What makes the world wobble

The Christian Science Monitor : We think of Earth as a rotating sphere. It would be more accurate to consider it a wobbling top. Like a top spinning on a table, Earth’s spin axis undergoes a complex mix of wobbles. Scientists have tracked the larger loops and dips for over a century. Thanks to the exquisite accuracy of the Global Positioning System (GPS) and a break from Mother Nature, they now have found patterns in which the axis traces small loops over an area no bigger than a sheet of typing paper every few days.