lib/sphere/positions.js
function distance(f1_φ, f1_λ, f2_φ, f2_λ) {
  return 2 * asin(sqrt(
      pow(sin((f1_φ - f2_φ) / 2), 2) +
      cos(f1_φ) * cos(f2_φ) * pow(sin((f1_λ - f2_λ) / 2), 2)
    ));
}
function distance(f1_φ, f1_λ, f2_φ, f2_λ) {
  return 2 * asin(sqrt(
      pow(sin((f1_φ - f2_φ) / 2), 2) +
      cos(f1_φ) * cos(f2_φ) * pow(sin((f1_λ - f2_λ) / 2), 2)
    ));
}
the spherical arclength of the icosahedron's edges. 
Returns the arc length between two positions.

@return {number} arc length