def sphere_dist(pickup_lat, pickup_lon, dropoff_lat, dropoff_lon):
  """
  Return distance along great radius between pickup and dropoff coordinates.
  """
  #Define earth radius (km)
  R_earth = 6371
  #Convert degrees to radians
  pickup_lat, pickup_lon, dropoff_lat, dropoff_lon = map(np.radians, [pickup_lat, pickup_lon, dropoff_lat, dropoff_lon])

  #Compute distances along lat, lon dimensions
  dlat = dropoff_lat - pickup_lat
  dlon = dropoff_lon - pickup_lon

  #Compute haversine distance
  a = np.sin(dlat/2.0)**2 + np.cos(pickup_lat) * np.cos(dropoff_lat) * np.sin(dlon/2.0)**2

  return 2 * R_earth * np.arcsin(np.sqrt(a))