Mass is constantly being redistributed around our planet, as Earth's atmosphere, oceans and other bodies of water on and under the surface melt, shift and stir. This mass redistribution alters Earth's center of gravity, which in turn speeds up and slows down the planet's spin—and so the length of the day—as well as changing the orientation of its spin axis. These changes to Earth's spin and orientation occur over relatively short timescales of days and weeks, and threaten communication between ground stations and missions in orbit and across the solar system.
source https://phys.org/news/2021-03-track-spacecraft-earth.html
