The integrand is the Poisson bracket of x and p. The Poisson bracket of two canonically conjugate quantities, like x and p, is equal to 1 in any canonical coordinate system. So 1 = H ′ ∫ ∫ 0 T { x , p } d t = H ′ T , and H ′ is the inverse period. The variable θ θ increases by an equal amount in each period for all values of J – it is an angle variable.