Temporal evolution of the semi-major axis a for a class of tidal models with k₂/Q ∝ χ^−α and with different values of the parameter α. The abscissa represents time in years, the vertical axis measures the deviation of the semi-major axis from its initial value a₀. Our plots serve to compare realistic rheologies (α = 0.2,0.3,0.4) with less physical ones – those with α = 0 (Kaula 1964) and α = −1 (Singer 1968; Mignard 1979).