This doesn't answer your question, but hopefully it sheds some light on it.
As I said in this comment the difference in distances isn't caused by the table type, it's caused by the light-time correction. However, I don't understand why that leads to such a large discrepancy.
The Horizons Observer table allows you to choose between astrometric and apparent values for RA & declination (Quantities 1 & 2), but it gives you no options for the range.
20. Target range & range rate (relative to observer)
Target apparent range ("delta", light-time aberrated) and range-rate ("delta-dot") relative to the observer.
The vector table gives you the options of plain geometric values (the default), astrometric (light-time corrected), or apparent (light-time and aberration corrected).
Aberration corrections:
- NONE (geometric state vectors)
- LT (light-time)
- LT+S (light-time & stellar aberration)
Here's a comparison. Each table lists the calendar date / time and Julian day in UTC, Delta T (= TDB - UTC) (in seconds), the
light-time from the centre of the Moon to the centre of the Earth, the range (in km), and the range-rate (in km/s). Note that the observer table reports light travel time in minutes, but the vector tables report it in seconds. Each heading is a link to the full Horizons output.
Vector table, GEOMETRIC cartesian states
*******************************************************************************
JDUT , Calendar Date (UT ), delta-T, LT, RG, RR,
******************************************************************************************************************************************
$$SOE
2440691.522222222, A.D. 1970-Apr-15 00:32:00.0000, 40.455330, 1.349003752066716E+00, 4.044211506833034E+05, 3.507416288023241E-03,
2440691.522916667, A.D. 1970-Apr-15 00:33:00.0000, 40.455332, 1.349004453018408E+00, 4.044213608233342E+05, 3.497251234683748E-03,
2440691.523611111, A.D. 1970-Apr-15 00:34:00.0000, 40.455334, 1.349005151935714E+00, 4.044215703534710E+05, 3.487086509335641E-03,
2440691.524305556, A.D. 1970-Apr-15 00:35:00.0000, 40.455336, 1.349005848818700E+00, 4.044217792737343E+05, 3.476922112318192E-03,
2440691.525000000, A.D. 1970-Apr-15 00:36:00.0000, 40.455337, 1.349006543667431E+00, 4.044219875841435E+05, 3.466758043990613E-03,
$$EOE
******************************************************************************************************************************************
Vector table, LT CORRECTED
*******************************************************************************
JDUT , Calendar Date (UT ), delta-T, LT, RG, RR,
******************************************************************************************************************************************
$$SOE
2440691.522222222, A.D. 1970-Apr-15 00:32:00.0000, 40.455330, 1.349132103821229E+00, 4.044596295712772E+05, 3.484037521090640E-03,
2440691.522916667, A.D. 1970-Apr-15 00:33:00.0000, 40.455332, 1.349132800160780E+00, 4.044598383286230E+05, 3.473861373118514E-03,
2440691.523611111, A.D. 1970-Apr-15 00:34:00.0000, 40.455334, 1.349133494463880E+00, 4.044600464754561E+05, 3.463685553580587E-03,
2440691.524305556, A.D. 1970-Apr-15 00:35:00.0000, 40.455336, 1.349134186730390E+00, 4.044602540117345E+05, 3.453510062668777E-03,
2440691.525000000, A.D. 1970-Apr-15 00:36:00.0000, 40.455337, 1.349134876960339E+00, 4.044604609374676E+05, 3.443334900674379E-03,
$$EOE
******************************************************************************************************************************************
Vector table, LT+S CORRECTED
*******************************************************************************
JDUT , Calendar Date (UT ), delta-T, LT, RG, RR,
******************************************************************************************************************************************
$$SOE
2440691.522222222, A.D. 1970-Apr-15 00:32:00.0000, 40.455330, 1.349132103821229E+00, 4.044596295712773E+05, 3.509638977137701E-03,
2440691.522916667, A.D. 1970-Apr-15 00:33:00.0000, 40.455332, 1.349132800160780E+00, 4.044598383286231E+05, 3.499474924335833E-03,
2440691.523611111, A.D. 1970-Apr-15 00:34:00.0000, 40.455334, 1.349133494463880E+00, 4.044600464754561E+05, 3.489311199569758E-03,
2440691.524305556, A.D. 1970-Apr-15 00:35:00.0000, 40.455336, 1.349134186730390E+00, 4.044602540117345E+05, 3.479147803031668E-03,
2440691.525000000, A.D. 1970-Apr-15 00:36:00.0000, 40.455337, 1.349134876960339E+00, 4.044604609374675E+05, 3.468984735012534E-03,
$$EOE
******************************************************************************************************************************************
Observer table
*******************************************************************************************************
Date__(UT)__HR:MN, Date_________JDUT, , , TDB-UT, 1-way_down_LT, delta, deldot,
*******************************************************************************************************
$$SOE
1970-Apr-15 00:32, 2440691.522222222, , , 40.455330, 0.02248554, 4.0445962957E+05, 0.0034840,
1970-Apr-15 00:33, 2440691.522916667, , , 40.455332, 0.02248555, 4.0445983833E+05, 0.0034739,
1970-Apr-15 00:34, 2440691.523611111, , , 40.455334, 0.02248556, 4.0446004648E+05, 0.0034637,
1970-Apr-15 00:35, 2440691.524305556, , , 40.455336, 0.02248557, 4.0446025401E+05, 0.0034535,
1970-Apr-15 00:36, 2440691.525000000, , , 40.455337, 0.02248558, 4.0446046094E+05, 0.0034433,
$$EOE
*******************************************************************************************************
The range and range-rate reported in the second vector table is the same as in the observer table.
The light-time and range-rate are both quite small, so it's certainly a puzzle where that ~38 km range difference is coming from. Surely, it's not a bug...
The JPL DE stores major body positions using its nominal Solar System barycentre as the origin. Perhaps that makes a difference between the geometric and light-time corrected values.
Here's a plot of the Light-time Corrected vector range minus the Geometric vector range over a few months, for 0:00 TDB, with a 1 day timestep. I've checked 5 decades of the 20th century, at various times of year, and it's always quite close to the Moon phases.
Here's the plotting script, which can be used to explore this discrepancy for other pairs of bodies. Select the verbose
checkbox to get a printed list of the differences.
Here's a small Sage / Python script which processes Horizons batch command lists and displays the output in a format that's easy to copy and paste. The Horizons Web GUI app can print such lists, but it doesn't accept them as input. You may find this script convenient for experimenting with those lists. I find it easier than tweaking parameters in the Horizons GUI.
The command list must start with the line!$$SOF
. You can use an exclamation mark !
to create a comment (which extends to the end of the line). Here's a typical example:
!$$SOF
MAKE_EPHEM='YES'
!REF_PLANE='FRAME'
EPHEM_TYPE='VECTORS'
VEC_TABLE='6'
VEC_CORR='LT'
VEC_DELTA_T='YES'
CSV_FORMAT='YES'
OBJ_DATA='NO'
COMMAND=301
CENTER=@399
START_TIME='1970-Apr-15 0:32'
STOP_TIME='1970-Apr-15 0:36'
STEP_SIZE='1m'
There are a few other versions of this script on Github
For further information, please see the Horizon API docs and the the Horizons batch example/instructions document.