```
''' distance_longitude_latitude101.py
given the longitudes and latitudes of two cities, calculate the distance
Uses the Haversine Formula recommended for calculating short
distances by NASA's Jet Propulsion Laboratory. For longer
distances this formula tends to overestimate trans-polar distances and
underestimate trans-equatorial distances.
tested with Python27 and Python34 by vegaseat 19jan2015
'''
from math import radians, cos, sin, asin, sqrt
def haversine(lon1, lat1, lon2, lat2):
'''
calculate the great circle distance between two points
on the Earth (coordinates specified in decimal degrees)
'''
# convert decimal degrees to radians
lon1, lat1, lon2, lat2 = map(radians, [lon1, lat1, lon2, lat2])
# Haversine formula
dlon = lon2 - lon1
dlat = lat2 - lat1
a = sin(dlat/2)**2 + cos(lat1) * cos(lat2) * sin(dlon/2)**2
c = 2 * asin(sqrt(a))
# 6367 km (3961 miles) is the radius of the Earth
km = 6367 * c
miles = 3961 * c
return km, miles
# Detroit, Michigan 48235
city1 = "Detroit"
lat1 = 42.4261
lon1 = -83.1951
# Chicago, Illinois 60290
city2 = "Chicago"
lat2 = 41.9613
lon2 = -87.8849
km, miles = haversine(lon1, lat1, lon2, lat2)
sf = "The distance between {} and {} is {:0.1f} miles (air)"
print(sf.format(city1, city2, miles))
''' result ...
The distance between Detroit and Chicago is 242.3 miles (air)
'''
```

The article starter has earned a lot of community kudos, and such articles offer a bounty for quality replies.