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Diffstat (limited to '')
| -rw-r--r-- | src/solar.c | 319 |
1 files changed, 319 insertions, 0 deletions
diff --git a/src/solar.c b/src/solar.c new file mode 100644 index 0000000..38278a0 --- /dev/null +++ b/src/solar.c @@ -0,0 +1,319 @@ +/** + * Copyright © 2016 Mattias Andrée <maandree@member.fsf.org> + * + * Permission is hereby granted, free of charge, to any person obtaining a + * copy of this software and associated documentation files (the "Software"), + * to deal in the Software without restriction, including without limitation + * the rights to use, copy, modify, merge, publish, distribute, sublicense, + * and/or sell copies of the Software, and to permit persons to whom the + * Software is furnished to do so, subject to the following conditions: + * + * The above copyright notice and this permission notice shall be included in + * all copies or substantial portions of the Software. + * + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR + * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL + * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER + * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING + * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER + * DEALINGS IN THE SOFTWARE. + */ +#include <math.h> +#include <time.h> +#include <errno.h> + + + +/** + * 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 defined(CLOCK_REALTIME_COARSE) + if (clock_gettime(CLOCK_REALTIME_COARSE, &now)) +#else + if (clock_gettime(CLOCK_REALTIME, &now)) +#endif + return 0.0; + tm = (double)(now.tv_nsec); + tm /= 1000000000.0; + tm += (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 = pow(0.0003032 * tm, 2.0) + 36000.76983 * tm + 280.46646; + return radians(fmod(rc, 360.0)); + /* + CANNIBALISERS: + The result of this function should always be positive, this + means that after division modulo 360 but before `radians`, + you will need to add 360 if the value is negative. This can + only happen if `tm` is negative, which can only happen for date + times before 2000-(01)Jan-01 12:00:00 UTC par division modulo + implementations with the signess of at least the left operand. + More precively, it happens between circa 1970-(01)Jan-11 + 16:09:02 UTC and circa 374702470660351740 seconds before + January 1, 1970 00:00 UTC, which is so far back in time + it cannot be reliable pinned down to the right year, but it + is without a shadow of a doubt looooong before the Earth + was formed, is right up there with the age of the Milky Way + and the universe itself. + */ +} + +/** + * 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) +{ + double rc = sun_geometric_mean_longitude(tm); + return rc + 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; + rc -= 0.00478 * sin(radians(-1934.136 * tm + 125.04)); + return radians(rc); +} + +/** + * 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; + rc = 26 + rc / 60; + rc = 23 + rc / 60; + return radians(rc); +} + +/** + * 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 = -1934.136 * tm + 125.04; + rc = 0.00256 * cos(radians(rc)); + rc += degrees(mean_ecliptic_obliquity(tm)); + return radians(rc); +} + +/** + * 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)); + rc *= sin(sun_apparent_longitude(tm)); + return asin(rc); +} + +/** + * 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, e, m, y, rc; + l = sun_geometric_mean_longitude(tm); + e = earth_orbit_eccentricity(tm); + m = sun_geometric_mean_anomaly(tm); + y = corrected_mean_ecliptic_obliquity(tm); + y = pow(tan(y / 2.0), 2.0); + 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(rm, latitude, longitude)); +} + |