Axial rotation causes Earth to protrude slightly at its midsection. While scientists have long known the magnitude of Earth’s oblateness, only since the 1970s have they been able to monitor minute changes in it, thanks to the global view provided by satellites. Such changes in the equatorial bulge reflect large-scale redistributions of mass. For example, the satellite measurements can detect such small effects as the seasonal movements of air masses in the atmosphere and the transport of water among oceans, atmosphere, and land.
On top of seasonal signals, the measured oblateness has shown a slight downward trend over the years, amounting each year to a few tens of parts per billion. Geophysicists attribute the decrease principally to postglacial rebound: Since the polar ice sheets melted away at the end of the last ice age about 10 000 years ago, the underlying mantle has been springing back up in a process...
