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

Functions
double	accum_prec (double t)
	Returns the general precession in longitude (Simon et al.

short	cel_pole (double jd_tt, enum novas_pole_offset_type type, double dpole1, double dpole2)

int	e_tilt (double jd_tdb, enum novas_accuracy accuracy, double restrict mobl, double restrict tobl, double restrict ee, double restrict dpsi, double *restrict deps)
	(primarily for internal use) Computes quantities related to the orientation of the Earth's rotation axis at the specified Julian date.

int	fund_args (double t, novas_delaunay_args *restrict a)
	Compute the fundamental (a.k.a.

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.

double	mean_obliq (double jd_tdb)
	Computes the mean obliquity of the ecliptic.

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.

short	precession (double jd_tdb_in, const double in, double jd_tdb_out, double out)
	Precesses equatorial rectangular coordinates from one epoch to another using the IAU2006 (P03) precession model of Capitaine et al.

Detailed Description

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

Date: Created on Mar 6, 2025

Author: G. Kaplan and Attila Kovacs

See also: cio.c, earth.c, frames.c, nutation.c, transform.c

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(), e_tilt(), NOVAS_JD_J2000

Since: 1.0

Author: Attila Kovacs

References TWOPI.

◆ cel_pole()

short cel_pole	(	double	jd_tt,
		enum novas_pole_offset_type	type,
		double	dpole1,
		double	dpole2 )

Deprecated: This old way of incorporating Earth orientation parameters into the true equator and equinox is now disfavored. Instead, the pole offsets should be used only to convert between the Terrestrial Intermediate Reference System (TIRS) / Pseudo Earth Fixed (PEF) and the International Terrestrial Reference System (ITRS) going forward, e.g. via novas_app_to_hor() / novas_hor_to_app() or else wobble().

Specifies the unmodeled celestial pole offsets for high-precision applications to be applied to the True of Date (TOD) equator, in the old, pre IAU 2006 methodology. Nonetheless, these offsets should be specified relative to the IAU2006 precession / nutation model to provide a correction to the modeled (precessed and nutated) position of Earth's pole, such those derived from observations and published by IERS.

The call sets the global variables PSI_COR and EPS_COR, for subsequent calls to e_tilt(). As such, it should be called to specify pole offsets prior to legacy NOVAS C equinox-specific calls. The global values of PSI_COR and EPS_COR specified via this function will be effective until explicitly changed again.

NOTES:

The pole offsets et this way will affect all future TOD-based calculations, until the pole is changed or reset again. Hence, you should be extremely careful using it (if at all), as it may become an unpredictable source of inaccuracy if implicitly applied without intent to do so.
The current UT1 - UTC time difference, and polar offsets, historical data and near-term projections are published in the <a href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html>IERS Bulletins
If Δδψ, Δδdε offsets are specified, these must be the residual corrections relative to the IAU 2006 precession/nutation model (not the Lieske et al. 1977 model!). As such, they are just a rotated version of the newer x_p, y_p offsets published by IERS.
The equivalent IAU 2006 standard method, which is preferred, is to apply x_p, y_p pole offsets only for converting between TIRS and ITRS, e.g. via novas_app_to_hor(),novas_hor_to_app(), or wobble()).
There is no need to define pole offsets this way when using the newer frame-based approach introduced in SuperNOVAS. If the pole offsets are specified on a per-frame basis during the initialization of each observing frame (see novas_make_frame(), the offsets will be applied for the TIRS / ITRS conversion only, and not to the TOD equator per se.

REFERENCES:

Kaplan, G. (2005), US Naval Observatory Circular 179.
Kaplan, G. (2003), USNO/AA Technical Note 2003-03.

Parameters

jd_tt	[day] Terrestrial Time (TT) based Julian date. Used only if 'type' is POLE_OFFSETS_X_Y (2), to transform dx and dy to the equivalent Δδψ and Δδε values.
type	POLE_OFFSETS_DPSI_DEPS (1) if the offsets are Δδψ, Δδε relative to the IAU 20006 precession/nutation model; or POLE_OFFSETS_X_Y (2) if they are dx, dy offsets relative to the IAU 2000 / 2006 precession-nutation model.
dpole1	[mas] Value of celestial pole offset in first coordinate, (Δδψ for or dx) in milliarcseconds, relative to the IAU2006 precession/nutation model.
dpole2	[mas] Value of celestial pole offset in second coordinate, (Δδε or dy) in milliarcseconds, relative to the IAU2006 precession/nutation model.

Returns: 0 if successful, or else 1 if 'type' is invalid.

See also: novas_make_frame(), wobble()

References POLE_OFFSETS_DPSI_DEPS, and POLE_OFFSETS_X_Y.

◆ e_tilt()

int e_tilt	(	double	jd_tdb,
		enum novas_accuracy	accuracy,
		double *restrict	mobl,
		double *restrict	tobl,
		double *restrict	ee,
		double *restrict	dpsi,
		double *restrict	deps )

(primarily for internal use) Computes quantities related to the orientation of the Earth's rotation axis at the specified Julian date.

In the pre-IAU2000 method, unmodelled corrections to earth orientation can be defined via cel_pole() prior to this call. However, we strongly recommend against that approach, and suggest you apply Earth orientation corrections only in novas_make_frame() or wobble().

NOTES:

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.

Parameters

	jd_tdb	[day] Barycentric Dynamical Time (TDB) based Julian date.
	accuracy	NOVAS_FULL_ACCURACY (0) or NOVAS_REDUCED_ACCURACY (1)
[out]	mobl	[deg] Mean obliquity of the ecliptic. It may be NULL if not required.
[out]	tobl	[deg] True obliquity of the ecliptic. It may be NULL if not required.
[out]	ee	[s] Equation of the equinoxes in seconds of time. It may be NULL if not required.
[out]	dpsi	[arcsec] Nutation in longitude. It may be NULL if not required.
[out]	deps	[arcsec] Nutation in obliquity. It may be NULL if not required.

Returns: 0 if successful, or -1 if the accuracy argument is invalid

See also: novas_gast(), nutation(), ira_equinox(), equ2ecl(), ecl2equ()

References mean_obliq(), NOVAS_FULL_ACCURACY, NOVAS_REDUCED_ACCURACY, and nutation_angles().

◆ fund_args()

int fund_args	(	double	t,
		novas_delaunay_args *restrict	a )

Compute the fundamental (a.k.a.

Delaunay) arguments (mean elements) of the Sun and Moon.

REFERENCES:

IERS Conventions 2010, Chapter 5, Eq. 5.43.
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(), e_tilt(), 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:

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:

Capitaine, N. et al. (2003), Astronomy and Astrophysics 412, 567-586, eq. (42).

Parameters

jd_tdb	[day] Barycentric Dynamic Time (TDB) based Julian date
equinox	NOVAS_MEAN_EQUINOX (0) or NOVAS_TRUE_EQUINOX (1, or non-zero)
accuracy	NOVAS_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_ra(); gcrs_to_cirs(), cirs_to_gcrs()

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:

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(), novas_get_time()

◆ 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:

Explanatory Supplement To The Astronomical Almanac, pp. 114-115.

Parameters

	jd_tdb	[day] Barycentric Dynamic Time (TDB) based Julian date
	direction	NUTATE_MEAN_TO_TRUE (0) or NUTATE_TRUE_TO_MEAN (-1; or non-zero)
	accuracy	NOVAS_FULL_ACCURACY (0) or NOVAS_REDUCED_ACCURACY (1)
	in	Position 3-vector, geocentric equatorial rectangular coordinates, referred to mean equator and equinox of epoch.
[out]	out	Position 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_get_time(), NOVAS_MOD, NOVAS_TOD

References e_tilt(), and NUTATE_MEAN_TO_TRUE.

◆ 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 using the IAU2006 (P03) precession model of Capitaine et al.

2003.

NOTE:

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.
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:

Explanatory Supplement To The Astronomical Almanac, pp. 103-104.
Capitaine, N. et al. (2003), Astronomy And Astrophysics 412, pp. 567-586.
Hilton, J. L. et al. (2006), IAU WG report, Celest. Mech., 94, pp. 351-367.
Capitaine, N., P.T. Wallace and J. Chapront (2005), “Improvement of the IAU 2000 precession model.” Astronomy & Astrophysics, Vol. 432, pp. 355–67.
Liu, J.-C., & Capitaine, N. (2017), A&A 597, A83

Parameters

	jd_tdb_in	[day] Barycentric Dynamic Time (TDB) based Julian date of the input epoch
	in	Position 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]	out	Position 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(); tt2tdb(), novas_get_time(), frame_tie(), novas_epoch(), NOVAS_MOD, NOVAS_JD_J2000, NOVAS_JD_B1950, NOVAS_JD_B1900

References precession().

Functions

Detailed Description

Function Documentation

◆ accum_prec()

◆ cel_pole()

◆ e_tilt()

◆ fund_args()

◆ ira_equinox()

◆ mean_obliq()

◆ nutation()

◆ precession()