Earth's center of mass — the balance point around which the entire planet, including its oceans, atmosphere and ice, pivots — drifts by only a few millimeters each year, according to a new NASA-led study that cuts previous estimates of that wobble roughly in half.
The research, published in Geophysical Journal International, used a new technique that strips out the elastic deformation of Earth's crust caused by shifting water and ice before calculating how the planet's mass center moves relative to its physical shape. Applying that method to decades of GPS and satellite tracking data, a team led by geophysicist Donald Argus of NASA's Jet Propulsion Laboratory found four independent measurements of the seasonal oscillation now agree within about 2.5 millimeters — resolving a gap of roughly 7 millimeters, nearly as large as the motion itself, between estimates published in 2017 and 2023.
Snow, Rain and Monsoons
The team traced the oscillation to the seasonal shuffling of water around the globe. Snow piling up across North America and Eurasia peaks each March and tugs Earth's center of mass about 3 millimeters toward the North Pole. A month later, floodwater in the Amazon basin — which swells to roughly 2,400 gigatons at its April peak — pulls the center about 2.2 millimeters toward South America. Later in the year, monsoon rains over Southeast Asia and a buildup of ocean mass in the South Pacific between August and October add further east-west and north-south nudges, with atmospheric pressure changes contributing a wobble that peaks roughly three months out of phase with the water-driven signal.
The work builds on data from missions including the GRACE-FO satellites, which track subtle changes in Earth's gravity field caused by moving mass, alongside a global network of GPS ground stations.
"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."
Donald Argus, NASA Jet Propulsion Laboratory
The revision matters beyond geophysics circles. Earth's center of mass anchors the reference frame that GPS, satellite altimetry and climate-monitoring instruments all use to fix locations and track change over time — including the sea-level-rise measurements that depend on knowing precisely where "zero" sits. A muddled estimate of that anchor point propagates small errors into everything measured against it, a point NASA's Jet Propulsion Laboratory emphasized in announcing the findings.
Argus's team says the next step is refining the atmospheric contribution further and extending the technique to longer time spans, to see whether the annual cycle itself is shifting as ice sheets and rainfall patterns change with the climate.
