calculate distance on unit sphere with lat long

gistfile1.py

def distance_on_unit_sphere(lat1, long1, lat2, long2):                          
    #http://www.johndcook.com/python_longitude_latitude.html                    
    degrees_to_radians = math.pi/180.0                                          
    phi1 = (90.0 - lat1)*degrees_to_radians                                     
    phi2 = (90.0 - lat2)*degrees_to_radians                                     
    theta1 = long1*degrees_to_radians                                           
    theta2 = long2*degrees_to_radians                                           
    cos = (math.sin(phi1)*math.sin(phi2)*math.cos(theta1 - theta2) +            
            math.cos(phi1)*math.cos(phi2))                                      
    try:                                                                        
        arc = math.acos(min(cos,1.0))                                           
    except ValueError:                                                          
        print cos                                                               
        print cos-1.0                                                           
        print lat1,lat2                                                         
        print long1,long2                                                       
        import sys                                                              
        sys.exit(1)                                                             
    feet = arc * 3960 * 5280                                                    
    feet = round(feet,0)                                                        
    return feet #returns the distance between two gps locations, in feet.