These two relations link the three parameters θ θ , φ φ and s that appear in the integral together. As θ θ increases from 0 to π π radians, φ φ varies from the initial value 0 to a maximal value before finally returning to zero at θ θ = π π . At the same time, s increases from the initial value r − − R to the final value r + R as θ θ increases from 0 to π π radians. This is illustrated in the following animation: