a1 = orig.latitude * (Math::PI/180.0)
b1 = orig.longitude * (Math::PI/180.0)
a2 = dest.latitude * (Math::PI/180.0)
b2 = dest.longitude * (Math::PI/180.0)

Math.acos(Math.cos(a1)*Math.cos(b1)*Math.cos(a2)*Math.cos(b2) +
                          Math.cos(a1)*Math.sin(b1)*Math.cos(a2)*Math.sin(b2) +
                          Math.sin(a1)*Math.sin(a2))