int __sunriset__( long year, long month, long day, double lon, double lat,
double altit, int upper_limb, double *trise, double *tset )
/***************************************************************************/
/* Note: year,month,date = calendar date, 1801-2099 only. */
/* Eastern longitude positive, Western longitude negative */
/* Northern latitude positive, Southern latitude negative */
/* The longitude value IS critical in this function! */
/* altit = the altitude which the Sun should cross */
/* Set to -35/60 degrees for rise/set, -6 degrees */
/* for civil, -12 degrees for nautical and -18 */
/* degrees for astronomical twilight. */
/* upper_limb: non-zero -> upper limb, zero -> center */
/* Set to non-zero (e.g. 1) when computing rise/set */
/* times, and to zero when computing start/end of */
/* twilight. */
/* *rise = where to store the rise time */
/* *set = where to store the set time */
/* Both times are relative to the specified altitude, */
/* and thus this function can be used to comupte */
/* various twilight times, as well as rise/set times */
/* Return value: 0 = sun rises/sets this day, times stored at */
/* *trise and *tset. */
/* +1 = sun above the specified "horizon" 24 hours. */
/* *trise set to time when the sun is at south, */
/* minus 12 hours while *tset is set to the south */
/* time plus 12 hours. "Day" length = 24 hours */
/* -1 = sun is below the specified "horizon" 24 hours */
/* "Day" length = 0 hours, *trise and *tset are */
/* both set to the time when the sun is at south. */
/* */
/**********************************************************************/
{
double d, /* Days since 2000 Jan 0.0 (negative before) */
sr, /* Solar distance, astronomical units */
sRA, /* Sun's Right Ascension */
sdec, /* Sun's declination */
sradius, /* Sun's apparent radius */
t, /* Diurnal arc */
tsouth, /* Time when Sun is at south */
sidtime; /* Local sidereal time */
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;
/* Compute local sideral time of this moment */
sidtime = revolution( GMST0(d) + 180.0 + lon );
/* Compute Sun's RA + Decl at this moment */
sun_RA_dec( d, &sRA, &sdec, &sr );
/* Compute time when Sun is at south - in hours UT */
tsouth = 12.0 - rev180(sidtime - sRA)/15.0;
/* Compute the Sun's apparent radius, degrees */
sradius = 0.2666 / sr;
/* Do correction to upper limb, if necessary */
if ( upper_limb )
altit -= sradius;
/* Compute the diurnal arc that the Sun traverses to reach */
/* 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; /* Sun always below altit */
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;
} /* __sunriset__ */
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;
}
