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NASA Watches Earth’s Weight, Finds Center of Mass
Earth's centre of mass, the balance point of the whole planet, is not fixed. Each year, water shifts between oceans, ice, rivers and land, and that redistribution moves the centre of mass back and forth by fractions of an inch relative to the planet's geometric centre. NASA scientists are tracking these oscillations because the centre of mass is the reference point for satellite navigation and for measuring elevation and sea level. If the reference wobbles with the seasons, any measurement tied to it needs to account for that. In effect, the planet's weight is being rebalanced all year by the movement of water from one hemisphere to another.
GPS-style positioning and sea-level records depend on a stable frame of reference, so tiny seasonal shifts matter for precision work measured in millimetres. It also shows how sensitively satellites can detect the planet's water cycle: moving mass around is something we can now weigh from orbit.
background
In physics, the center of mass of a distribution of mass in space (sometimes referred to as the barycenter or balance point) is the unique point at any given time where the weighted relative position of the distributed mass sums to zero. For a rigid body containing its center of mass, this is the point to which a force may be applied to cause a linear acceleration without an angular acceleration. Calculations in mechanics are often simplified when formulated with respect to the center of mass. It is a hypothetical point where the entire mass of an object may be assumed to be concentrated to visualise its motion. In other words, the center of mass is the particle equivalent of a given object for the application of Newton's laws of motion.
From Wikipedia: Center of mass (CC BY-SA 4.0) ↗
Credit: NASA
