The Chandler wobble is a wobbling motion that occurs as the Earth spins. This wobble causes latitude to vary by 20 feet every 433 days. The Earth returns to its mean position every 6.5 years.
The Chandler wobble was discovered by American astronomer Seth Carlo Chandler in the late 19th century. Theories as to the causes of the wobble include tides, the Earth's liquid interior, constant ocean winds pushing varying amounts of water, major earthquakes, changing ocean pressures and melting ice. The magnitude of the 8.8 earthquake in Chile in 2010 changed the way the Earth wobbles and the length of each day, according to NPR. Global positioning systems adjust to the affect the wobble has on latitude, but star charts must be updated to reflect the new reference points for the geographic poles.