Earth's Center of Mass Wobbles a Few Millimeters Every Year as Snow Piles Up in March and the Amazon Fills in April. NASA Just Measured It and Found the Wobble Is Half What Everyone Thought.

Every year, enough water moves between the land, the oceans and the air to drag Earth's center of mass back and forth by a few millimeters relative to the planet's geometric center. A team led by NASA's Jet Propulsion Laboratory has now measured that seasonal wobble more precisely than ever, and the answer, published in Geophysical Journal International, is that the swing is roughly half as large as the scientific community assumed eight years ago.

If Earth were a rigid marble, its center of mass would sit exactly at its geometric center. It is not. Ice, water and dense winter air pile up in different places at different times of year, and the planet's crust sags and rebounds under those loads. The result is that the center of mass, which geodesists call the geocenter, circles the geometric center by several millimeters over the course of a year. That matters because every satellite in orbit is, in effect, tethered by gravity to the center of mass, not the geometric center. GPS positioning, sea-level measurements and elevation maps all depend on knowing where that point is.

The two most recent international estimates of the motion, made in 2017 and 2023, differ from each other by 7 millimeters, about the height of three stacked nickels and nearly as large as the motion they were trying to pin down. "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 JPL geoscientist Donald Argus, who led the work. "Our findings suggest that the mass of Earth's water and air moving between the hemispheres is smaller than previously thought."

The classic tool for the job is satellite laser ranging. Two dense metal spheres, LAGEOS 1 and 2, launched in 1976 and 1992, weigh about 900 pounds each and are studded with reflective prisms; ground stations in more than 20 countries bounce lasers off them and time the return. But the stations are unevenly distributed across the globe, which skews the answer. Argus and colleagues from JPL's orbit-determination team, the University of Nevada, the University of Montana and Germany's Helmholtz Centre for Geosciences folded in GPS tracking and orbital data from several low-Earth-orbit satellites to widen the set of targets, and they explicitly modeled how the weight of water and ice deforms the crust, carrying the ground stations along with it.

The seasonal picture that emerges is vivid. Snow across North America and Eurasia peaks in March and pulls the center of mass about 3 millimeters toward the North Pole. A month later, water in the Amazon basin crests at 2,400 gigatons and swings it 2.2 millimeters toward South America. Southeast Asia's monsoon adds 600 gigatons in November. Between August and October the oceans swell with meltwater and rain and the center of mass drifts toward the South Pacific, whose sheer size dwarfs the seasonal contributions of the Mediterranean, Red, North, Baltic and Barents seas. Cold, dense winter air tips the balance over Arabia, Asia and North Africa around Dec. 21 and over South America and southern Africa around June 21. The mass estimates agree with independent measurements from the twin GRACE-FO satellites, which have been weighing Earth's water since 2018.

"While these movements might appear tiny, our modern world relies on extremely accurate positioning measurements," said co-author Felix Landerer of JPL. "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." The same reference frame underpins the satellite record of sea-level rise, where a millimeter of systematic error is the difference between a trend and a controversy.