Let θ be the geographical longitude and φ be the latitude of two points 1 and 2 and Δθ and Δφ be their absolute differences: then the central angle between
a) θ1 - θ2, φ1 - φ2
b) sin(Δθ) + sin(Δφ)
c) cos(Δθ) + cos(Δφ)
d) cos(Δθ/2) * cos(Δφ/2)