We may express cos ⁡ ⁡ γ γ in terms of the coordinates of the observation point and charge position using the spherical law of cosines (Fig. 2) cos ⁡ ⁡ γ γ = cos ⁡ ⁡ θ θ cos ⁡ ⁡ θ θ ′ + sin ⁡ ⁡ θ θ sin ⁡ ⁡ θ θ ′ cos ⁡ ⁡ ( ϕ ϕ − − ϕ ϕ ′ )