expanding the array of targets.
Crucially, the updated technique accounts for how the immense weight of shifting water and ice deforms Earth’s crust, taking ground stations along for the ride as the solid surface flexes. The study was authored by researchers 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. Our findings suggest that the mass of Earth’s water and air moving between the hemisphere is smaller than previously thought.”
Donald Argus, JPL geoscientist
Mapping the Planetary Heartbeat Across Continents and Oceans
The research team cataloged how continental water, atmospheric pressure, and ocean currents drive the seasonal wobble. Snow accumulation across North America and Eurasia peaks in March, shifting Earth’s mass center about 3 millimeters toward the North Pole. A month later, rainwater in the Amazon River basin peaks at 2,400 gigatons, swinging the balance point 2.2 millimeters toward South America.
Later in the year, monsoons in Southeast Asia reach a maximum of 600 gigatons in November, adding a subtle contribution to the annual cycle. Meanwhile, the atmosphere also exerts influence: dense winter air tips the balance over Arabia, Asia, and northern Africa around December 21, and over South America and South Africa around June 21.
To verify these dynamics, the team cross-checked the calculations with NASA’s GRACE-FO mission, whose twin satellites measure microscopic changes in gravity as water flows above and below ground. The European Centre for Medium-Range Weather Forecasts model also provided essential data for estimating atmospheric shifts.
Why Millimeter-Scale Precision Matters for Modern Navigation
While these shifts unfold on a microscopic scale, our technology depends entirely on keeping track of them. Reference systems underpin everything from automated shipping logistics to precision agriculture.
“While these movements might appear tiny, our modern world relies on extremely accurate positioning measurements. By unraveling and understanding the mechanisms that change reference systems, we can build better reference systems that ultimately benefit mapping and navigation — from global shipping logistics to precision agriculture.”
Felix Landerer, study coauthor at JPL
As scientists continue refining these geodetic frameworks, the new models provide a sharper, more reliable baseline for understanding how our dynamic planet distributes its weight from season to season.