1 - cos(x)

1 - cos(x) = 2*sin(x/2)^2
           = 2*haversin(x)

1 - cos(x) = sin(x)^2/(1 + cos(x))

Distance = acos(SIN(lat1)*SIN(lat2)+COS(lat1)*COS(lat2)*COS(lon2-lon1))*6371

dLat = (lat2-lat1)
  dLon = (lon2-lon1)
  a = sin(dLat/2) * sin(dLat/2) + cos(lat1) * cos(lat2) * sin(dLon/2) * sin(dLon/2)
  distance = 6371 * 2 * atan2(sqrt(a), sqrt(1-a))