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Diffstat (limited to 'src/solar.c')
| -rw-r--r-- | src/solar.c | 312 |
1 files changed, 312 insertions, 0 deletions
diff --git a/src/solar.c b/src/solar.c new file mode 100644 index 0000000..8a57507 --- /dev/null +++ b/src/solar.c @@ -0,0 +1,312 @@ +/** + * Copyright © 2016 Mattias Andrée <maandree@member.fsf.org> + * + * This program is free software: you can redistribute it and/or modify + * it under the terms of the GNU General Public License as published by + * the Free Software Foundation, either version 3 of the License, or + * (at your option) any later version. + * + * This program is distributed in the hope that it will be useful, + * but WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + * GNU General Public License for more details. + * + * You should have received a copy of the GNU General Public License + * along with this program. If not, see <http://www.gnu.org/licenses/>. + */ +#include <math.h> +#include <time.h> +#include <errno.h> + + + +#if !defined(CLOCK_REALTIME_COARSE) +# define CLOCK_REALTIME_COARSE CLOCK_REALTIME +#endif + + + +/** + * Get current Julian Centuries time (100 Julian days since J2000.) + * + * @return The current Julian Centuries time. + * + * @throws 0 On success. + * @throws Any error specified for clock_gettime(3) on error. + */ +static double +julian_centuries() +{ + struct timespec now; + double tm; + if (clock_gettime(CLOCK_REALTIME_COARSE, &now)) return 0.0; + tm = (double)(now.tv_nsec) / 1000000000.0 + (double)(now.tv_sec); + tm = (tm / 86400.0 + 2440587.5 - 2451545.0) / 36525.0; + return errno = 0, tm; +} + +/** + * Convert a Julian Centuries timestamp to a Julian Day timestamp. + * + * @param tm The time in Julian Centuries + * @return The time in Julian Days + */ +static inline double +julian_centuries_to_julian_day(double tm) +{ + return tm * 36525.0 + 2451545.0; +} + + +/** + * Convert an angle (or otherwise) from degrees to radians. + * + * @param deg The angle in degrees. + * @param The angle in radians. + */ +static inline double +radians(double deg) +{ + return deg * (double)M_PI / 180.0; +} + +/** + * Convert an angle (or otherwise) from radians to degrees. + * + * @param rad The angle in radians. + * @param The angle in degrees. + */ +static inline double +degrees(double rad) +{ + return rad * 180.0 / (double)M_PI; +} + + +/** + * Calculates the Sun's elevation from the solar hour angle + * + * @param longitude The longitude in degrees eastwards. + * from Greenwich, negative for westwards. + * @param declination The declination, in radians. + * @param hour_angle The solar hour angle, in radians. + * @return The Sun's elevation, in radians. + */ +static inline double +elevation_from_hour_angle(double latitude, double declination, double hour_angle) +{ + double rc = cos(radians(latitude)); + rc *= cos(hour_angle) * cos(declination); + rc += sin(radians(latitude)) * sin(declination); + return asin(rc); +} + +/** + * Calculates the Sun's geometric mean longitude. + * + * @param tm The time in Julian Centuries. + * @return The Sun's geometric mean longitude in radians. + */ +static inline double +sun_geometric_mean_longitude(double tm) +{ + double rc = fmod(pow(0.0003032 * tm, 2.0) + 36000.76983 * tm + 280.46646, 360.0); +#if defined(TIMETRAVELLER) + rc = rc < 0.0 ? (rc + 360.0) : rc; +#endif + return radians(rc); +} + +/** + * Calculates the Sun's geometric mean anomaly. + * + * @param tm The time in Julian Centuries. + * @return The Sun's geometric mean anomaly in radians. + */ +static inline double +sun_geometric_mean_anomaly(double tm) +{ + return radians(pow(-0.0001537 * tm, 2.0) + 35999.05029 * tm + 357.52911); +} + +/** + * Calculates the Earth's orbit eccentricity. + * + * @param tm The time in Julian Centuries. + * @return The Earth's orbit eccentricity. + */ +static inline double +earth_orbit_eccentricity(double tm) +{ + return pow(-0.0000001267 * tm, 2.0) - 0.000042037 * tm + 0.016708634; +} + +/** + * Calculates the Sun's equation of the centre, the difference + * between the true anomaly and the mean anomaly. + * + * @param tm The time in Julian Centuries. + * @return The Sun's equation of the centre, in radians. + */ +static inline double +sun_equation_of_centre(double tm) +{ + double a = sun_geometric_mean_anomaly(tm), rc; + rc = sin(1.0 * a) * (pow(-0.000014 * tm, 2.0) - 0.004817 * tm + 1.914602); + rc += sin(2.0 * a) * (-0.000101 * tm + 0.019993); + rc += sin(3.0 * a) * 0.000289; + return radians(rc); +} + +/** + * Calculates the Sun's real longitudinal position. + * + * @param tm The time in Julian Centuries. + * @return The longitude, in radians. + */ +static inline double +sun_real_longitude(double tm) +{ + return sun_geometric_mean_longitude(tm) + sun_equation_of_centre(tm); +} + +/** + * Calculates the Sun's apparent longitudinal position. + * + * @param tm The time in Julian Centuries. + * @return The longitude, in radians. + */ +static inline double +sun_apparent_longitude(double tm) +{ + double rc = degrees(sun_real_longitude(tm)) - 0.00569; + return radians(rc - 0.00478 * sin(radians(-1934.136 * tm + 125.04))); +} + +/** + * Calculates the mean ecliptic obliquity of the Sun's + * apparent motion without variation correction. + * + * @param tm The time in Julian Centuries. + * @return The uncorrected mean obliquity, in radians. + */ +static double +mean_ecliptic_obliquity(double tm) +{ + double rc = pow(0.001813 * tm, 3.0) - pow(0.00059 * tm, 2.0) - 46.815 * tm + 21.448; + return radians(23.0 + (26.0 + rc / 60.0) / 60.0); +} + +/** + * Calculates the mean ecliptic obliquity of the Sun's + * parent motion with variation correction. + * + * @param tm The time in Julian Centuries. + * @return The mean obliquity, in radians. + */ +static double +corrected_mean_ecliptic_obliquity(double tm) +{ + double rc = 0.00256 * cos(radians(-1934.136 * tm + 125.04)); + return radians(rc + degrees(mean_ecliptic_obliquity(tm))); +} + +/** + * Calculates the Sun's declination. + * + * @param tm The time in Julian Centuries. + * @return The Sun's declination, in radian. + */ +static inline double +solar_declination(double tm) +{ + double rc = sin(corrected_mean_ecliptic_obliquity(tm)); + return asin(rc * sin(sun_apparent_longitude(tm))); +} + +/** + * Calculates the equation of time, the discrepancy + * between apparent and mean solar time. + * + * @param tm The time in Julian Centuries. + * @return The equation of time, in degrees. + */ +static inline double +equation_of_time(double tm) +{ + double l = sun_geometric_mean_longitude(tm); + double e = earth_orbit_eccentricity(tm); + double m = sun_geometric_mean_anomaly(tm); + double y = pow(tan(corrected_mean_ecliptic_obliquity(tm) / 2.0), 2.0); + double rc = y * sin(2.0 * l); + rc += (4.0 * y * cos(2.0 * l) - 2.0) * e * sin(m); + rc -= pow(0.5 * y, 2.0) * sin(4.0 * l); + rc -= pow(1.25 * e, 2.0) * sin(2.0 * m); + return 4.0 * degrees(rc); +} + +/** + * Calculates the Sun's elevation as apparent. + * from a geographical position. + * + * @param tm The time in Julian Centuries. + * @param latitude The latitude in degrees northwards from + * the equator, negative for southwards. + * @param longitude The longitude in degrees eastwards from + * Greenwich, negative for westwards. + * @return The Sun's apparent elevation at the specified time as seen + * from the specified position, measured in radians. + */ +static inline double +solar_elevation_from_time(double tm, double latitude, double longitude) +{ + double rc = julian_centuries_to_julian_day(tm); + rc = (rc - round(rc) - 0.5) * 1440; + rc = 720.0 - rc - equation_of_time(tm); + rc = radians(rc / 4.0 - longitude); + return elevation_from_hour_angle(latitude, solar_declination(tm), rc); +} + + +/** + * Calculates the Sun's elevation as apparent. + * from a geographical position. + * + * @param latitude The latitude in degrees northwards from + * the equator, negative for southwards. + * @param longitude The longitude in degrees eastwards from + * Greenwich, negative for westwards. + * @return The Sun's apparent elevation as seen, right now, + * from the specified position, measured in degrees. + * + * @throws 0 On success. + * @throws Any error specified for clock_gettime(3) on error. + */ +double +solar_elevation(double latitude, double longitude) +{ + double tm = julian_centuries(); + return errno ? -1 : degrees(solar_elevation_from_time(tm, latitude, longitude)); +} + + +/** + * Exit if time the is before year 0 in J2000. + * + * @return 0 on success, -1 on error. + */ +#if defined(TIMETRAVELLER) +int +check_timetravel(void) +{ + struct timespec now; + if (clock_gettime(CLOCK_REALTIME, &now)) return -1; + if (now.tv_nsec < (time_t)946728000L) + fprintf(stderr, "We have detected that you are a time-traveller" + "(or your clock is not configured correctly.)" + "Please recompile with -DTIMETRAVELLER" + "(or correct your clock.)"), exit(1); + return 0; +} +#endif + |