Swiss ephemeris 4 Computer ephemeris for developers of astrological software 4


Test 5: Topocentric Position of a Planet



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Test 5: Topocentric Position of a Planet


  • To calculate the topocentric position of a planet proceed as follows:

  • Scroll up to “Current Settings”, change Target Body and select Venus.

  • After clicking on “Select Indicated Body”, change Time Span, edit

  • “Start Time” to “2015-09-01 05:00 TT” and

  • “Stop Time” to “2015-09-03”.

  • We choose a date near the heliacal rising of Venus, where the parallax effect is greater.



  • Then change Table Settings, activate only the checkboxes

  • 2. Apparent RA & DEC and

  • 31. Observer ecliptic lon. & lat.

  • and uncheck all other options.

  • In the lower options table activate the option “extra precision”.

  • Scroll down and click on the button “Use Settings Above”.



  • Finally change Observer Location. In the field “Lookup Named Location” enter “Jerusalem” and click on the Button “Search”.




    1. Now the current settings should look as follows:






    1. Please check carefully if you have exactly these data and otherwise correct them.

    2. Then click on “Generate Ephemeris”. The output is as follows:







    3. In the current settings you see the geographic coordinates of Jerusalem as used by Horizons. You have to transform them into decimal values in order to use them with swetest.

    4. Now call swetest as follows:



    5. swetest -b01.09.2015 -t5 -p3 -fTPADlb -n2 -ejplde431.eph -jplhora -topo35.233305,31.766694,0

    6. date (dmy) 1.9.2015 greg. 5:00:00 ET version 2.05.02b04

    7. TT: 2457266.708333333

    8. geo. long 35.233305, lat 31.766694, alt 0.000000

    9. Epsilon (true) 23°26' 5.3157

    10. Nutation 0° 0' 1.3321 -0° 0' 8.7886

    11. 01.09.2015 5:00:00 ET Venus 9h 0'32.0966 9° 3'13.6147 134.9240534 -7.6248475

    12. 02.09.2015 5:00:00 ET Venus 8h59'56.4969 9°12'43.0988 134.7370775 -7.5147577



    13. This is very precise again. The geocentric positions would be:



    14. swetest -b01.09.2015 -t5 -p3 -fTPADlb -n2 -ejplde431.eph -jplhora

    15. date (dmy) 1.9.2015 greg. 5:00:00 ET version 2.05.02b04

    16. TT: 2457266.708333333

    17. Epsilon (true) 23°26' 5.3157

    18. Nutation 0° 0' 1.3321 -0° 0' 8.7886

    19. 01.09.2015 5:00:00 ET Venus 9h 0'30.9799 9° 3'25.0943 134.9186940 -7.6230971

    20. 02.09.2015 5:00:00 ET Venus 8h59'55.4154 9°12'54.3289 134.7318831 -7.5130275



    21. Note, if you specify the time as UT and call the Swiss Ephemeris with -ut5 (instead of -t5), then the deviation is slightly greater, because Horizons uses UTC, whereas the Swiss Ephemeris uses UT1. If the input date is in the current or a future year, there also may be differences in Delta T values used by JPL and the Swiss Ephemeris.



    22. Significant deviations from Horizons only appear with the topocentric Moon, where our error can amount to 0.2 arcsec. We have not studied this difference so far, so do not know its exact cause.



        1. Test 6: Heliocentric Positions


    In “Current Settings” select:

    “Target Body”: “Mars”.

    “Observer Location”: “@sun” (for heliocentric positions)

    “Table Settings”: “1. Astrometric RA & DEC”



    Then click “Generate Ephemeris”. The output is:

    Swetest provides the following data, using the same parameters we used in Test 1, but adding the parameter -hel for heliocentric positions:
    swetest -b25.10.2016 -p4 -fTPAD -hel -n3 -ejplde431.eph -j2000 -icrs -noaberr -nodefl

    date (dmy) 25.10.2016 greg. 0:00:00 ET version 2.05.02b04

    TT: 2457686.500000000

    Epsilon (true) 23°26'13.5306

    Nutation 0° 0' 0.0000 0° 0' 0.0000

    25.10.2016 Mars 22h23'32.8216 -11°58'15.5314

    26.10.2016 Mars 22h25'57.3574 -11°44' 6.7775

    27.10.2016 Mars 22h28'21.6564 -11°29'53.4295


    The precision is not as good as with geocentric positions. Horizons explains its output as follows:
    “R.A.___(ICRF/J2000.0)___DEC =

    J2000.0 astrometric right ascension and declination of target center. Adjusted for light-time.”


    The Swiss Ephemeris also adjusts for light-time, assuming that the body of the Sun is transparent and ignoring relativistic effects. What exactly Horizons does is not clear.
    Now change “Table Settings” and activate

    1. “2. Apparent RA & DEC” and

    2. “31. Observer ecliptic lon. & lat.”

    Now the output of Horizons looks as follows:



    Then call swetest with the following parameters:
    swetest -b25.10.2016 -p4 -fTPADlb -hel -n3 -ejplde431.eph -j2000 -icrs -noaberr -nodefl

    date (dmy) 25.10.2016 greg. 0:00:00 ET version 2.05.02b04

    TT: 2457686.500000000

    Epsilon (true) 23°26'13.5306

    Nutation 0° 0' 0.0000 0° 0' 0.0000

    25.10.2016 Mars 22h23'32.8216 -11°58'15.5314 333.2945513 -1.7951663

    26.10.2016 Mars 22h25'57.3574 -11°44' 6.7775 333.9296391 -1.7901765

    27.10.2016 Mars 22h28'21.6564 -11°29'53.4295 334.5647707 -1.7849664


    Right ascension and declination are completely different. However, Horizons now defines these as follows:

    “R.A._(airls-apparent)__DEC. =

    Airless apparent right ascension and declination of the target center with respect to the center/site body's true-equator and the meridian containing the Earth equinox of J2000.0. Adjusted for light-time, the gravitational deflection of light, stellar aberration, precession and nutation.”
    Thus the positions given by JPL are not relative to the Earth equatorial coordinate system, but relative to the coordinate system defined by the equator of the Sun. Therefore these values cannot be compared.
    How about ecliptic longitudes and latitudes? Here we have differences of about 0.027 arcsec.
    JPL explains its values as follows:

    ObsEcLon ObsEcLat =



    Observer-centered J2000 ecliptic longitude and latitude of the target center's apparent position, adjusted for light-time, the gravitational deflection of light and stellar aberration.”
    The Swiss Ephemeris does not include “gravitational deflection of light and stellar aberration” in the above calculation.


      1. D. How to compare the Swiss Ephemeris with Ephemerides of the Astronomical Almanac (apparent positions)

        1. Test 7: Astronomical Almanac online


    1. Get a recent "Astronomical Almanac" from the library or your bookshelf. If you are too lazy to do that, go on the following page:

    2. http://asa.usno.navy.mil/SecE/Section_E.html

    3. and click on "Geocentric equatorial coordinates".



    4. The position of Mars for today (25 Oct 2016), 0:00 TT, is given as:

    5. Mars 19 23 24.488 -24 07 29.03 1.2088279



    6. Then call swetest using the following parameters:



    7. swetest -b25.10.2016 -p4 -fTPADR -ejplde431.eph

    8. date (dmy) 25.10.2016 greg. 0:00:00 ET version 2.05.02b04

    9. TT: 2457686.500000000

    10. Epsilon (true) 23°26' 5.1018

    11. Nutation -0° 0' 7.1942 -0° 0' 8.4288

    12. 25.10.2016 Mars 19h23'24.4884 -24° 7'29.0249 1.208903220



    13. There is a difference in the distance value R. The reason is that AA combines apparent RA and DE with true distance.

    14. To arrive at the same distance value, call swetest as follows:



    15. swetest -b25.10.2016 -p4 -fTPR -ejplde431.eph -true

    16. date (dmy) 25.10.2016 greg. 0:00:00 ET version 2.05.02b04

    17. TT: 2457686.500000000

    18. Epsilon (true) 23°26' 5.1018

    19. Nutation -0° 0' 7.1942 -0° 0' 8.4288

    20. 25.10.2016 Mars 1.208827910



    21. which is identical to AA, but has more digits.


        1. Test 8: Astronomical Almanac printed


    22. If you are not too lazy to get a printed AA of a recent year or manage to get pages B68-B70 from AA 2016 in google books, there you will find an additional digit both in right ascension and declination.



    23. Page B68 gives an example how to calculate the apparent position of Venus for 17 April 2016 12:00 UT1, assuming Delta T as 68s.

    24. On p. B69, the corresponding TT is given as JD 2457496.000787.

    25. On p. B70, the result is given as RA = 0h55m33s.8912, DE = 4°23'25".333.



    26. The Swiss Ephemeris provides the same result if called with the following parameters:




    1. swetest -bj2457496.000787 -p3 -fTPAD -ejplde431.eph

    2. date (dmy) 17.4.2016 greg. 12:01:08 ET version 2.05.02b04

    3. TT: 2457496.000787000

    4. Epsilon (true) 23°26' 5.0046

    5. Nutation -0° 0' 3.8526 -0° 0' 8.7703

    6. 17.04.2016 12:01:08 ET Venus 0h55'53.8912 4°23'25.3326



    7. You may find that there is a difference of about 0.052 arcsec between JPL Horizons and Astronomical Almanac. For more information on this, please read the following paragraph in the general documentation of the Swiss Ephemeris:

    8. http://www.astro.com/swisseph/swisseph.htm?lang=g#_Toc443485363

    9. (2.1.2.2 Swiss Ephemeris and JPL Horizons System)






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