/*******************************************************************************
* NAME:
* AppSolarCoordinates
*
* PURPOSE:
* Computes the apparent coordinates of the sun
*
* REFERENCES;
* Meeus, Jean. "Astronomical Algorithms, 1st ed." Willmann-Bell. Inc. 1991.
* pp. 151-153
*
* INPUT ARGUMENTS:
* JD (double)
* Julian Day for day/time to calculate at TD
*
* OUTPUT ARGUMENTS:
* *alpha (double)
* apparent right ascention in degrees
* *delta (double)
* apparent declination in degrees
*
* RETURNED VALUE:
* none yet
*
* GLOBALS USED:
* none
*
* FUNCTIONS CALLED:
* SinD, CosD, Revolution, atan2
*
* DATE/PROGRAMMER/NOTE:
* 09-16-1999 Todd A. Guillory created
* 07-04-2000 Todd A. Guillory condensed equations some more
* 07-27-2000 Todd A. Guillory checked with example 24.a
*
********************************************************************************/
void app_solar_coordinates( double JD, double *alpha, double *delta)
{
double T, /* Julian Centuries */
L0, /* geometric mean longitude of the sun */
M, /* mean anomoly */
e, /* eccentricity of Earth's orbit */
C, /* Sun's equation of center */
Long, /* true longitude of the sun */
v, /* true anomaly of the sun */
R, /* Radius in AU */
omega, /* nutation */
lamda, /* apparent longitude of the sun */
ep0; /* mean obliquity of the ecliptic */
/* Get Julian Centuries */
T = julian_centuries(JD);
/* calculate the geometric mean longitude of the sun */
L0 = 280.46645 + 36000.76983 * T + 0.0003032 * T * T;
/* calculate the mean anomaly of the sun */
M = 357.52910 + 35999.05030 * T - 0.0001559 * T * T - 0.00000048 * T * T * T;
/* calculate the eccentricity of the Earth's Orbit */
e = 0.016708617 - 0.000042037 * T - 0.0000001236 * T * T;
/* calculate the sun's equation of center */
C = (1.914600 - 0.004817 * T - 0.000014 * T * T) * SinD(M)
+ (0.019993 - 0.000101 * T) * SinD(2*M)
+ 0.000290 * SinD(3*M);
/* calculate the sun's true longitude */
Long = L0 + C;
/* calculate the true anomaly */
v = M + C;
/* calculate the sun's radius vector, distance of the earth in AUs */
R = (1.000001018 * ( 1 - e * e)) / ( 1 + e * CosD(v) );
omega = 125.04 - 1934.136 * T;
/* calculate the apparent longitude of the sun */
lamda = Revolution(Long - 0.00569 - 0.00478 * SinD(omega));
/* calculate the mean obliquity of the ecliptic */
ep0 = ((23*60)+26)*60+21.448 - 46.8150 * T - 0.00059 * T * T + 0.001813 * T * T * T;
ep0 /= 3600;
/* correct mean obliquity of the ecliptic */
ep0 = ep0 + 0.00256 * CosD(omega);
/* calculate right ascension and declination */
*alpha = Revolution(atan2(CosD(ep0) * SinD(lamda), CosD(lamda)) * kRadDeg);
*delta = asin(SinD(ep0) * SinD(lamda)) * kRadDeg;
}
double SunTimes::GMST0(double d)
{
double sidtim0;
sidtim0 = Revolution((180.0 + 356.0470 + 282.9404) +
(0.9856002585 + 4.70935E-5) * d);
return sidtim0;
}
void SunTimes::Sunpos(double d, double &lon, double &r)
{
double M,//太阳的平均近点角，从太阳观察到的地球（=从地球看到太阳的）距近日点（近地点）的角度。
w, //近日点的平均黄道经度。
e, //地球椭圆公转轨道离心率。
E, //太阳的偏近点角。计算公式见下面。
x, y,
v; //真近点角，太阳在任意时刻的真实近点角。
M = Revolution(356.0470 + 0.9856002585 * d);//自变量的组成：2000.0时刻太阳黄经为356.0470度,此后每天约推进一度（360度/365天
w = 282.9404 + 4.70935E-5 * d;//近日点的平均黄经。
e = 0.016709 - 1.151E-9 * d;//地球公转椭圆轨道离心率的时间演化。以上公式和黄赤交角公式一样，不必深究。
E = M + e * Radge * Sind(M) * (1.0 + e * Cosd(M));
x = Cosd(E) - e;
y = sqrt(1.0 - e * e) * Sind(E);
r = sqrt(x * x + y * y);
v = Atan2d(y, x);
lon = v + w;
if (lon >= 360.0)
lon -= 360.0;
}
int SunTimes::SunRiset(int year, int month, int day, double lon, double lat,
double altit, int upper_limb, double &trise, double &tset)
{
double d, /* Days since 2000 Jan 0.0 (negative before) */
//以历元2000.0起算的日数。
sr, /* Solar distance, astronomical units */
//太阳距离，以天文单位计算（约1.5亿公里）。
sRA, /* Sun's Right Ascension */
//同前，太阳赤经。
sdec, /* Sun's declination */
//太阳赤纬。
sradius, /* Sun's apparent radius */
//太阳视半径，约16分（受日地距离、大气折射等诸多影响）
t, /* Diurnal arc */
//周日弧，太阳一天在天上走过的弧长。
tsouth, /* Time when Sun is at south */
sidtime; /* Local sidereal time */
//当地恒星时，即地球的真实自转周期。比平均太阳日（日常时间）长3分56秒。
int rc = 0; /* Return cde from function - usually 0 */
/* Compute d of 12h local mean solar time */
d = Days_since_2000_Jan_0(year, month, day) + 0.5 - lon / 360.0;
//计算观测地当日中午时刻对应2000.0起算的日数。
/* Compute local sideral time of this moment */
sidtime = Revolution(GMST0(d) + 180.0 + lon);
//计算同时刻的当地恒星时（以角度为单位）。以格林尼治为基准，用经度差校正。
/* Compute Sun's RA + Decl at this moment */
sRA = 0.0;
sdec = 0.0;
sr = 0.0;
Sun_RA_dec(d, sRA, sdec, sr);
//计算同时刻太阳赤经赤纬。
/* Compute time when Sun is at south - in hours UT */
tsouth = 12.0 - Rev180(sidtime - sRA) / 15.0;
//计算太阳日的正午时刻，以世界时（格林尼治平太阳时）的小时计。
/* Compute the Sun's apparent radius, degrees */
sradius = 0.2666 / sr;
//太阳视半径。0.2666是一天文单位处的太阳视半径（角度）。
/* Do correction to upper limb, if necessary */
if (upper_limb != 0)
altit -= sradius;
//如果要用上边缘，就要扣除一个视半径。
/* Compute the diurnal arc that the Sun traverses to reach */
//计算周日弧。直接利用球面三角公式。如果碰到极昼极夜问题，同前处理。
/* the specified altitide altit: */
double cost;
cost = (Sind(altit) - Sind(lat) * Sind(sdec)) /
(Cosd(lat) * Cosd(sdec));
if (cost >= 1.0)
{
rc = -1;
t = 0.0;
}
else
{
if (cost <= -1.0)
{
rc = +1;
t = 12.0; /* Sun always above altit */
}
else
t = Acosd(cost) / 15.0; /* The diurnal arc, hours */
}
/* Store rise and set times - in hours UT */
trise = tsouth - t;
tset = tsouth + t;
return rc;
}
