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/* Correction for light time from object to earth
* including gravitational retardation due to the Sun.
* AA page B36.
*/
#include "kep.h"
int lightt( elemnt, q, e )
double e[], q[]; /* rectangular position vectors */
struct orbit *elemnt; /* orbital elements of object q */
{
double p[3], p0[3], ptemp[3];
double P, Q, E, t, x, y;
int i, k;
/* save initial q-e vector for display */
for( i=0; i<3; i++ )
{
p0[i] = q[i] - e[i];
}
E = 0.0;
for( i=0; i<3; i++ )
E += e[i]*e[i];
E = sqrt(E);
for( k=0; k<2; k++ )
{
P = 0.0;
Q = 0.0;
for( i=0; i<3; i++ )
{
y = q[i];
x = y - e[i];
p[i] = x;
Q += y * y;
P += x * x;
}
P = sqrt(P);
Q = sqrt(Q);
/* Note the following blows up if object equals sun. */
t = (P + 1.97e-8 * log( (E+P+Q)/(E-P+Q) ) )/173.1446327;
kepler( TDT-t, elemnt, q, ptemp );
}
if( prtflg )
printf( "light time %.4fm, ", 1440.0*t );
/* Final object-earth vector and the amount by which it changed.
*/
for( i=0; i<3; i++ )
{
x = q[i] - e[i];
p[i] = x;
dp[i] = x - p0[i];
}
showcor( "aberration", p0, dp );
/* Calculate dRA/dt and dDec/dt.
* The desired correction of apparent coordinates is relative
* to the equinox of date, but the coordinates here are
* for J2000. This introduces a slight error.
*
* Estimate object-earth vector t days ago. We have
* p(?) = q(J-t) - e(J), and must adjust to
* p(J-t) = q(J-t) - e(J-t) = q(J-t) - (e(J) - Vearth * t)
* = p(?) + Vearth * t.
*/
velearth(TDT);
for( i=0; i<3; i++ )
p[i] += vearth[i]*t;
deltap( p, p0, &dradt, &ddecdt ); /* see dms.c */
dradt /= t;
ddecdt /= t;
return(0);
}
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