#Determine distance from latitude and longitude
#The first two arguments are the latitude and 
#longitude of the first point, in degrees.
#The next two are the latitude and longitude of
#the second point, also in degrees.  The return
#value is the distance, in miles, between the two
#points.  
def greatCircleDistance(lat1,long1,lat2,long2):
  #the radius of the earth, in miles:
  r=3959
  #convert from degrees to radians:
  convert=0.017453292519943295
  lat1=convert*lat1
  lat2=convert*lat2
  long1=convert*long1
  long2=convert*long2
  #dlat=lat2-lat1
  dlong=long2-long1
  a=sin(lat1)*sin(lat2)+cos(lat1)*cos(lat2)*cos(long2-long1)
  return r*acos(a)