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Time/sunpos.c
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Time/sunpos.c
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/*
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* sunpos.c
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* kirk johnson
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* july 1993
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*
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* code for calculating the position on the earth's surface for which
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* the sun is directly overhead (adapted from _practical astronomy
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* with your calculator, third edition_, peter duffett-smith,
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* cambridge university press, 1988.)
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*
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* RCS $Id$
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*
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* Copyright (C) 1989, 1990, 1993, 1994, 1995 Kirk Lauritz Johnson
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*
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* Parts of the source code (as marked) are:
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* Copyright (C) 1989, 1990, 1991 by Jim Frost
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* Copyright (C) 1992 by Jamie Zawinski <jwz@lucid.com>
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*
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* Permission to use, copy, modify and freely distribute xearth for
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* non-commercial and not-for-profit purposes is hereby granted
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* without fee, provided that both the above copyright notice and this
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* permission notice appear in all copies and in supporting
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* documentation.
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*
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* The author makes no representations about the suitability of this
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* software for any purpose. It is provided "as is" without express or
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* implied warranty.
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*
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* THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
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* INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS,
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* IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, INDIRECT
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* OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
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* LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT,
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* NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
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* CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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*
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* $Id$
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* (Log is kept at end of this file)
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*/
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#include <math.h>
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#include <stdio.h>
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#include <sys/time.h>
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#include "sunpos.h"
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#include "../constants.h"
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#undef E
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/*
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* the epoch upon which these astronomical calculations are based is
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* 1990 january 0.0, 631065600 seconds since the beginning of the
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* "unix epoch" (00:00:00 GMT, Jan. 1, 1970)
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*
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* given a number of seconds since the start of the unix epoch,
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* DaysSinceEpoch() computes the number of days since the start of the
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* astronomical epoch (1990 january 0.0)
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*/
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#define EpochStart (631065600)
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#define DaysSinceEpoch(secs) (((secs)-EpochStart)*(1.0/(24*3600)))
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/*
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* assuming the apparent orbit of the sun about the earth is circular,
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* the rate at which the orbit progresses is given by RadsPerDay --
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* FG_2PI radians per orbit divided by 365.242191 days per year:
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*/
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#define RadsPerDay (FG_2PI/365.242191)
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/*
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* details of sun's apparent orbit at epoch 1990.0 (after
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* duffett-smith, table 6, section 46)
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*
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* Epsilon_g (ecliptic longitude at epoch 1990.0) 279.403303 degrees
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* OmegaBar_g (ecliptic longitude of perigee) 282.768422 degrees
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* Eccentricity (eccentricity of orbit) 0.016713
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*/
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#define Epsilon_g (279.403303*(FG_2PI/360))
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#define OmegaBar_g (282.768422*(FG_2PI/360))
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#define Eccentricity (0.016713)
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/*
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* MeanObliquity gives the mean obliquity of the earth's axis at epoch
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* 1990.0 (computed as 23.440592 degrees according to the method given
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* in duffett-smith, section 27)
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*/
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#define MeanObliquity (23.440592*(FG_2PI/360))
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static double solve_keplers_equation(double);
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static double sun_ecliptic_longitude(time_t);
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static void ecliptic_to_equatorial(double, double, double *, double *);
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static double julian_date(int, int, int);
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static double GST(time_t);
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/*
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* solve Kepler's equation via Newton's method
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* (after duffett-smith, section 47)
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*/
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static double solve_keplers_equation(double M) {
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double E;
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double delta;
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E = M;
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while (1) {
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delta = E - Eccentricity*sin(E) - M;
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if (fabs(delta) <= 1e-10) break;
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E -= delta / (1 - Eccentricity*cos(E));
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}
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return E;
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}
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/* compute ecliptic longitude of sun (in radians) (after
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* duffett-smith, section 47) */
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static double sun_ecliptic_longitude(time_t ssue) {
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/* time_t ssue; seconds since unix epoch */
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double D, N;
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double M_sun, E;
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double v;
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D = DaysSinceEpoch(ssue);
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N = RadsPerDay * D;
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N = fmod(N, FG_2PI);
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if (N < 0) N += FG_2PI;
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M_sun = N + Epsilon_g - OmegaBar_g;
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if (M_sun < 0) M_sun += FG_2PI;
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E = solve_keplers_equation(M_sun);
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v = 2 * atan(sqrt((1+Eccentricity)/(1-Eccentricity)) * tan(E/2));
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return (v + OmegaBar_g);
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}
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/* convert from ecliptic to equatorial coordinates (after
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* duffett-smith, section 27) */
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static void ecliptic_to_equatorial(double lambda, double beta,
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double *alpha, double *delta) {
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/* double lambda; ecliptic longitude */
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/* double beta; ecliptic latitude */
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/* double *alpha; (return) right ascension */
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/* double *delta; (return) declination */
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double sin_e, cos_e;
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sin_e = sin(MeanObliquity);
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cos_e = cos(MeanObliquity);
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*alpha = atan2(sin(lambda)*cos_e - tan(beta)*sin_e, cos(lambda));
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*delta = asin(sin(beta)*cos_e + cos(beta)*sin_e*sin(lambda));
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}
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/* computing julian dates (assuming gregorian calendar, thus this is
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* only valid for dates of 1582 oct 15 or later) (after duffett-smith,
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* section 4) */
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static double julian_date(int y, int m, int d) {
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/* int y; year (e.g. 19xx) */
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/* int m; month (jan=1, feb=2, ...) */
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/* int d; day of month */
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int A, B, C, D;
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double JD;
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/* lazy test to ensure gregorian calendar */
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if (y < 1583) {
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printf("WHOOPS! Julian dates only valid for 1582 oct 15 or later\n");
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}
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if ((m == 1) || (m == 2)) {
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y -= 1;
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m += 12;
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}
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A = y / 100;
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B = 2 - A + (A / 4);
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C = 365.25 * y;
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D = 30.6001 * (m + 1);
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JD = B + C + D + d + 1720994.5;
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return JD;
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}
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/* compute greenwich mean sidereal time (GST) corresponding to a given
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* number of seconds since the unix epoch (after duffett-smith,
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* section 12) */
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static double GST(time_t ssue) {
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/* time_t ssue; seconds since unix epoch */
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double JD;
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double T, T0;
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double UT;
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struct tm *tm;
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tm = gmtime(&ssue);
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JD = julian_date(tm->tm_year+1900, tm->tm_mon+1, tm->tm_mday);
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T = (JD - 2451545) / 36525;
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T0 = ((T + 2.5862e-5) * T + 2400.051336) * T + 6.697374558;
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T0 = fmod(T0, 24.0);
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if (T0 < 0) T0 += 24;
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UT = tm->tm_hour + (tm->tm_min + tm->tm_sec / 60.0) / 60.0;
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T0 += UT * 1.002737909;
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T0 = fmod(T0, 24.0);
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if (T0 < 0) T0 += 24;
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return T0;
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}
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/* given a particular time (expressed in seconds since the unix
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* epoch), compute position on the earth (lat, lon) such that sun is
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* directly overhead. (lat, lon are reported in radians */
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void fgSunPosition(time_t ssue, double *lon, double *lat) {
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/* time_t ssue; seconds since unix epoch */
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/* double *lat; (return) latitude */
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/* double *lon; (return) longitude */
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double lambda;
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double alpha, delta;
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double tmp;
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lambda = sun_ecliptic_longitude(ssue);
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ecliptic_to_equatorial(lambda, 0.0, &alpha, &delta);
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tmp = alpha - (FG_2PI/24)*GST(ssue);
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if (tmp < -FG_PI) {
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do tmp += FG_2PI;
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while (tmp < -FG_PI);
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} else if (tmp > FG_PI) {
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do tmp -= FG_2PI;
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while (tmp < -FG_PI);
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}
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*lon = tmp;
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*lat = delta;
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}
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/* $Log$
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/* Revision 1.1 1997/08/01 15:27:56 curt
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/* Initial revision.
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/*
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*/
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53
Time/sunpos.h
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53
Time/sunpos.h
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@ -0,0 +1,53 @@
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/*
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* sunpos.h
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* kirk johnson
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* july 1993
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*
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* code for calculating the position on the earth's surface for which
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* the sun is directly overhead (adapted from _practical astronomy
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* with your calculator, third edition_, peter duffett-smith,
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* cambridge university press, 1988.)
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*
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* RCS $Id$
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*
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* Copyright (C) 1989, 1990, 1993, 1994, 1995 Kirk Lauritz Johnson
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*
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* Parts of the source code (as marked) are:
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* Copyright (C) 1989, 1990, 1991 by Jim Frost
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* Copyright (C) 1992 by Jamie Zawinski <jwz@lucid.com>
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*
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* Permission to use, copy, modify and freely distribute xearth for
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* non-commercial and not-for-profit purposes is hereby granted
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* without fee, provided that both the above copyright notice and this
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* permission notice appear in all copies and in supporting
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* documentation.
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*
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* The author makes no representations about the suitability of this
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* software for any purpose. It is provided "as is" without express or
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* implied warranty.
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*
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* THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
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* INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS,
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* IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, INDIRECT
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* OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
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* LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT,
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* NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
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* CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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*/
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#ifndef SUNPOS_H
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#define SUNPOS_H
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#include <sys/time.h>
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/* given a particular time (expressed in seconds since the unix
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* epoch), compute position on the earth (lat, lon) such that sun is
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* directly overhead. (lat, lon are reported in radians */
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void fgSunPosition(time_t ssue, double *lon, double *lat);
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#endif /* SUNPOS_H */
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