RESEARCH NOTE The Earth's equatorial principal axes and moments of inertia

Let the Earth's equatorial principal moments of inertia be A and B, where A < B, and the corresponding principal axes be a and b. Explicit formulae are here derived for determining the orientation of a and b axes and the difference B- A using CZ2 and s2, the two gravitational harmonic coefficients of degree 2 and order 2. For the Earth, the a axis lies along the (14.93"W ' 165.07"E) diameter, and the b axis lies perpendicular to it along the (75.07"E ' 104.93"W) diameter. The difference B- A is 7.260 x MR2. These quantities for other planets are contrasted, and geophysi-cal implications are discussed. Key words: Earth's moment of inertia, Earth's rotation