Find Nearest Cities, given Latitude, Longitude

/* You must install JSONCpp library to use this program.
   Compile this file using this command:
   c++ -o nearestcity nearestcity.cc -ljsoncpp
*/
#include <iostream>
#include <cstdlib>
#include <iomanip>
#include <fstream>
#include <limits>
#include <cmath>
#include <queue>
#include <json/json.h>

double find_distance(double lat1, double lon1, double lat2, double lon2);

int main(int argc, char **argv) {
    Json::Value cities;
    // download from http://bulk.openweathermap.org/sample/city.list.json.gz and unzip
    std::ifstream cities_file("city.list.json", std::ifstream::binary);
    cities_file >> cities;
    double query_lat = std::atof(argv[1]), query_lon = std::atof(argv[2]);
    double dist_min = std::numeric_limits<double>::max();
    std::queue<int> min_indices;

    for ( int i = 0 ; i < cities.size() ; i++ ){
        double curr_dist = find_distance(cities[i]["coord"]["lat"].asDouble(), cities[i]["coord"]["lon"].asDouble(), query_lat, query_lon);

        if ( curr_dist < dist_min ) {
            dist_min = curr_dist;
            min_indices.push(i);
            if ( min_indices.size() > 5 )
                min_indices.pop();
        }
    }

    while ( !min_indices.empty() ) {
        int min_index = min_indices.front();
        std::cout << cities[min_index]["id"] << std::endl;
        std::cout << cities[min_index]["name"] << " " << cities[min_index]["country"] << std::endl;
        min_indices.pop();
    }

    return 0;
}

double find_distance(double lat1, double lon1, double lat2, double lon2) {
    static double rad_per_deg = std::acos(0) / 90.0;
    lat1 *= rad_per_deg;
    lon1 *= rad_per_deg;
    lat2 *= rad_per_deg;
    lon2 *= rad_per_deg;

    //ref: https://wp.me/p7skFn-8S for derivation of find_distance
    return std::acos(std::sin(lat1) * std::sin(lat2) + std::cos(lat1) * std::cos(lat2) * std::cos(lon1 - lon2));
}