Snow pulls north, the Amazon pulls south
In March, snow over North America and Eurasia shifts Earth's centre of mass about 3 millimetres toward the North Pole. In April, 2400 gigatons of rainwater in the Amazon basin swings it 2.2 millimetres toward South America. In November the Southeast Asian monsoon adds 600 gigatons; between August and October the oceans pull the same point toward the South Pacific.[1]
These figures are the geometry of seasonal water and air. As load moves among continents and seas, the centre of mass moves with it; satellites orbit that point, so a millimetre is the denominator of maps and navigation.[1]
The seven-millimetre gap had outgrown the motion
The 2017 and 2023 international estimates differed by 7 millimetres. Once Argus's team folded GPS into low-Earth-orbit ranges and let the crust flex under water and ice, the yearly back-and-forth fell to about half what had been believed eight years ago. That gap was close to the size of the motion itself; the old denominator had been inflating the swing.[1]
The usable lever is which loads enter the number. Ground stations remain unevenly spread; the new chain is explicit about which loads bend the crust. If the next international terrestrial frame takes up this GPS-and-loading mix, the published seasonal amplitude should move toward Argus's halved scale; if it does not, the 7 millimetre split remains a methods artefact.[1]