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136 lines
3.5 KiB
136 lines
3.5 KiB
#define _USE_MATH_DEFINES
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#include <Rcpp.h>
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#include <math.h>
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using namespace Rcpp;
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#define rad2deg(rad) ((rad * 180.0) / M_PI)
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#define deg2rad(deg) ((deg * M_PI) / 180.0)
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#define get_julian(time) (((double)time / 86400.0) + 2440587.5)
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#define get_gmst(j_day) (fmod((18.697374558 + 24.06570982441908 * (j_day - 2451545.0)), 24.0))
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inline std::vector < double > sun_ecliptic_position(double j_day) {
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// compute the position of the Sun in ecliptic coordinates days since start of J2000.0
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double n = j_day - 2451545.0;
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// mean longitude of the Sun
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double L = fmod((280.460 + 0.9856474 * n), 360.0);
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// mean anomaly of the Sun
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double g = fmod((357.528 + 0.9856003 * n), 360.0);
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// ecliptic longitude of Sun
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double lambda = L + 1.915 * sin(deg2rad(g)) + 0.02 * sin(2 * deg2rad(g));
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// distance from Sun in AU
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double R = 1.00014 - 0.01671 * cos(deg2rad(g)) - 0.0014 * cos(2 * deg2rad(g));
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std::vector< double > ret(2);
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ret[0] = lambda;
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ret[1] = R;
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return(ret);
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}
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inline double ecliptic_obliquity(double j_day) {
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double n = j_day - 2451545.0;
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// Julian centuries since J2000.0
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double T = n / 36525.0;
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// compute epsilon
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return(23.43929111 -
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T * (46.836769 / 3600.0
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- T * (0.0001831 / 3600.0
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+ T * (0.00200340 / 3600.0
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- T * (0.576e-6 / 3600.0
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- T * 4.34e-8 / 3600.0)))));
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}
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std::vector< double > sun_equatorial_position(double lng, double obliq) {
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double alpha = rad2deg(atan(cos(deg2rad(obliq)) * tan(deg2rad(lng))));
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double delta = rad2deg(asin(sin(deg2rad(obliq)) * sin(deg2rad(lng))));
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double lQuadrant = floor(lng / 90.0) * 90.0;
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double raQuadrant = floor(alpha / 90.0) * 90.0;
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alpha = alpha + (lQuadrant - raQuadrant);
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std::vector< double > ret(2);
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ret[0] = alpha;
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ret[1] = delta;
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return(ret);
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}
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inline double hour_angle(double lng, std::vector< double >sun_pos, double gst) {
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return((gst + lng / 15.0) * 15.0 - sun_pos[0]);
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}
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inline double longitude(double ha, std::vector< double >sun_pos) {
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return(rad2deg(atan(-cos(deg2rad(ha)) / tan(deg2rad(sun_pos[1])))));
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}
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//' Compute a single termiantor band
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//'
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//' Returns a dataframe of latitude and longitude for the line that separates illuminated
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//' day and dark night for any given time
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//'
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//' @md
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//' @param time time (numeric from `POSIXct`) for the computation (bands are time-dependent)
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//' @param from,to,by latitude sequence setup
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//' @return data frame
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//' @references <https://github.com/joergdietrich/Leaflet.Terminator/blob/master/L.Terminator.js>,
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//' <https://github.com/JoGall/terminator/blob/master/terminator.R>
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//' @export
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// [[Rcpp::export]]
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DataFrame terminator(int time, double from = -180, double to = 180, double by = 0.1) {
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// calculate latitude and longitude of terminator within specified range using time (in POSIXct format, e.g. `Sys.time()`)
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double j_day = get_julian(time);
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double gst = get_gmst(j_day);
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std::vector< double > sunEclPos = sun_ecliptic_position(j_day);
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double eclObliq = ecliptic_obliquity(j_day);
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std::vector< double > sunEqPos = sun_equatorial_position(sunEclPos[0], eclObliq);
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std::vector< double > out_lat, out_lon;
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out_lat.reserve(4000);
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out_lon.reserve(4000);
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int n=0;
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for (double i=from; i<=to; i+=by) {
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n += 1;
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out_lat.push_back(i);
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out_lon.push_back(
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longitude(
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hour_angle(i, sunEqPos, gst),
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sunEqPos
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)
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);
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}
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out_lat.resize(n);
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out_lon.resize(n);
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return(
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DataFrame::create(
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Named("lat") = out_lat,
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Named("lon") = out_lon
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)
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);
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}
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