In astrophysics, the Darwin–Radau equation gives an approximate relation between the moment of inertia factor of a planetary body and its rotational speed and shape. The moment of inertia factor is directly related to the largest principal moment of inertia, C. It is assumed that the rotating body is in hydrostatic equilibrium and is an ellipsoid of revolution. The Darwin–Radau equation states
where q is the geodynamical constant
and ε is the geometrical flattening
where Rp is the mean polar radius and Re is the mean equatorial radius.
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