def METHOD_NAME(self, lat, lng):
    """
    http://www.johndcook.com/python_longitude_latitude.html
    Distance in miles multiply by 3960
    Distance in kilometers multiply by 6373
    """
    import math
    # if we don't have latitude or longitude
    # we return a none type object instead of int
    if not all((self.latitude, self.longitude)):
        return None
    # Convert latitude and longitude to
    # spherical coordinates in radians.
    degrees_to_radians = math.pi/180.0
    # phi = 90 - latitude
    phi1 = (90.0 - self.latitude)*degrees_to_radians
    phi2 = (90.0 - lat)*degrees_to_radians
    # theta = longitude
    theta1 = self.longitude*degrees_to_radians
    theta2 = lng*degrees_to_radians
    cos = (math.sin(phi1)*math.sin(phi2)*math.cos(theta1 - theta2) +
           math.cos(phi1)*math.cos(phi2))
    try:
        arc = math.acos(cos)
    except:
        arc = 0
    # Remember to multiply arc by the radius of the earth
    # in your favorite set of units to get length.
    return arc * 3960