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-rw-r--r--src/solar.c312
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diff --git a/src/solar.c b/src/solar.c
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-/**
- * 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
-
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