sunriset.c

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/*

SUNRISET.C - computes Sun rise/set times, start/end of twilight, and
             the length of the day at any date and latitude

Written as DAYLEN.C, 1989-08-16

Modified to SUNRISET.C, 1992-12-01

(c) Paul Schlyter, 1989, 1992

Released to the public domain by Paul Schlyter, December 1992

*/


#include <stdio.h>
#include <math.h>


/* A macro to compute the number of days elapsed since 2000 Jan 0.0 */
/* (which is equal to 1999 Dec 31, 0h UT)                           */

#define days_since_2000_Jan_0(y,m,d) \
    (367L*(y)-((7*((y)+(((m)+9)/12)))/4)+((275*(m))/9)+(d)-730530L)

/* Some conversion factors between radians and degrees */

#ifndef PI
 #define PI        3.1415926535897932384
#endif

#define RADEG     ( 180.0 / PI )
#define DEGRAD    ( PI / 180.0 )

/* The trigonometric functions in degrees */

#define sind(x)  sin((x)*DEGRAD)
#define cosd(x)  cos((x)*DEGRAD)
#define tand(x)  tan((x)*DEGRAD)

#define atand(x)    (RADEG*atan(x))
#define asind(x)    (RADEG*asin(x))
#define acosd(x)    (RADEG*acos(x))
#define atan2d(y,x) (RADEG*atan2(y,x))


/* Following are some macros around the "workhorse" function __daylen__ */
/* They mainly fill in the desired values for the reference altitude    */
/* below the horizon, and also selects whether this altitude should     */
/* refer to the Sun's center or its upper limb.                         */


/* This macro computes the length of the day, from sunrise to sunset. */
/* Sunrise/set is considered to occur when the Sun's upper limb is    */
/* 35 arc minutes below the horizon (this accounts for the refraction */
/* of the Earth's atmosphere).                                        */
#define day_length(year,month,day,lon,lat)  \
        __daylen__( year, month, day, lon, lat, -35.0/60.0, 1 )

/* This macro computes the length of the day, including civil twilight. */
/* Civil twilight starts/ends when the Sun's center is 6 degrees below  */
/* the horizon.                                                         */
#define day_civil_twilight_length(year,month,day,lon,lat)  \
        __daylen__( year, month, day, lon, lat, -6.0, 0 )

/* This macro computes the length of the day, incl. nautical twilight.  */
/* Nautical twilight starts/ends when the Sun's center is 12 degrees    */
/* below the horizon.                                                   */
#define day_nautical_twilight_length(year,month,day,lon,lat)  \
        __daylen__( year, month, day, lon, lat, -12.0, 0 )

/* This macro computes the length of the day, incl. astronomical twilight. */
/* Astronomical twilight starts/ends when the Sun's center is 18 degrees   */
/* below the horizon.                                                      */
#define day_astronomical_twilight_length(year,month,day,lon,lat)  \
        __daylen__( year, month, day, lon, lat, -18.0, 0 )


/* This macro computes times for sunrise/sunset.                      */
/* Sunrise/set is considered to occur when the Sun's upper limb is    */
/* 35 arc minutes below the horizon (this accounts for the refraction */
/* of the Earth's atmosphere).                                        */
#define sun_rise_set(year,month,day,lon,lat,rise,set)  \
        __sunriset__( year, month, day, lon, lat, -35.0/60.0, 1, rise, set )

/* This macro computes the start and end times of civil twilight.       */
/* Civil twilight starts/ends when the Sun's center is 6 degrees below  */
/* the horizon.                                                         */
#define civil_twilight(year,month,day,lon,lat,start,end)  \
        __sunriset__( year, month, day, lon, lat, -6.0, 0, start, end )

/* This macro computes the start and end times of nautical twilight.    */
/* Nautical twilight starts/ends when the Sun's center is 12 degrees    */
/* below the horizon.                                                   */
#define nautical_twilight(year,month,day,lon,lat,start,end)  \
        __sunriset__( year, month, day, lon, lat, -12.0, 0, start, end )

/* This macro computes the start and end times of astronomical twilight.   */
/* Astronomical twilight starts/ends when the Sun's center is 18 degrees   */
/* below the horizon.                                                      */
#define astronomical_twilight(year,month,day,lon,lat,start,end)  \
        __sunriset__( year, month, day, lon, lat, -18.0, 0, start, end )


/* Function prototypes */

double __daylen__( int year, int month, int day, double lon, double lat,
                   double altit, int upper_limb );

int __sunriset__( int year, int month, int day, double lon, double lat,
                  double altit, int upper_limb, double *rise, double *set );

void sunpos( double d, double *lon, double *r );

void sun_RA_dec( double d, double *RA, double *dec, double *r );

double revolution( double x );

double rev180( double x );

double GMST0( double d );



/* A small test program */

main()
{
      int year,month,day;
      double lon, lat;
      double daylen, civlen, nautlen, astrlen;
      double rise, set, civ_start, civ_end, naut_start, naut_end,
             astr_start, astr_end;
      int    rs, civ, naut, astr;
      char buf[80];

      printf( "Longitude (+ is east) and latitude (+ is north) : " );
      fgets(buf, 80, stdin);
      sscanf(buf, "%lf %lf", &lon, &lat );

      for(;;)
      {
            printf( "Input date ( yyyy mm dd ) (ctrl-C exits): " );
            fgets(buf, 80, stdin);
            sscanf(buf, "%d %d %d", &year, &month, &day );

            daylen  = day_length(year,month,day,lon,lat);
            civlen  = day_civil_twilight_length(year,month,day,lon,lat);
            nautlen = day_nautical_twilight_length(year,month,day,lon,lat);
            astrlen = day_astronomical_twilight_length(year,month,day,
                  lon,lat);

            printf( "Day length:                 %5.2f hours\n", daylen );
            printf( "With civil twilight         %5.2f hours\n", civlen );
            printf( "With nautical twilight      %5.2f hours\n", nautlen );
            printf( "With astronomical twilight  %5.2f hours\n", astrlen );
            printf( "Length of twilight: civil   %5.2f hours\n",
                  (civlen-daylen)/2.0);
            printf( "                  nautical  %5.2f hours\n",
                  (nautlen-daylen)/2.0);
            printf( "              astronomical  %5.2f hours\n",
                  (astrlen-daylen)/2.0);

            rs   = sun_rise_set         ( year, month, day, lon, lat,
                                          &rise, &set );
            civ  = civil_twilight       ( year, month, day, lon, lat,
                                          &civ_start, &civ_end );
            naut = nautical_twilight    ( year, month, day, lon, lat,
                                          &naut_start, &naut_end );
            astr = astronomical_twilight( year, month, day, lon, lat,
                                          &astr_start, &astr_end );

            printf( "Sun at south %5.2fh UT\n", (rise+set)/2.0 );

            switch( rs )
            {
                case 0:
                    printf( "Sun rises %5.2fh UT, sets %5.2fh UT\n",
                             rise, set );
                    break;
                case +1:
                    printf( "Sun above horizon\n" );
                    break;
                case -1:
                    printf( "Sun below horizon\n" );
                    break;
            }

            switch( civ )
            {
                case 0:
                    printf( "Civil twilight starts %5.2fh, "
                            "ends %5.2fh UT\n", civ_start, civ_end );
                    break;
                case +1:
                    printf( "Never darker than civil twilight\n" );
                    break;
                case -1:
                    printf( "Never as bright as civil twilight\n" );
                    break;
            }

            switch( naut )
            {
                case 0:
                    printf( "Nautical twilight starts %5.2fh, "
                            "ends %5.2fh UT\n", naut_start, naut_end );
                    break;
                case +1:
                    printf( "Never darker than nautical twilight\n" );
                    break;
                case -1:
                    printf( "Never as bright as nautical twilight\n" );
                    break;
            }

            switch( astr )
            {
                case 0:
                    printf( "Astronomical twilight starts %5.2fh, "
                            "ends %5.2fh UT\n", astr_start, astr_end );
                    break;
                case +1:
                    printf( "Never darker than astronomical twilight\n" );
                    break;
                case -1:
                    printf( "Never as bright as astronomical twilight\n" );
                    break;
            }
      return 0;
      }
}


/* The "workhorse" function for sun rise/set times */

int __sunriset__( int year, int month, int day, double lon, double lat,
                  double altit, int upper_limb, double *trise, double *tset )
/***************************************************************************/
/* Note: year,month,date = calendar date, 1801-2099 only.             */
/*       Eastern longitude positive, Western longitude negative       */
/*       Northern latitude positive, Southern latitude negative       */
/*       The longitude value IS critical in this function!            */
/*       altit = the altitude which the Sun should cross              */
/*               Set to -35/60 degrees for rise/set, -6 degrees       */
/*               for civil, -12 degrees for nautical and -18          */
/*               degrees for astronomical twilight.                   */
/*         upper_limb: non-zero -> upper limb, zero -> center         */
/*               Set to non-zero (e.g. 1) when computing rise/set     */
/*               times, and to zero when computing start/end of       */
/*               twilight.                                            */
/*        *rise = where to store the rise time                        */
/*        *set  = where to store the set  time                        */
/*                Both times are relative to the specified altitude,  */
/*                and thus this function can be used to compute       */