distance.py

import math

KM_MULTIPLIER = 6373
MILE_MULTIPLIER = 3960

# http://www.johndcook.com/python_longitude_latitude.html
def distance_on_unit_sphere(lat1, long1, lat2, long2, units='km'):

    # Convert latitude and longitude to 
    # spherical coordinates in radians.
    degrees_to_radians = math.pi/180.0
        
    # phi = 90 - latitude
    phi1 = (90.0 - lat1)*degrees_to_radians
    phi2 = (90.0 - lat2)*degrees_to_radians
        
    # theta = longitude
    theta1 = long1*degrees_to_radians
    theta2 = long2*degrees_to_radians
        
    # Compute spherical distance from spherical coordinates.
        
    # For two locations in spherical coordinates 
    # (1, theta, phi) and (1, theta, phi)
    # cosine( arc length ) = 
    #    sin phi sin phi' cos(theta-theta') + cos phi cos phi'
    # distance = rho * arc length
    
    cos = (math.sin(phi1)*math.sin(phi2)*math.cos(theta1 - theta2) + 
           math.cos(phi1)*math.cos(phi2))
    arc = math.acos( cos )

    # Remember to multiply arc by the radius of the earth 
    # in your favorite set of units to get length.

    if units.lower() == 'km':
        return arc * KM_MULTIPLIER
    return arc * MILE_MUTIPLIER