simple haversine implementation from description on wikipedia

haversine.py

from math import pi, sin, cos, atan2, sqrt

def haversine_distance(loc1, loc2):
    """Haversine formula - give coordinates as (lat_decimal, lon_decimal) tuples. Returns distance in miles"""
    earth_radius = 6371.0 # km 

    to_radians = lambda x : x * pi / 180.0
    lat1, lon1 = map(to_radians, loc1)
    lat2, lon2 = map(to_radians, loc2)

    # haversine formula
    hdlat = (lat2 - lat1) / 2.0
    hdlon = (lon2 - lon1) / 2.0
    a = sin(hdlat)**2 + cos(lat1) * cos(lat2) * sin(hdlon)**2
    c = 2.0 * atan2(sqrt(a), sqrt(1.0 - a))
    km = earth_radius * c
    return km * 0.621371