docs: def space_angle(phi1, theta1, phi2, theta2):
      """Also called Great-circle-distance --
      use long-ass formula from wikipedia (last in section):
      https://en.wikipedia.org/wiki/Great-circle_distance#Computational_formulas
  
      Space angle only makes sense in lon-lat, so convert zenith -> latitude.
      """
      from numpy import pi, sin, cos, arctan2, sqrt, square
      lamb1 = pi / 2 - theta1
      lamb2 = pi / 2 - theta2
      lambdelt = lamb2 - lamb1
      under = sin(phi1) * sin(phi2) + cos(phi1) * cos(phi2) * cos(lambdelt)
      over = sqrt(
          np.square((cos(phi2) * sin(lambdelt))) +
          square(cos(phi1) * sin(phi2) - sin(phi1) * cos(phi2) * cos(lambdelt))
      )
      angle = arctan2(over, under)
      return angle