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    Re: Planet positions and light travel time
    From: Dave Walden
    Date: 2017 Dec 18, 06:39 -0800

    TMI, here's the USNO NOVAS ephemeris routine:

     
          SUBROUTINE DLIGHT (POS1,PE,DIFLT)
    *
    *     THIS SUBROUTINE RETURNS THE DIFFERENCE IN LIGHT-TIME, FOR A STAR,
    *     BETWEEN THE BARYCENTER OF THE SOLAR SYSTEM AND THE OBSERVER (OR
    *     THE GEOCENTER).
    *
    *          POS1   = POSITION VECTOR OF STAR, WITH RESPECT TO ORIGIN AT
    *                   SOLAR SYSTEM BARYCENTER (IN)
    *          PE     = POSITION VECTOR OF OBSERVER (OR THE GEOCENTER),
    *                   WITH RESPECT TO ORIGIN AT SOLAR SYSTEM BARYCENTER,
    *                   COMPONENTS IN AU (IN)
    *          DIFLT  = DIFFERENCE IN LIGHT TIME, IN THE SENSE STAR TO
    *                   BARYCENTER MINUS STAR TO EARTH, IN DAYS (OUT)
    *
    *     -OR-
    *
    *     THIS SUBROUTINE RETURNS THE LIGHT-TIME FROM THE OBSERVER (OR THE
    *     GEOCENTER) TO A POINT ON A LIGHT RAY THAT IS CLOSEST TO A
    *     SPECIFIC SOLAR SYSTEM BODY.
    *
    *          POS1   = POSITION VECTOR TOWARD OBSERVED OBJECT, WITH RESPECT
    *                   TO ORIGIN AT OBSERVER (OR THE GEOCENTER) (IN)
    *          PE     = POSITION VECTOR OF SOLAR SYSTEM BODY, WITH RESPECT
    *                   TO ORIGIN AT OBSERVER (OR THE GEOCENTER), COMPONENTS
    *                   IN AU (IN)
    *          DIFLT  = LIGHT TIME TO POINT ON LINE DEFINED BY POS1 THAT IS
    *                   CLOSEST TO SOLAR SYSTEM BODY (POSITIVE IF LIGHT
    *                   PASSES BODY BEFORE HITTING OBSERVER, I.E., IF
    *                   POS1 IS WITHIN 90 DEGREES OF PE)(OUT)
    *
    *
          DOUBLE PRECISION POS1,PE,DIFLT,C,DIS,U1,DSQRT
          DIMENSION POS1(3), PE(3), U1(3)
          SAVE
     
          DATA NTIMES / 0 /
     
          NTIMES = NTIMES + 1
          IF (NTIMES.EQ.1) THEN
    *         GET C, THE SPEED OF LIGHT IN AU/DAY
              CALL ASTCON ('C(AU/DAY)',1.D0,C)
          END IF
     
    *     FROM POS1, FORM UNIT VECTOR U1 IN DIRECTION OF STAR OR
    *     LIGHT SOURCE
          DIS = DSQRT ( POS1(1)**2 + POS1(2)**2 + POS1(3)**2 )
          DO 20 J=1,3
       20 U1(J) = POS1(J) / DIS
     
    *     LIGHT-TIME RETURNED IS THE PROJECTION OF VECTOR PE ONTO THE UNIT
    *     VECTOR U1 (FORMED FROM POS1), DIVIDED BY THE SPEED OF LIGHT
          DIFLT = ( PE(1)*U1(1) + PE(2)*U1(2) + PE(3)*U1(3) ) / C
     
          RETURN
     
          END
     

       
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