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)
double	planet_lon (double t, enum novas_planet planet)

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.

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.
accuracy	NOVAS_FULL_ACCURACY (0) or NOVAS_REDUCED_ACCURACY (1)

Returns

[rad] Complementary terms, in radians.

See also

e_tilt()

cel_pole()

nutation()

sidereal_time()

References accum_prec(), novas_delaunay_args::D, novas_delaunay_args::F, fund_args(), NOVAS_FULL_ACCURACY, NOVAS_MERCURY, NOVAS_NEPTUNE, 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
equinox	NOVAS_MEAN_EQUINOX (0) or NOVAS_TRUE_EQUINOX (1; or non-zero)
accuracy	NOVAS_FULL_ACCURACY (0) or NOVAS_REDUCED_ACCURACY (1)

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()

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()

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
planet	Novas 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.