24
4.11
EQU2HOR
void equ2hor (double tjd, double deltat, double x, double y,
site_info *location, double ra, double dec,
short int ref_option,
double *zd, double *az, double *rar, double *decr)
PURPOSE:
This function transforms apparent equatorial coordinates (right
ascension and declination) to horizon coordinates (zenith
distance and azimuth). It uses a method that properly accounts
for polar motion, which is significant at the sub-arcsecond
level. This function can also adjust coordinates for atmospheric
refraction.
INPUT
ARGUMENTS:
tjd (double)
TT (or TDT) Julian date.
deltat (double)
Difference TT (or TDT)-UT1 at 'tjd', in seconds.
x (double)
Conventionally-defined x coordinate of celestial ephemeris
pole with respect to IERS reference pole, in arcseconds.
y (double)
Conventionally-defined y coordinate of celestial ephemeris
pole with respect to IERS reference pole, in arcseconds.
*location (struct site_info)
Pointer to structure containing observer's location (defined
in novas.h).
ra (double)
Topocentric right ascension of object of interest, in hours,
referred to true equator and equinox of date.
dec (double)
Topocentric declination of object of interest, in degrees,
referred to true equator and equinox of date.
ref_option (short int)
= 0 ... no refraction
= 1 ... include refraction, using 'standard' atmospheric
conditions.
= 2 ... include refraction, using atmospheric parameters
input in the 'location' structure.
OUTPUT
ARGUMENTS:
*zd (double)
Topocentric zenith distance in degrees, affected by
refraction if 'ref_option' is non-zero.
*az (double)
Topocentric azimuth (measured east from north) in degrees.
*rar (double)
Topocentric right ascension of object of interest, in hours,
referred to true equator and equinox of date, affected by
refraction if 'ref_option' is non-zero.
*decr (double)
Topocentric declination of object of interest, in degrees,
referred to true equator and equinox of date, affected by
refraction if 'ref_option' is non-zero.
RETURNED
VALUE:
None.
