Secular and Long-Period Variations in the Length of Day and the Harmonic Coefficient J₂ of Geopotential

The characters of the secular and long-period variations in the length of day and the harmonic coefficient J₂ of geopotential were analyzed based on the recent observational result of the length of day,the result of J₂ variations in the recent twenty years and the ancient astronomical result of the Earth rotation.Then the relation and difference between variations of the length of day and J₂ were discussed according to the theory of the Earth rotation and gravity.Furthermore we stressed the importance of J₂ and the integrated use of the length of day and J₂ for geodynamics.