Haversine formula, from https://news.ycombinator.com/item?id=9282102

haversine.py

import math
from collections import namedtuple

def haversine_distance(origin, destination):
    """ Haversine formula to calculate the distance between two lat/long points on a sphere """

    radius = 6371 # FAA approved globe radius in km

    dlat = math.radians(destination.lat-origin.lat)
    dlon = math.radians(destination.lng-origin.lng)
    a = math.sin(dlat/2) * math.sin(dlat/2) + math.cos(math.radians(origin.lat)) \
      * math.cos(math.radians(destination.lat)) * math.sin(dlon/2) * math.sin(dlon/2)
    c = 2 * math.atan2(math.sqrt(a), math.sqrt(1-a))
    d = radius * c

    # Return distance in km
    # return int(math.floor(d))
    return d

if __name__ == "__main__":

    LatLng = namedtuple('LatLng', 'lat, lng')

    origin = LatLng(51.507222, -0.1275) # London
    destination = LatLng(37.966667, 23.716667) # Athens