JPL's new tracking mix halves the estimated annual wander of Earth's center of mass

This measurement is limited by geometry. LAGEOS 1 and 2, launched in 1976 and 1992, exist for this one job: two 408-kilogram metal spheres studded with reflective prisms, ranged by laser from stations spread across more than 20 countries ^[8]. Each range measurement is excellent. The stations are unevenly distributed over the planet, and NASA identifies that distribution as a limitation of satellite laser ranging ^[9]. A network that samples some regions densely and others barely pins down the origin of the frame less tightly than its per-shot precision would suggest.

Donald Argus, the JPL geoscientist who led the work, took two steps against that ^[6]. His group added GPS tracking and orbital data from several satellites in low Earth orbit, giving more targets in more places ^[10]. It also accounted for the weight of water and ice bending the crust, which carries the ground stations up and down with the load ^[11]. The second step is a model, so the new amplitude depends on how well that deformation is described as well as on the tracking. Coauthors came from JPL's satellite orbit determination team, the University of Nevada, the University of Montana, and the Helmholtz Centre for Geosciences in Germany ^[12].

"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 ^[13]. He added: "Our findings suggest that the mass of Earth's water and air moving between the hemisphere is smaller than previously thought" ^[14].

The study splits the oscillation into oceans, atmosphere and continental water, the last of which includes land ice, snow, lake and river water, soil moisture and groundwater ^[17]. Snow over North America and Eurasia peaks in March and pulls the mass center about 3 millimetres toward the North Pole ^[18]. A month later, Amazon basin rainwater peaks at 2,400 gigatons and pulls it 2.2 millimetres toward South America ^[19]. That works out to roughly 1,100 gigatons of water per millimetre of displacement ^[22]. The southeast Asian monsoon peaks in November at 600 gigatons, a quarter of the Amazon figure ^[24], and the study describes its addition to the annual oscillation as slight ^[20]. Where the water sits matters as much as how much of it there is. From August to October, meltwater and rain swelling the oceans move the center of mass toward the South Pacific ^[21].

For scale, the 7-millimetre gap between the 2017 and 2023 international estimates is about 2.3 times the March snow displacement ^[5][23]. The published account makes the practical case qualitatively. "While these movements might appear tiny, our modern world relies on extremely accurate positioning measurements," said Felix Landerer, a JPL coauthor ^[15][25], who said better reference systems "ultimately benefit mapping and navigation" ^[16]. NASA did not quantify how much of a 7-millimetre origin error reaches a single navigation fix, or a decadal sea-level trend.

I would treat the halved amplitude as the better-instrumented number and still provisional. It rests in part on a crustal loading model ^[11], and the two estimates it supersedes disagreed with each other by nearly as much as the motion they were measuring ^[5].