Difference δν = ν_ML − ν₃ = (a − c)_ML/R − (a − c)₃/R computed for the homogeneous case (MacLaurin ML) and a two-layer model, developed to the order of 3. The horizontal lines represent the uncertainties in the mean densities. The error bars on the rotation periods are too small to be legible on this plot. σ = 0 corresponds to a homogeneous body of density equal to 920 kg m⁻³, whereas σ = 1 corresponds to a homogeneous body of density 3000 kg m⁻³.