 The angle of Earth 's axial tilt is relatively stable over long periods of time . Its axial tilt does undergo nutation ; a slight , irregular motion with a main period of 18 @.@ 6 years . The orientation ( rather than the angle ) of Earth 's axis also changes over time , precessing around in a complete circle over each 25 @,@ 800 year cycle ; this precession is the reason for the difference between a sidereal year and a tropical year . Both of these motions are caused by the varying attraction of the Sun and the Moon on Earth 's equatorial bulge . The poles also migrate a few meters across Earth 's surface . This polar motion has multiple , cyclical components , which collectively are termed <unk> motion . In addition to an annual component to this motion , there is a 14 @-@ month cycle called the Chandler wobble . Earth 's rotational velocity also varies in a phenomenon known as length @-@ of @-@ day variation . 
