Programming interface to the Swiss Ephemeris - programmiastral.com

Swiss Ephemeris 24
If you first compute tjd on the basis of the registered Local Apparent Time, you convert it to Local Mean
Time with:
tjd_mean = tjd_app + e;
8. Time functions
/* delta t from Julian day number */
double swe_deltat(double tjd);
/* get tidal acceleration used in swe_deltat() */
double swe_get_tid_acc(void);
/* set tidal acceleration to be used in swe_deltat() */
void swe_set_tid_acc(double t_acc);
The Julian day number, you compute from a birth date, will be Universal Time (UT, former GMT) and can
be used to compute the star time and the houses. However, for the planets and the other factors, you have to
convert UT to Ephemeris time (ET):
8.1 swe_deltat()
tjde = tjd + swe_deltat(tjd); where tjd = Julian day in UT, tjde = in ET
For precision fanatics: The value of delta t depends on the tidal acceleration in the motion of the moon. Its
default value corresponds to the state-of-the-art JPL ephemeris (e.g. DE406, s. swephexp.h). If you use
another JPL ephemeris, e.g. DE200, you may wish the tidal constant of DE200. This makes a difference of 0.5
time seconds in 1900 and 4 seconds in 1800 (= 0.2” in the position of the sun). However, this effect is limited to
the period 1620 - ~1997. To change the tidal acceleration, use the function
8.2 swe_set_tid_acc(), swe_get_tid_acc()
swe_set_tid_acc(acceleration); // Do this before calling deltat() !
The values that acceleration can have are listed in swephexp.h. (e.g. SE_TIDAL_200, etc.)
To find out the built-in value of the tidal acceleration, you can call
acceleration = swe_get_tidacc();
9. The function swe_set_topo() for topocentric planet positions
void swe_set_topo(double geolon, double geolat, double altitude);
/* eastern longitude is positive, western longitude is negative,
northern latitude is positive, southern latitude is negative */
This function must be called before topocentric planet positions for a certain birth place can be computed. It tells
Swiss Ephemeris, what geographic position is to be used. Geographic longitude geolon and latitude geolat must
be in degrees, the altitude above sea must be in meters. Neglecting the altitude can result in an error of
about 2 arc seconds with the moon and at an altitude 3000 m. After calling swe_set_topo(), add
SEFLG_TOPOCTR to iflag and call swe_calc() as with an ordinary computation. E.g.:
swe_set_topo(geo_lon, geo_lat, altitude_above_sea);
iflag | = SEFLG_TOPOCTR;
for (i = 0; i < NPLANETS; i++) {
iflgret = swe_calc( tjd, ipl, iflag, xp, serr );
printf(”%f\n”, xp[0]);
}
The parameters set by swe_set_topo() survive swe_close().