Earth’s centre of mass, or geocentre, shifts slightly throughout the year as water, ice and air move across the planet. NASA scientists have developed a more precise method to measure these seasonal movements, which occur on the scale of just a few millimetres.
The technique, led by geoscientist Donald Argus at NASA’s Jet Propulsion Laboratory, combines GPS observations with orbital measurements from multiple low-Earth-orbit satellites. It also accounts for changes in Earth’s crust caused by shifting loads of water and ice.
The geocentre is important for highly accurate satellite navigation, mapping, elevation measurements and other positioning systems. Earlier estimates of seasonal movement differed by around 7 millimetres, highlighting the difficulty of measuring such small changes.
Researchers found that oceans, atmospheric circulation and water stored on land—including snow, glaciers, lakes, rivers, soil moisture and groundwater—are the main drivers. Snow accumulation across North America and Eurasia can shift the centre about 3 millimetres towards the North Pole around March, while seasonal water storage in the Amazon basin can move it roughly 2.2 millimetres towards South America.
Melting ice, rainfall and changes in ocean water contribute later in the year, with the Pacific Ocean having the largest influence. Seasonal variations in atmospheric mass also affect the geocentre.
The findings are consistent with measurements from the GRACE-FO mission. NASA plans to extend this monitoring through the GRACE-C mission, targeted for launch in late 2028.
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