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e16-epplets/epplets/CalcEphem.c

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#include "CalcEphem.h"
/* Moon.c */
double Moon(double T, double *LAMBDA, double *BETA, double *R,
double *AGE);
double NewMoon(double, double, double);
static double kepler(double, double);
static double jd(int, int, int, double);
static double hour24(double);
static double angle2pi(double);
static double angle360(double);
static double
frac(double x)
{
x -= (int)x;
return ((x < 0) ? x + 1.0 : x);
}
static int
DayofYear(int year, int month, int day)
{
return ((int)(jd(year, month, day, 0.0) - jd(year, 1, 0, 0.0)));
}
static int
DayofWeek(int year, int month, int day, char dowstr[])
{
double JD, A, Afrac;
int n, iA;
JD = jd(year, month, day, 0.0);
A = (JD + 1.5) / 7.0;
iA = (int)A;
Afrac = A - (double)iA;
n = (int)(Afrac * 7.0 + 0.5);
switch (n)
{
case 0:
strcpy(dowstr, "Sunday");
break;
case 1:
strcpy(dowstr, "Monday");
break;
case 2:
strcpy(dowstr, "Tuesday");
break;
case 3:
strcpy(dowstr, "Wednesday");
break;
case 4:
strcpy(dowstr, "Thursday");
break;
case 5:
strcpy(dowstr, "Friday");
break;
case 6:
strcpy(dowstr, "Saturday");
break;
}
return (n);
}
void
CalcEphem(date, UT, c)
long int date; /* integer containing the date (e.g. 960829) */
double UT; /* Universal Time */
CTrans *c; /* structure containing all the relevent coord trans info */
{
int year, month, day;
double TU, TU2, TU3, T0;
double varep, varpi;
double eccen, epsilon;
double days, M, E, nu, lambnew;
double r0, earth_sun_distance;
double RA, DEC, RA_Moon, DEC_Moon;
double TDT;
double AGE;
double LambdaMoon, BetaMoon, R;
double Ta, Tb, Tc;
double SinGlat, CosGlat, Tau, lmst, x, y, z;
double SinTau, CosTau, SinDec, CosDec;
c->UT = UT;
year = (int)(date / 10000);
month = (int)((date - year * 10000) / 100);
day = (int)(date - year * 10000 - month * 100);
c->year = year;
c->month = month;
c->day = day;
c->doy = DayofYear(year, month, day);
c->dow = DayofWeek(year, month, day, c->dowstr);
/*
* Compute Greenwich Mean Sidereal Time (gmst)
* The TU here is number of Julian centuries
* since 2000 January 1.5
* From the 1996 astronomical almanac
*/
TU = (jd(year, month, day, 0.0) - 2451545.0) / 36525.0;
TU2 = TU * TU;
TU3 = TU2 * TU;
T0 = (6.0 + 41.0 / 60.0 + 50.54841 / 3600.0) + 8640184.812866 / 3600.0 * TU
+ 0.093104 / 3600.0 * TU2 - 6.2e-6 / 3600.0 * TU3;
T0 = hour24(T0);
c->gmst = hour24(T0 + UT * 1.002737909);
lmst = 24.0 * frac((c->gmst - c->Glon / 15.0) / 24.0);
/*
*
* Construct Transformation Matrix from GEI to GSE systems
*
*
* First compute:
* mean ecliptic longitude of sun at epoch TU (varep)
* elciptic longitude of perigee at epoch TU (varpi)
* eccentricity of orbit at epoch TU (eccen)
*
* The TU here is the number of Julian centuries since
* 1900 January 0.0 (= 2415020.0)
*/
TDT = UT + 59.0 / 3600.0;
TU = (jd(year, month, day, TDT) - 2415020.0) / 36525.0;
varep = (279.6966778 + 36000.76892 * TU + 0.0003025 * TU * TU) * RadPerDeg;
varpi = (281.2208444 + 1.719175 * TU + 0.000452778 * TU * TU) * RadPerDeg;
eccen = 0.01675104 - 0.0000418 * TU - 0.000000126 * TU * TU;
c->eccentricity = eccen;
/*
* Compute the Obliquity of the Ecliptic at epoch TU
* The TU in this formula is the number of Julian
* centuries since epoch 2000 January 1.5
*/
TU = (jd(year, month, day, TDT) - jd(2000, 1, 1, 12.0)) / 36525.0;
epsilon = (23.43929167 - 0.013004166 * TU - 1.6666667e-7 * TU * TU
- 5.0277777778e-7 * TU * TU * TU) * RadPerDeg;
c->epsilon = epsilon;
/*
* Compute:
* Number of Days since epoch 1990.0 (days)
* The Mean Anomaly (M)
* The True Anomaly (nu)
* The Eccentric Anomaly via Keplers equation (E)
*
*
*/
days = jd(year, month, day, TDT) - jd(year, month, day, TDT);
M = angle2pi(2.0 * M_PI / 365.242191 * days + varep - varpi);
E = kepler(M, eccen);
nu = 2.0 * atan(sqrt((1.0 + eccen) / (1.0 - eccen)) * tan(E / 2.0));
lambnew = angle2pi(nu + varpi);
c->lambda_sun = lambnew;
/*
* Compute distance from earth to the sun
*/
r0 = 1.495985e8; /* in km */
earth_sun_distance =
r0 * (1 - eccen * eccen) / (1.0 + eccen * cos(nu)) / 6371.2;
c->earth_sun_dist = earth_sun_distance;
/*
* Compute Right Ascension and Declination of the Sun
*/
RA =
angle360(atan2(sin(lambnew) * cos(epsilon), cos(lambnew)) * 180.0 / M_PI);
DEC = asin(sin(epsilon) * sin(lambnew)) * 180.0 / M_PI;
c->RA_sun = RA;
c->DEC_sun = DEC;
/*
* Compute Moon Phase and AGE Stuff. The AGE that comes out of Moon()
* is actually the Phase converted to days. Since AGE is actually defined
* to be time since last NewMoon, we need to figure out what the JD of the
* last new moon was. Thats done below....
*/
TU = (jd(year, month, day, TDT) - 2451545.0) / 36525.0;
c->MoonPhase = Moon(TU, &LambdaMoon, &BetaMoon, &R, &AGE);
LambdaMoon *= RadPerDeg;
BetaMoon *= RadPerDeg;
RA_Moon =
angle360(atan2
(sin(LambdaMoon) * cos(epsilon) - tan(BetaMoon) * sin(epsilon),
cos(LambdaMoon)) * DegPerRad);
DEC_Moon =
asin(sin(BetaMoon) * cos(epsilon) +
cos(BetaMoon) * sin(epsilon) * sin(LambdaMoon)) * DegPerRad;
c->RA_moon = RA_Moon;
c->DEC_moon = DEC_Moon;
/*
* Compute Alt/Az coords
*/
Tau = (15.0 * lmst - RA_Moon) * RadPerDeg;
CosGlat = cos(c->Glat * RadPerDeg);
SinGlat = sin(c->Glat * RadPerDeg);
CosTau = cos(Tau);
SinTau = sin(Tau);
SinDec = sin(DEC_Moon * RadPerDeg);
CosDec = cos(DEC_Moon * RadPerDeg);
x = CosDec * CosTau * SinGlat - SinDec * CosGlat;
y = CosDec * SinTau;
z = CosDec * CosTau * CosGlat + SinDec * SinGlat;
c->A_moon = DegPerRad * atan2(y, x);
c->h_moon = DegPerRad * asin(z);
c->Visible = (c->h_moon < 0.0) ? 0 : 1;
/*
* Compute accurate AGE of the Moon
*/
Tb = TU - AGE / 36525.0; /* should be very close to minimum */
Ta = Tb - 0.4 / 36525.0;
Tc = Tb + 0.4 / 36525.0;
c->MoonAge = (TU - NewMoon(Ta, Tb, Tc)) * 36525.0;
/*
* Compute Earth-Moon distance
*/
c->EarthMoonDistance = R;
}
static double
kepler(M, e)
double M, e;
{
int n = 0;
double E, Eold, eps = 1.0e-8;
E = M + e * sin(M);
do
{
Eold = E;
E = Eold + (M - Eold + e * sin(Eold)) / (1.0 - e * cos(Eold));
++n;
}
while ((fabs(E - Eold) > eps) && (n < 100));
return (E);
}
/*
* Compute the Julian Day number for the given date.
* Julian Date is the number of days since noon of Jan 1 4713 B.C.
*/
static double
jd(ny, nm, nd, UT)
int ny, nm, nd;
double UT;
{
double A, B, C, D, JD, day;
day = nd + UT / 24.0;
if ((nm == 1) || (nm == 2))
{
ny = ny - 1;
nm = nm + 12;
}
if (((double)ny + nm / 12.0 + day / 365.25) >=
(1582.0 + 10.0 / 12.0 + 15.0 / 365.25))
{
A = ((int)(ny / 100.0));
B = 2.0 - A + (int)(A / 4.0);
}
else
{
B = 0.0;
}
if (ny < 0.0)
{
C = (int)((365.25 * (double)ny) - 0.75);
}
else
{
C = (int)(365.25 * (double)ny);
}
D = (int)(30.6001 * (double)(nm + 1));
JD = B + C + D + day + 1720994.5;
return (JD);
}
static double
hour24(hour)
double hour;
{
int n;
if (hour < 0.0)
{
n = (int)(hour / 24.0) - 1;
return (hour - n * 24.0);
}
else if (hour > 24.0)
{
n = (int)(hour / 24.0);
return (hour - n * 24.0);
}
else
{
return (hour);
}
}
static double
angle2pi(angle)
double angle;
{
int n;
double a;
a = 2.0 * M_PI;
if (angle < 0.0)
{
n = (int)(angle / a) - 1;
return (angle - n * a);
}
else if (angle > a)
{
n = (int)(angle / a);
return (angle - n * a);
}
else
{
return (angle);
}
}
static double
angle360(angle)
double angle;
{
int n;
if (angle < 0.0)
{
n = (int)(angle / 360.0) - 1;
return (angle - n * 360.0);
}
else if (angle > 360.0)
{
n = (int)(angle / 360.0);
return (angle - n * 360.0);
}
else
{
return (angle);
}
}
#if 0
static void
Radec_to_Cart(ra, dec, r)
double ra, dec; /* RA and DEC */
Vector *r; /* returns corresponding cartesian unit vector */
{
/*
* Convert ra/dec from degrees to radians
*/
ra *= RadPerDeg;
dec *= RadPerDeg;
/*
* Compute cartesian coordinates (in GEI)
*/
r->x = cos(dec) * cos(ra);
r->y = cos(dec) * sin(ra);
r->z = sin(dec);
}
int
LeapYear(year)
int year;
{
if ((year % 100 == 0) && (year % 400 != 0))
return (0);
else if (year % 4 == 0)
return (1);
else
return (0);
}
#endif
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