SuperNOVAS v1.3
The NOVAS C library, made better
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equinox.c File Reference

Functions

double accum_prec (double t)
 
double ee_ct (double jd_tt_high, double jd_tt_low, enum novas_accuracy accuracy)
 
int fund_args (double t, novas_delaunay_args *restrict a)
 
double ira_equinox (double jd_tdb, enum novas_equinox_type equinox, enum novas_accuracy accuracy)
 
double mean_obliq (double jd_tdb)
 
int nutation (double jd_tdb, enum novas_nutation_direction direction, enum novas_accuracy accuracy, const double *in, double *out)
 
int nutation_angles (double t, enum novas_accuracy accuracy, double *restrict dpsi, double *restrict deps)
 
double planet_lon (double t, enum novas_planet planet)
 
short precession (double jd_tdb_in, const double *in, double jd_tdb_out, double *out)
 

Detailed Description

Date
Created on Mar 6, 2025
Author
G. Kaplan and Attila Kovacs

Various function for calculating the equator and equinox of date, and related quatities.

Function Documentation

◆ accum_prec()

double accum_prec ( double  t)

Returns the general precession in longitude (Simon et al. 1994), equivalent to 5028.8200 arcsec/cy at J2000.

Parameters
t[cy] Julian centuries since J2000
Returns
[rad] the approximate precession angle [-π:π].
See also
planet_lon()
nutation_angles()
ee_ct()
NOVAS_JD_J2000
Since
1.0
Author
Attila Kovacs

References TWOPI.

◆ ee_ct()

double ee_ct ( double  jd_tt_high,
double  jd_tt_low,
enum novas_accuracy  accuracy 
)

Computes the "complementary terms" of the equation of the equinoxes. The input Julian date can be split into high and low order parts for improved accuracy. Typically, the split is into integer and fractiona parts. If the precision of a single part is sufficient, you may set the low order part to 0.

The series used in this function was derived from the first reference. This same series was also adopted for use in the IAU's Standards of Fundamental Astronomy (SOFA) software (i.e., subroutine eect00.for and function eect00.c).

The low-accuracy series used in this function is a simple implementation derived from the first reference, in which terms smaller than 2 microarcseconds have been omitted.

NOTES:

  1. This function caches the results of the last calculation in case it may be re-used at no extra computational cost for the next call.

REFERENCES:

  1. Capitaine, N., Wallace, P.T., and McCarthy, D.D. (2003). Astron. & Astrophys. 406, p. 1135-1149. Table 3.
  2. IERS Conventions (2010), Chapter 5, p. 60, Table 5.2e.
    (Table 5.2e presented in the printed publication is a truncated series. The full series, which is used in NOVAS, is available on the IERS Conventions Center website: ftp://tai.bipm.org/iers/conv2010/chapter5/tab5.2e.txt)
Parameters
jd_tt_high[day] High-order part of TT based Julian date.
jd_tt_low[day] Low-order part of TT based Julian date.
accuracyNOVAS_FULL_ACCURACY (0) or NOVAS_REDUCED_ACCURACY (1)
Returns
[rad] Complementary terms, in radians.
See also
e_tilt()
cel_pole()
nutation()
sidereal_time()
Deprecated:
(for intrernal use) There is no good reason why this function should be exposed to users of the library. It is intended only for use by e_tilt() internally.

References accum_prec(), novas_delaunay_args::D, novas_delaunay_args::F, fund_args(), NOVAS_FULL_ACCURACY, NOVAS_MERCURY, NOVAS_NEPTUNE, NOVAS_REDUCED_ACCURACY, novas_delaunay_args::Omega, and planet_lon().

◆ fund_args()

int fund_args ( double  t,
novas_delaunay_args *restrict  a 
)

Compute the fundamental arguments (mean elements) of the Sun and Moon.

REFERENCES:

  1. Simon et al. (1994) Astronomy and Astrophysics 282, 663-683, esp. Sections 3.4-3.5.
Parameters
t[cy] TDB time in Julian centuries since J2000.0
[out]a[rad] Fundamental arguments data to populate (5 doubles) [0:2π]
Returns
0 if successful, or -1 if the output pointer argument is NULL.
See also
nutation_angles()
ee_ct()
NOVAS_JD_J2000

References novas_norm_ang().

◆ ira_equinox()

double ira_equinox ( double  jd_tdb,
enum novas_equinox_type  equinox,
enum novas_accuracy  accuracy 
)

Compute the intermediate right ascension of the equinox at the input Julian date, using an analytical expression for the accumulated precession in right ascension. For the true equinox, the result is the equation of the origins.

NOTES:

  1. Fixes bug in NOVAS C 3.1, which returned the value for the wrong 'equinox' if 'equinox = 1' was requested for the same 'jd_tbd' and 'accuracy' as a the preceding call with 'equinox = 0'. As a result, the caller ended up with the mean instead of the expected true equinox R.A. value.

REFERENCES:

  1. Capitaine, N. et al. (2003), Astronomy and Astrophysics 412, 567-586, eq. (42).
Parameters
jd_tdb[day] Barycentric Dynamic Time (TDB) based Julian date
equinoxNOVAS_MEAN_EQUINOX (0) or NOVAS_TRUE_EQUINOX (1, or non-zero)
accuracyNOVAS_FULL_ACCURACY (0) or NOVAS_REDUCED_ACCURACY (1, or non-zero)
Returns
[h] Intermediate right ascension of the equinox, in hours (+ or -). If 'equinox' = 1 (i.e true equinox), then the returned value is the equation of the origins.
See also
cio_location()
gcrs_to_cirs()
Deprecated:
(for internal use) There is no good reason why this function should be exposed to users. It is intended only for cio_location() internally.

References e_tilt(), NOVAS_FULL_ACCURACY, NOVAS_REDUCED_ACCURACY, and NOVAS_TRUE_EQUINOX.

◆ mean_obliq()

double mean_obliq ( double  jd_tdb)

Computes the mean obliquity of the ecliptic.

REFERENCES:

  1. Capitaine et al. (2003), Astronomy and Astrophysics 412, 567-586.
Parameters
jd_tdb[day] Barycentric Dynamic Time (TDB) based Julian date
Returns
[arcsec] Mean obliquity of the ecliptic in arcseconds.
See also
e_tilt()
equ2ecl()
ecl2equ()
tt2tdb()

◆ nutation()

int nutation ( double  jd_tdb,
enum novas_nutation_direction  direction,
enum novas_accuracy  accuracy,
const double *  in,
double *  out 
)

Nutates equatorial rectangular coordinates from mean equator and equinox of epoch to true equator and equinox of epoch. Inverse transformation may be applied by setting flag 'direction'.

This is the old (pre IAU 2006) method of nutation calculation. If you follow the now standard IAU 2000/2006 methodology you will want to use nutation_angles() instead.

REFERENCES:

  1. Explanatory Supplement To The Astronomical Almanac, pp. 114-115.
Parameters
jd_tdb[day] Barycentric Dynamic Time (TDB) based Julian date
directionNUTATE_MEAN_TO_TRUE (0) or NUTATE_TRUE_TO_MEAN (-1; or non-zero)
accuracyNOVAS_FULL_ACCURACY (0) or NOVAS_REDUCED_ACCURACY (1)
inPosition 3-vector, geocentric equatorial rectangular coordinates, referred to mean equator and equinox of epoch.
[out]outPosition vector, geocentric equatorial rectangular coordinates, referred to true equator and equinox of epoch. It can be the same as the input position.
Returns
0 if successful, or -1 if one of the vector arguments is NULL.
See also
nutation_angles()
tt2tdb()
NOVAS_TOD

References e_tilt(), and NUTATE_MEAN_TO_TRUE.

◆ nutation_angles()

int nutation_angles ( double  t,
enum novas_accuracy  accuracy,
double *restrict  dpsi,
double *restrict  deps 
)

Returns the values for nutation in longitude and nutation in obliquity for a given TDB Julian date. The nutation model selected depends upon the input value of 'accuracy'. See notes below for important details.

This function selects the nutation model depending first upon the input value of 'accuracy'. If 'accuracy' is NOVAS_FULL_ACCURACY (0), the IAU 2000A nutation model is used. Otherwise the model set by set_nutation_lp_provider() is used, or else the default nu2000k().

See the prologs of the nutation functions in file 'nutation.c' for details concerning the models.

REFERENCES:

  1. Kaplan, G. (2005), US Naval Observatory Circular 179.
Parameters
t[cy] TDB time in Julian centuries since J2000.0
accuracyNOVAS_FULL_ACCURACY (0) or NOVAS_REDUCED_ACCURACY (1)
[out]dpsi[arcsec] Nutation in longitude in arcseconds.
[out]deps[arcsec] Nutation in obliquity in arcseconds.
Returns
0 if successful, or -1 if the output pointer arguments are NULL
See also
set_nutation_lp_provider()
nutation()
iau2000b()
nu2000k()
cio_basis()
NOVAS_CIRS
NOVAS_JD_J2000

References get_nutation_lp_provider(), iau2000a(), and NOVAS_FULL_ACCURACY.

◆ planet_lon()

double planet_lon ( double  t,
enum novas_planet  planet 
)

Returns the planetary longitude, for Mercury through Neptune, w.r.t. mean dynamical ecliptic and equinox of J2000, with high order terms omitted (Simon et al. 1994, 5.8.1-5.8.8).

Parameters
t[cy] Julian centuries since J2000
planetNovas planet id, e.g. NOVAS_MARS.
Returns
[rad] The approximate longitude of the planet in radians [-π:π], or NAN if the planet id is out of range.
See also
accum_prec()
nutation_angles()
ee_ct()
NOVAS_JD_J2000
Since
1.0
Author
Attila Kovacs

References NOVAS_EARTH, NOVAS_JUPITER, NOVAS_MARS, NOVAS_MERCURY, NOVAS_NEPTUNE, NOVAS_SATURN, NOVAS_URANUS, NOVAS_VENUS, and TWOPI.

◆ precession()

short precession ( double  jd_tdb_in,
const double *  in,
double  jd_tdb_out,
double *  out 
)

Precesses equatorial rectangular coordinates from one epoch to another. Unlike the original NOVAS routine, this routine works for any pairing of the time arguments.

This function calculates precession for the old (pre IAU 2000) methodology. Its main use for NOVAS users is to allow converting older catalog coordinates e.g. to J2000 coordinates, which then can be converted to the now standard ICRS system via frame_tie().

NOTE:

  1. Unlike the original NOVAS C 3.1 version, this one does not require that one of the time arguments must be J2000. You can precess from any date to any other date, and the intermediate epoch of J2000 will be handled internally as needed.

  2. This function caches the results of the last calculation in case it may be re-used at no extra computational cost for the next call.

REFERENCES:

  1. Explanatory Supplement To The Astronomical Almanac, pp. 103-104.
  2. Capitaine, N. et al. (2003), Astronomy And Astrophysics 412, pp. 567-586.
  3. Hilton, J. L. et al. (2006), IAU WG report, Celest. Mech., 94, pp. 351-367.
Parameters
jd_tdb_in[day] Barycentric Dynamic Time (TDB) based Julian date of the input epoch
inPosition 3-vector, geocentric equatorial rectangular coordinates, referred to mean dynamical equator and equinox of the initial epoch.
jd_tdb_out[day] Barycentric Dynamic Time (TDB) based Julian date of the output epoch
[out]outPosition 3-vector, geocentric equatorial rectangular coordinates, referred to mean dynamical equator and equinox of the final epoch. It can be the same vector as the input.
Returns
0 if successful, or -1 if either of the position vectors is NULL.
See also
nutation()
frame_tie()
novas_epoch()
tt2tdb()
cio_basis()
NOVAS_TOD
NOVAS_JD_J2000
NOVAS_JD_B1950
NOVAS_JD_B1900

References precession().