Distance between lat/lon geo point

point_distance.py

def point_distance(a, b):
    degrees_to_radians = math.pi / 180
    phi_a = (90 - a.lat) * degrees_to_radians
    phi_b = (90 - b.lat) * degrees_to_radians
    theta_a = a.lon * degrees_to_radians
    theta_b = b.lon * degrees_to_radians
    cos = (math.sin(phi_a) * math.sin(phi_b) * math.cos(theta_a - theta_b) +
           math.cos(phi_a) * math.cos(phi_b))
    arc = math.acos(cos)
    return arc