In this way the equation –
r₁³ / r₂³ = t₁² / t₂²
assumes the form-
r₁³ / r₂³ = (t₁ / t₂) / (t₂ / t₁)
The right-hand side of the equation is now constituted by the double ratio of the linear values of the periods of two planets, and this is something with which we can connect a quite concrete idea.