Various functions relating to Earth position and rotation. More...

Functions
double	era (double jd_ut1_high, double jd_ut1_low)
	Returns the value of the Earth Rotation Angle (θ) for a given UT1 Julian date.

short	geo_posvel (double jd_tt, double ut1_to_tt, enum novas_accuracy accuracy, const observer restrict obs, double restrict pos, double *restrict vel)
	Computes the geocentric GCRS position and velocity of an observer.

int	limb_angle (const double pos_src, const double pos_obs, double restrict limb_ang, double restrict nadir_ang)
	Determines the angle of an object above or below the Earth's limb (horizon).

int	novas_diurnal_eop (double gmst, const novas_delaunay_args restrict delaunay, double restrict dxp, double restrict dyp, double restrict dut1)
	Calculate corrections to the Earth orientation parameters (EOP) due to short term (diurnal and semidiurnal) libration and the ocean tides.

int	novas_diurnal_eop_at_time (const novas_timespec restrict time, double restrict dxp, double restrict dyp, double restrict dut1)
	Calculate corrections to the Earth orientation parameters (EOP) due to short term (diurnal and semidiurnal) libration and the ocean tides at a given astromtric time.

int	novas_diurnal_libration (double gmst, const novas_delaunay_args restrict delaunay, double restrict dxp, double restrict dyp, double restrict dut1)
	Calculate diurnal and semi-diurnal libration corrections to the Earth orientation parameters (EOP) for the non-rigid Earth.

int	novas_diurnal_ocean_tides (double gmst, const novas_delaunay_args restrict delaunay, double restrict dxp, double restrict dyp, double restrict dut1)
	Calculate corrections to the Earth orientation parameters (EOP) due to the ocean tides.

double	novas_gast (double jd_ut1, double ut1_to_tt, enum novas_accuracy accuracy)
	Returns the Greenwich Apparent Sidereal Time (GAST) for a given UT1 date.

double	novas_gmst (double jd_ut1, double ut1_to_tt)
	Returns the Greenwich Mean Sidereal Time (GMST) for a given UT1 date, using eq.

short	sidereal_time (double jd_ut1_high, double jd_ut1_low, double ut1_to_tt, enum novas_equinox_type gst_type, enum novas_earth_rotation_measure erot, enum novas_accuracy accuracy, double *restrict gst)

int	terra (const on_surface restrict location, double gast, double restrict pos, double *restrict vel)
	Computes the position and velocity vectors of a terrestrial observer with respect to the center of the Earth, based on the GRS80 reference ellipsoid, used for the International Terrestrial Reference Frame (ITRF) and its realizations.

int	wobble (double jd_tt, enum novas_wobble_direction direction, double xp, double yp, const double in, double out)
	Corrects a vector in the ITRS (rotating Earth-fixed system) for polar motion, and also corrects the longitude origin (by a tiny amount) to the Terrestrial Intermediate Origin (TIO).

Detailed Description

Various functions relating to Earth position and rotation.

Date: Created on Mar 6, 2025

Author: G. Kaplan and Attila Kovacs

See also: frames.c, itrf.c

Function Documentation

◆ sidereal_time()

short sidereal_time	(	double	jd_ut1_high,
		double	jd_ut1_low,
		double	ut1_to_tt,
		enum novas_equinox_type	gst_type,
		enum novas_earth_rotation_measure	erot,
		enum novas_accuracy	accuracy,
		double *restrict	gst )

Deprecated: Use novas_gmst() or novas_gast() instead to get the same results simpler.

Computes the Greenwich sidereal time, either mean or apparent, at the specified Julian date. The Julian date can be broken into two parts if convenient, but for the highest precision, set 'jd_high' to be the integral part of the Julian date, and set 'jd_low' to be the fractional part.

NOTES:

Contains fix for known sidereal time units bug.
As of version 1.5.0, the function uses Eq 42 of Capitaine et al. (2003) exclusively. In prior versions, including NOVAS C the same method was provided twice, with the only difference being an unnecessary change of coordinate basis to GCRS. This latter, mor roundabout way of getting the same answer has been eliminated.<.li>

REFERENCES:

Capitaine, N. et al. (2003), Astronomy and Astrophysics 412, 567-586, eq. (42).
Kaplan, G. (2005), US Naval Observatory Circular 179.

Parameters

	jd_ut1_high	[day] High-order part of UT1 Julian date.
	jd_ut1_low	[day] Low-order part of UT1 Julian date. (You can leave it at zero if 'jd_high' specified the date with sufficient precision)
	ut1_to_tt	[s] TT - UT1 Time difference in seconds
	gst_type	NOVAS_MEAN_EQUINOX (0) or NOVAS_TRUE_EQUINOX (1), depending on whether wanting mean or apparent GST, respectively.
	erot	Unused as of 1.5.0.
	accuracy	NOVAS_FULL_ACCURACY (0) or NOVAS_REDUCED_ACCURACY (1)
[out]	gst	[h] Greenwich (mean or apparent) sidereal time, in hours [0:24]. (In case the returned error code is >1 the gst value will be set to NAN.)

Returns: 0 if successful, or -1 if the 'gst' argument is NULL, 1 if 'accuracy' is

See also: novas_gmst(), novas_gast(), novas_time_gst(); era(), get_ut1_to_tt()

References NOVAS_FULL_ACCURACY, novas_gast(), novas_gmst(), NOVAS_REDUCED_ACCURACY, and NOVAS_TRUE_EQUINOX.

Functions

Detailed Description

Function Documentation

◆ sidereal_time()