light_t.c File Reference

Correct for light-travel time. More...

#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include "aephem.h"

Functions
void	ae_light_t_orbit (double jd_tt, double o[], const struct ae_orbit_t *orb, double q[])
	Get the apparent position of an object, corrected for light-travel time. More...
int	ae_light_t_jpl (struct ae_jpl_handle_t *jh, double jd_tt, double o[], int obj_num, double q[], double v_q[], char is_planetary)
	Get the apparent position of an object, corrected for light-travel time. More...
double	ae_light_t (double p[], double q[], int do_retardation)
	Get light-travel time between two objects. More...

Detailed Description

Correct for light-travel time.

Function Documentation

double ae_light_t	(	double	p[],
		double	q[],
		int	do_retardation
	)

Get light-travel time between two objects.

Given the position of two objects in heliocentric equatorial rectangular coordinates, returns the light-travel time between them. To correct for light-travel time, this should be done iteratively a couple of times. The routines ae_light_t_jpl() and ae_light_t_orbit() do this automatically.

This routine can approximate the effect due to the sun's gravitational retardation (AA, p. 36). However, it is usually a small effect. It will blow up if one of the objects is the sun.

Parameters

p	The first object, in heliocentric rectangular coordinates, in AU.
q	The second object, in heliocentric rectangular coordinates, in AU.
do_retardation	If 0, neglect gravitational retardation from the sun; otherwise, compute gravitational retardation from the sun.

Returns

The light-travel time, in days.

References AE_CLIGHT_AUD.

Referenced by ae_light_t_jpl(), ae_light_t_orbit(), and aes_subobs_point().

int ae_light_t_jpl	(	struct ae_jpl_handle_t *	jh,
		double	jd_tt,
		double	o[],
		int	obj_num,
		double	q[],
		double	v_q[],
		char	is_planetary
	)

Get the apparent position of an object, corrected for light-travel time.

The routine does three iterations of light-time correction. Gravitational retardation from the sun is neglected. (However, it can be added if necessary by using ae_light_t() directly.)

This function uses JPL ephemerides. For orbital elements, use ae_light_t_orbit().

Parameters

jh	A handle initialised by ae_jpl_init().
jd_tt	The Julian date in TT.
o	The heliocentric rectangular position of the earth, in AU.
obj_num	The object for which to get coordinates. If a planetary ephemeris file is being used, one can use the predefined ae_ss_bodies_t constants for clarity.
q	For returning the heliocentric rectangular position of the object in AU.
v_q	For returning the velocity of the object in AU. Set to NULL if you do not require the velocity.
is_planetary	If 0, assume these are not planetary ephemerides and barycentric coordinates are returned. Otherwise, it will be assumed that a Solar System ephemeris file is being used and that the object numbers are as in ae_ss_bodies_t; heliocentric coordinates will be returned.

Returns

0 on success; on failure, a negative error code from ae_retcode_t.

References ae_jpl_get_coords(), and ae_light_t().

Referenced by ae_geocentric_from_jpl().

void ae_light_t_orbit	(	double	jd_tt,
		double	o[],
		const struct ae_orbit_t *	orb,
		double	q[]
	)

Get the apparent position of an object, corrected for light-travel time.

The routine does three iterations of light-time correction. Gravitational retardation from the sun is neglected. (However, it can be added if necessary by using ae_light_t() directly.)

This routine uses orbital elements. For JPL ephemerides, use ae_light_t_jpl().

Parameters

jd_tt	The Julian date in TT.
o	The heliocentric rectangular position of the observer, in AU.
orb	The orbital elements of the object.
q	For returning the heliocentric rectangular position of the object in AU.

References ae_kepler(), and ae_light_t().

Referenced by ae_geocentric_from_orbit().