Let λ 1 , ϕ 1 {\displaystyle \lambda _{1},\phi _{1}} and λ 2 , ϕ 2 {\displaystyle \lambda _{2},\phi _{2}} be the geographical longitude and latitude of two points 1 and 2, and Δ λ , Δ ϕ {\displaystyle \Delta \lambda ,\Delta \phi } be their absolute differences; then Δ σ {\displaystyle \Delta \sigma } , the central angle between them, is given by the spherical law of cosines if one of the poles is used as an auxiliary third point on the sphere:[2]