Earth's centre of mass is constantly moving in time with the seasons, experts have revealed. A team of scientists at NASA said our planet is continually sloshing and sagging under the weight of water, ice and air. These are all affected by winter freezes, springtime thaws, churning oceans and changing atmospheric pressure. Because of this, Earth's centre of mass swivels around its geometric centre by as much as several millimetres, they revealed. And it could be throwing off satellite data and navigation systems. 'While these movements might appear tiny, our modern world relies on extremely accurate positioning measurements,' co-author Felix Landerer, from NASA's Jet Propulsion Laboratory, said. 'By unravelling 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.' Driven by gravity, satellites naturally orbit Earth's centre of mass as if bound by invisible tethers. Therefore, tiny changes in the distance between satellites and ground stations reflect Earth's shifting centre of mass. Dense metal satellites resembling enormous disco balls are already orbiting Earth, and have done so for decades.

But they are limited by the uneven distribution of ground stations around Earth – which can make measurements slightly less accurate. For the study, published in Geophysical Journal International, lead author Donald Argus and his team developed a new technique to track the size of Earth's swivel based on ultraprecise satellite tracking. They added GPS tracking, as well as orbital data from several other satellites in low Earth orbit to provide a diverse array of 'targets'. It also considers how the weight of water and ice can deform the Earth's crust – and how this can affect ground stations, too. The scientists tracked Earth's centre of mass oscillating with the seasons, and said this is likely caused by changes to the oceans, atmosphere and continental water as the months go by. They found that snow accumulation in North America and Eurasia reaches a maximum in March and shifts Earth's centre of mass about three millimetres toward the North Pole. A month later, in April, rainwater in the Amazon River basin peaks at 2,400 gigatons, swinging Earth's centre of mass 2.2 millimetres toward South America.

Monsoon water in southeast Asia attains a maximum of 600 gigatons seven months later in November, adding slightly to the annual oscillation. Between August and October, the oceans swell with meltwater and rain, and Earth's centre of mass shifts toward the South Pacific Ocean. They also found that cold, dense, winter air helps tip the scales over Arabia, Asia, and northern Africa around December 21 each year, and over South America and South Africa around June 21.

Overall, they discovered that the Earth's centre of mass is actually moving less than previously estimated. 'We're now estimating the size of the movement of Earth's mass centre back and forth each year to be about half of what we believed it to be eight years ago,' Dr Argus said. 'Our findings suggest that the mass of Earth's water and air moving between the hemispheres is smaller than previously thought.' Alongside Earth's centre of mass, scientists have confirmed the geographical North Pole is also moving – and could shift nearly 90 feet (27 metres) by 2100. As the planet's ice sheets and glaciers melt, this is causing a rapid redistribution of the Earth's mass, causing the pole to shift. Researchers measured the movement of the poles between 1908 and 2000 and compared this with projections of ice melt to see how far they might move in the future. In the worst-case scenario, in which greenhouse gas emissions are not reduced, the dramatic melting of the ice sheets will have moved the poles 89 feet between 1900 and 2100. In a more optimistic scenario in which greenhouse gas emissions are reduced, the North Pole will still move as much as 39 feet (12 metres).