Date: Thu, 09 Oct 2008 20:16:58 -0400 From: Michael Butler <imb@protected-networks.net> To: FreeBSD Stable <freebsd-stable@freebsd.org> Subject: Re: sidetrack [was Re: 'at now' not working as expected] Message-ID: <48EE9EFA.3080009@protected-networks.net> In-Reply-To: <48EE9D0E.8000002@protected-networks.net> References: <884679.22561.qm@web110112.mail.gq1.yahoo.com> <48E75BB7.2060206@madpilot.net> <e7db6d980810081817g6673b593pf9bb7e940562a340@mail.gmail.com> <20081009145337.P16723@sola.nimnet.asn.au> <48EDE8DC.8030108@webzone.net.au> <e7db6d980810091618j12731413ladd7e656e07eb17c@mail.gmail.com> <200810092322.m99NMD3l043255@apollo.backplane.com> <48EE9D0E.8000002@protected-networks.net>
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I wrote:
> The attached program (not mine - credits in the header) does this
> effectively given your current position as input,
Inserted as text since it got stripped last time ..
> ------------------------------------------------------------------------
/*
SUNRISET.C - computes Sun rise/set times, start/end of twilight, and
the length of the day at any date and latitude
Written as DAYLEN.C, 1989-08-16
Modified to SUNRISET.C, 1992-12-01
(c) Paul Schlyter, 1989, 1992
This program may be used by anyone for any purpose, iff:
1. it is not being sold for profit
2. this notice is not removed
*/
#include <stdio.h>
#include <math.h>
/* A macro to compute the number of days elapsed since 2000 Jan 0.0 */
/* (which is equal to 1999 Dec 31, 0h UT) */
#define days_since_2000_Jan_0(y,m,d) \
(367L*(y)-((7*((y)+(((m)+9)/12)))/4)+((275*(m))/9)+(d)-730530L)
/* Some conversion factors between radians and degrees */
#ifndef PI
#define PI 3.1415926535897932384
#endif
#define RADEG ( 180.0 / PI )
#define DEGRAD ( PI / 180.0 )
/* The trigonometric functions in degrees */
#define sind(x) sin((x)*DEGRAD)
#define cosd(x) cos((x)*DEGRAD)
#define tand(x) tan((x)*DEGRAD)
#define atand(x) (RADEG*atan(x))
#define asind(x) (RADEG*asin(x))
#define acosd(x) (RADEG*acos(x))
#define atan2d(y,x) (RADEG*atan2(y,x))
/* Following are some macros around the "workhorse" function __daylen__ */
/* They mainly fill in the desired values for the reference altitude */
/* below the horizon, and also selects whether this altitude should */
/* refer to the Sun's center or its upper limb. */
/* This macro computes the length of the day, from sunrise to sunset. */
/* Sunrise/set is considered to occur when the Sun's upper limb is */
/* 35 arc minutes below the horizon (this accounts for the refraction */
/* of the Earth's atmosphere). */
#define day_length(year,month,day,lon,lat) \
__daylen__( year, month, day, lon, lat, -35.0/60.0, 1 )
/* This macro computes the length of the day, including civil twilight. */
/* Civil twilight starts/ends when the Sun's center is 6 degrees below */
/* the horizon. */
#define day_civil_twilight_length(year,month,day,lon,lat) \
__daylen__( year, month, day, lon, lat, -6.0, 0 )
/* This macro computes the length of the day, incl. nautical twilight. */
/* Nautical twilight starts/ends when the Sun's center is 12 degrees */
/* below the horizon. */
#define day_nautical_twilight_length(year,month,day,lon,lat) \
__daylen__( year, month, day, lon, lat, -12.0, 0 )
/* This macro computes the length of the day, incl. astronomical
twilight. */
/* Astronomical twilight starts/ends when the Sun's center is 18 degrees
*/
/* below the horizon. */
#define day_astronomical_twilight_length(year,month,day,lon,lat) \
__daylen__( year, month, day, lon, lat, -18.0, 0 )
/* This macro computes times for sunrise/sunset. */
/* Sunrise/set is considered to occur when the Sun's upper limb is */
/* 35 arc minutes below the horizon (this accounts for the refraction */
/* of the Earth's atmosphere). */
#define sun_rise_set(year,month,day,lon,lat,rise,set) \
__sunriset__( year, month, day, lon, lat, -35.0/60.0, 1, rise, set )
/* This macro computes the start and end times of civil twilight. */
/* Civil twilight starts/ends when the Sun's center is 6 degrees below */
/* the horizon. */
#define civil_twilight(year,month,day,lon,lat,start,end) \
__sunriset__( year, month, day, lon, lat, -6.0, 0, start, end )
/* This macro computes the start and end times of nautical twilight. */
/* Nautical twilight starts/ends when the Sun's center is 12 degrees */
/* below the horizon. */
#define nautical_twilight(year,month,day,lon,lat,start,end) \
__sunriset__( year, month, day, lon, lat, -12.0, 0, start, end )
/* This macro computes the start and end times of astronomical twilight.
*/
/* Astronomical twilight starts/ends when the Sun's center is 18 degrees
*/
/* below the horizon. */
#define astronomical_twilight(year,month,day,lon,lat,start,end) \
__sunriset__( year, month, day, lon, lat, -18.0, 0, start, end )
/* Function prototypes */
double
__daylen__ (int year, int month, int day, double lon, double lat,
double altit, int upper_limb);
int
__sunriset__ (int year, int month, int day, double lon, double lat,
double altit, int upper_limb, double *rise, double *set);
void sunpos (double d, double *lon, double *r);
void sun_RA_dec (double d, double *RA, double *dec, double *r);
double revolution (double x);
double rev180 (double x);
double GMST0 (double d);
/* A small test program */
void
main (void)
{
int year, month, day;
double lon, lat;
double daylen, civlen, nautlen, astrlen;
double rise, set, civ_start, civ_end, naut_start, naut_end, astr_start,
astr_end;
int rs, civ, naut, astr;
printf ("Longitude (+ is east) and latitude (+ is north) : ");
scanf ("%lf %lf", &lon, &lat);
for (;;)
{
printf ("Input date ( yyyy mm dd ): ");
if (scanf ("%d %d %d", &year, &month, &day) != 3)
exit (0);
daylen = day_length (year, month, day, lon, lat);
civlen = day_civil_twilight_length (year, month, day, lon, lat);
nautlen = day_nautical_twilight_length (year, month, day, lon, lat);
astrlen = day_astronomical_twilight_length (year, month, day, lon, lat);
printf ("Day length: %5.2f hours\n", daylen);
printf ("With civil twilight %5.2f hours\n", civlen);
printf ("With nautical twilight %5.2f hours\n", nautlen);
printf ("With astronomical twilight %5.2f hours\n", astrlen);
printf ("Length of twilight: civil %5.2f hours\n", (civlen -
daylen) / 2.0);
printf (" nautical %5.2f hours\n", (nautlen -
daylen) / 2.0);
printf (" astronomical %5.2f hours\n", (astrlen -
daylen) / 2.0);
rs = sun_rise_set (year, month, day, lon, lat, &rise, &set);
civ = civil_twilight (year, month, day, lon, lat, &civ_start, &civ_end);
naut = nautical_twilight (year, month, day, lon, lat, &naut_start,
&naut_end);
astr = astronomical_twilight (year, month, day, lon, lat,
&astr_start, &astr_end);
printf ("Sun at south %5.2fh UT\n", (rise + set) / 2.0);
switch (rs)
{
case 0:
printf ("Sun rises %5.2fh UT, sets %5.2fh UT\n", rise, set);
break;
case +1:
printf ("Sun above horizon\n");
break;
case -1:
printf ("Sun below horizon\n");
break;
}
switch (civ)
{
case 0:
printf ("Civil twilight starts %5.2fh, ends %5.2fh UT\n", civ_start,
civ_end);
break;
case +1:
printf ("Never darker than civil twilight\n");
break;
case -1:
printf ("Never as bright as civil twilight\n");
break;
}
switch (naut)
{
case 0:
printf ("Nautical twilight starts %5.2fh, ends %5.2fh UT\n",
naut_start, naut_end);
break;
case +1:
printf ("Never darker than nautical twilight\n");
break;
case -1:
printf ("Never as bright as nautical twilight \n");
break;
}
switch (astr)
{
case 0:
printf ("Astronomical twilight starts %5.2fh, ends %5.2fh UT\n",
astr_start, astr_end);
break;
case +1:
printf ("Never darker than astronomical twilight \n ");
break;
case -1:
printf ("Never as bright as astronomical twilight \n ");
break;
}
}
}
/* The "workhorse" function for sun rise/set times */
int
__sunriset__ (int year, int month, int 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__ */
/* The "workhorse" function */
double
__daylen__ (int year, int month, int day, double lon, double lat,
double altit, int upper_limb)
/**********************************************************************/
/* 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 not critical. Set it to the correct */
/* longitude if you're picky, otherwise set to to, say, 0.0 */
/* The latitude however IS critical - be sure to get it correct */
/* 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 day length */
/* and to zero when computing day+twilight length. */
/**********************************************************************/
{
double d, /* Days since 2000 Jan 0.0 (negative before) */
obl_ecl, /* Obliquity (inclination) of Earth's axis */
sr, /* Solar distance, astronomical units */
slon, /* True solar longitude */
sin_sdecl, /* Sine of Sun's declination */
cos_sdecl, /* Cosine of Sun's declination */
sradius, /* Sun's apparent radius */
t; /* Diurnal arc */
/* Compute d of 12h local mean solar time */
d = days_since_2000_Jan_0 (year, month, day) + 0.5 - lon / 360.0;
/*
* Compute obliquity of ecliptic (inclination of Earth's axis)
*/
obl_ecl = 23.4393 - 3.563E-7 * d;
/* Compute Sun's position */
sunpos (d, &slon, &sr);
/* Compute sine and cosine of Sun's declination */
sin_sdecl = sind (obl_ecl) * sind (slon);
cos_sdecl = sqrt (1.0 - sin_sdecl * sin_sdecl);
/* 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) * sin_sdecl) / (cosd (lat) * cos_sdecl);
if (cost >= 1.0)
t = 0.0; /* Sun always below altit */
else if (cost <= -1.0)
t = 24.0; /* Sun always above altit */
else
t = (2.0 / 15.0) * acosd (cost); /* The diurnal arc, hours */
}
return t;
} /* __daylen__ */
/* This function computes the Sun's position at any instant */
void
sunpos (double d, double *lon, double *r)
/******************************************************/
/* Computes the Sun's ecliptic longitude and distance */
/* at an instant given in d, number of days since */
/* 2000 Jan 0.0. The Sun's ecliptic latitude is not */
/* computed, since it's always very near 0. */
/******************************************************/
{
double M, /* Mean anomaly of the Sun */
w, /* Mean longitude of perihelion */
/* Note: Sun's mean longitude = M + w */
e, /* Eccentricity of Earth's orbit */
E, /* Eccentric anomaly */
x, y, /* x, y coordinates in orbit */
v; /* True anomaly */
/* Compute mean elements */
M = revolution (356.0470 + 0.9856002585 * d);
w = 282.9404 + 4.70935E-5 * d;
e = 0.016709 - 1.151E-9 * d;
/* Compute true longitude and radius vector */
E = M + e * RADEG * 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); /* Solar distance */
v = atan2d (y, x); /* True anomaly */
*lon = v + w; /* True solar longitude */
if (*lon >= 360.0)
*lon -= 360.0; /* Make it 0..360 degrees */
}
void
sun_RA_dec (double d, double *RA, double *dec, double *r)
{
double lon, obl_ecl, x, y, z;
/* Compute Sun's ecliptical coordinates */
sunpos (d, &lon, r);
/* Compute ecliptic rectangular coordinates (z=0) */
x = *r * cosd (lon);
y = *r * sind (lon);
/*
* Compute obliquity of ecliptic (inclination of Earth's axis)
*/
obl_ecl = 23.4393 - 3.563E-7 * d;
/*
* Convert to equatorial rectangular coordinates - x is uchanged
*/
z = y * sind (obl_ecl);
y = y * cosd (obl_ecl);
/* Convert to spherical coordinates */
*RA = atan2d (y, x);
*dec = atan2d (z, sqrt (x * x + y * y));
} /* sun_RA_dec */
/******************************************************************/
/* This function reduces any angle to within the first revolution */
/* by subtracting or adding even multiples of 360.0 until the */
/* result is >= 0.0 and < 360.0 */
/******************************************************************/
#define INV360 ( 1.0 / 360.0 )
double
revolution (double x)
/*****************************************/
/* Reduce angle to within 0..360 degrees */
/*****************************************/
{
return (x - 360.0 * floor (x * INV360));
} /* revolution */
double
rev180 (double x)
/*********************************************/
/* Reduce angle to within +180..+180 degrees */
/*********************************************/
{
return (x - 360.0 * floor (x * INV360 + 0.5));
} /* revolution */
/*******************************************************************/
/* This function computes GMST0, the Greenwhich Mean Sidereal Time */
/* at 0h UT (i.e. the sidereal time at the Greenwhich meridian at */
/* 0h UT). GMST is then the sidereal time at Greenwich at any */
/* time of the day. I've generelized GMST0 as well, and define it */
/* as: GMST0 = GMST - UT -- this allows GMST0 to be computed at */
/* other times than 0h UT as well. While this sounds somewhat */
/* contradictory, it is very practical: instead of computing */
/* GMST like: */
/* */
/* GMST = (GMST0) + UT * (366.2422/365.2422) */
/* */
/* where (GMST0) is the GMST last time UT was 0 hours, one simply */
/* computes: */
/* */
/* GMST = GMST0 + UT */
/* */
/* where GMST0 is the GMST "at 0h UT" but at the current moment! */
/* Defined in this way, GMST0 will increase with about 4 min a */
/* day. It also happens that GMST0 (in degrees, 1 hr = 15 degr) */
/* is equal to the Sun's mean longitude plus/minus 180 degrees! */
/* (if we neglect aberration, which amounts to 20 seconds of arc */
/* or 1.33 seconds of time) */
/* */
/*******************************************************************/
double
GMST0 (double d)
{
double sidtim0;
/* Sidtime at 0h UT = L (Sun's mean longitude) + 180.0 degr */
/* L = M + w, as defined in sunpos(). Since I'm too lazy to */
/* add these numbers, I'll let the C compiler do it for me. */
/* Any decent C compiler will add the constants at compile */
/* time, imposing no runtime or code overhead. */
sidtim0 = revolution ((180.0 + 356.0470 + 282.9404) +
(0.9856002585 + 4.70935E-5) * d);
return sidtim0;
} /* GMST0 */
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