Just a little mathematical exercise in geography, namely the use of longitude and latitiude coordinates to calculate the distance between two given cities. You can obtain the coordinates for just about any earthly city from WWW.ASTRO.COM. The coordinates are in a format like 35n13 (35 degrees, 13 minutes north). The north/south indicator makes this a latitude. This value has to be converted to radians. The southerly direction is negative.

The same goes for longitudes, which have an east/west indicator. The westerly direction is negative.

You can expand the dictionary with additional cities, just stick with the format used in the code snippet.

# calculate the air distance between two given cities
# the map coordinates of each city are contained in a dictionary
# locations obtained from: WWW.ASTRO.COM
# longitude is east/west with zero at Greenwhich/London England
# latitude is north/south with zero at the equator
# tested with Python25     vegseat     10may2007

import math

city_locations = {
'Amarillo': ('35n13', '101w50', 'Texas USA'),
'Austin': ('30n16', '97w45', 'Texas USA'),
'Berlin': ('52n30', '13e22', 'Germany'),
'Boston': ('42n22', '71w04', 'Massachusetts USA'),
'Chicago': ('41n51', '87w39', 'Illinois USA'),
'Delhi': ('28n40', '77e13', 'India'),
'Denver': ('39n44', '104w59', 'Colorado USA'),
'Detroit': ('42n20', '83w03', 'Michigan USA'),
'Jerusalem': ('31n46', '35e14', 'Israel'),
'London': ('51n30', '0w10', 'England'),
'Madrid': ('40n24', '3w41', 'Spain'),
'Melbourne': ('37s49', '144e58', 'Australia'),
'Orlando': ('28n32', '81w23', 'Florida USA'),
'Paris': ('48n52', '2e20', 'France'),
'Reno': ('39n32', '119w49', 'Nevada USA'),
'Salt Lake City': ('40n46', '111w53', 'Utah USA'),
'Seattle': ('47n36', '122w20', 'Washington USA'),
'Tokyo': ('35n42', '139e46', 'Japan')

def city_coordinates(city):
    given the coordinates of the city in WWW.ASTRO.COM format
    return the coordinates in radians, negative to the south and west
    if city in city_locations:
        lat1 = city_locations[city][0]
        lng1 = city_locations[city][1]
        if 'n' in lat1:
            hr, mn = lat1.split('n')
            deg = float(hr) + float(mn)/60
            lat_rad = math.radians(deg)
        elif 's' in lat1:
            hr, mn = lat1.split('s')
            deg = float(hr) + float(mn)/60
            lat_rad = -math.radians(deg)
        if 'e' in lng1:
            hr, mn = lng1.split('e')
            deg = float(hr) + float(mn)/60
            lng_rad = math.radians(deg)
        elif 'w' in lng1:
            hr, mn = lng1.split('w')
            deg = float(hr) + float(mn)/60
            lng_rad = -math.radians(deg)
        print "city = %s not found" % city
    # coordinates in radians    
    return (lat_rad, lng_rad)

def city_distance(city1, city2):
    calculate the distance (air) in miles between 2 cities given their coordinates
    # get coordinates in radians
    lat1, lng1 = city_coordinates(city1)
    lat2, lng2 = city_coordinates(city2)
    # circumference in miles at equator, if you want km, use km value here
    circ = 24830.0  
    a = lng1 - lng2
    if a < 0.0:
        a = -a
    if a > math.pi:
        a = 2.0 * math.pi - a
    angle = math.acos(math.sin(lat2) * math.sin(lat1) + 
        math.cos(lat2) * math.cos(lat1) * math.cos(a))
    distance = circ * angle / (2.0 * math.pi)
    print "Shortest distance between %s and %s is %0.1f miles" % (city1, city2, distance)
# mind you these are air miles
city_distance('Detroit', 'Denver')
city_distance('Amarillo', 'Denver')
city_distance('Boston', 'Seattle')
city_distance('Berlin', 'Tokyo')
city_distance('Denver', 'Jerusalem')
city_distance('Detroit', 'Delhi')
city_distance('Detroit', 'Melbourne')

print '-'*60

# these three cities almost line up straight (should add up)
city_distance('Boston', 'Detroit')
city_distance('Detroit', 'Chicago')
city_distance('Boston', 'Chicago')