Found Moon maximum declination error difference when compared with NASA JPL data, for a close date.

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Anonymous
2013-12-27
2017-07-05
  • Anonymous - 2013-12-27

    Hi everyone, I have found the error goes bigger in a southern latitude. (35ºSouth), please correct me if i am wrong.

    i did several tests:

    At 0º 0m 0s North
    at 0º 0m 0s East.
    From NASA JPL we have maximum moon dec at:
    2013-Nov-19 02:07 m 04 47 01.04 +19 37 23.5 NASA JPL
    From Stellarium we have maximum dec at:
    2013 Nov 19 at 02:08 aprox. +19 37 26.6
    1 minute and 3 seconds different its no bad.

    when i change from location:
    90ºW and 20º15m N i got:
    2013-Nov-19 07:55 m 04 59 54.67 +19 25 50.4 NASA JPL
    and from stellarium:
    07:55 +19 25 45.8
    0 minute time and 5 secs arc difference

    the same but with southern latitud.
    90ºW and 20º15m South
    2013-Nov-19 07:53 m 04 59 51.98 +20 01 43.5 NASA JPL
    and from stellarium:
    07:54:18 +20 01 52.4
    1 minute time and 9 second arc difference

    but with 90W and 35º 15m South
    2013-Nov-19 06:51 m 04 57 34.79 +20 10 12.3 NASA JPL
    and with stellarium:
    06:59 +20 12 08.8
    over 8 minutes time and almost 2 minutes arc in dec. Too Much for a close date!.

    I found all max declinations by steping by hand at 1 second rate.

    The error value for 35º15m is too much and its not a very southern latitude, its normal???

    Thanks for your help.
    Igor.

     
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    Anonymous - 2017-07-05

    Bonjour,
    Pourquoi la déclinaison de la lune varie-t'elle selon la position de l'observateur ?
    Merci

    Hello,
    Why the declination of the moon does vary following the observer's position ?
    Thank you

     
    • Alexander Wolf

      Alexander Wolf - 2017-07-05

      Short answer: refraction.

       
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    Anonymous - 2017-07-05

    Sorry, but I do not understand your answer. How can the refraction interact with the position of the moon ?

     
    • Alexander Wolf

      Alexander Wolf - 2017-07-05

      On the screenshot you show position of the Moon with atmosphere and without atmosphere - what exactly you don't understand here?

       
  • gzotti

    gzotti - 2017-07-05

    No, this is parallax. See latitude: +60 and -60. This is the effect of the observer not in Earth's centre but on the surface. The moon is so close that this makes a big difference.

     
    • Alexander Wolf

      Alexander Wolf - 2017-07-05

      Ah, you are right! I didn't saw coordinates - of course it's parallax.

       
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    Anonymous - 2017-07-05

    Hello,
    @Alexander : It is not atmosphere, but day and night.
    @gzotti : Sorry, but I do not understand how can parallax interact with the position of the moon. I thought that the position of a celestial body, at the same time, was an absolute data, the same for all the people on earth, no matter where they are, and even no matter if they see or not this body. Please do you know where I can find an explanation of the influence of parallax on the position of the moon ?

     
  • gzotti

    gzotti - 2017-07-05

    I have already explained:

    This is the effect of the observer not in Earth's centre but on the surface. The moon is so close that this makes a big difference.

    More info: Wikipedia? Any intro book to astronomy? Draw a sketch? Put yourself on the globe, the moon 60 globe radii away, the stars and sky coordinate grid infinitely farther away.

     
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    Anonymous - 2017-07-05

    Hum, we're talking about declination, which is the body's (here the Moon) latitude. This is not an observer's measurement but an almanac data, completely independant of the observer's position. So I confirm the question which has not been answered here: why does the declination vary with observer's postion?

     
    • Alexander Wolf

      Alexander Wolf - 2017-07-05

      Google: lunar parallax, geocentric coordinates, topocentric coordinates

       
    • gzotti

      gzotti - 2017-07-05

      Declination is deviation from the (celestial) equator, latitude is deviation from the ecliptic. Very much not the same.
      Observers are not interested in almanac values but coordinates to apply to their telescopes.
      Most observers skipped topographic correction (described either in the Almanac's introduction or companion volume) because the object was just arcminutes from the estimated position. The Moon rarely requires computation to locate in the sky.
      Declination varies with observer position because the observer sits on the surface of a planet, not in its center. You can put yourself into Earth's center by switching off topographic correction in the settings. Note that you will most certainly get values different from other almanacs by a few arcseconds. The reasons for these are still to be traced down.

       
  • VReijs

    VReijs - 2017-07-05

    It coudl be that Stellarium presents is the topoccentric decliantion (aka with the parallax included). If it was the geocentric declination then it should not change.
    For the Sun and stars the topocentric and geocentric declianation are the same (for th Sun a little less the same;-).
    Perhaps Stellarium should add the word topocentric to the Declincation term (certaily because some people will indeed think only in geocentric declination).

    All the best,

    Victor

     
    • gzotti

      gzotti - 2017-07-05

      Before around 2014 only topocentric coordinates were given, and no switch possible. Do we want to clutter the display with even more words? Or can we just learn and remember something?

       
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    Anonymous - 2017-07-05

    Thanks Victor, could be an explanation. This being said, it is a significant effect, and doing a sight reduction with this value modify greatly the intercept. So the next question is: is it correct to do sight reduction with this value and if yes, how to compute it, manually (tables) or computerised?

     
    • gzotti

      gzotti - 2017-07-05

      What kind of sight reduction do you want to do? Until we are confident we do everything correct, at least compare results with other software for detailed numerical work.

       
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    Anonymous - 2017-07-05

    @gzotti: sextant sight reduction. Another software gives also a declination varying with observer's position (Navastro). But manual procedure (with HO-249) takes directly the declination in the almanac, depending only on time. The parallax correction applies on the sextant's measured height.

     
  • gzotti

    gzotti - 2017-07-05

    Accounting for lunar parallax is standard since before sextant times. Of course the printed Almanac can only list geocentric positions, but navigators have to correct with a standard computing procedure with their estimated position. This should hopefully be described in the Nautical Almanac. (Although I don't have one to check.) Planetary parallax is much smaller, but was important in 18-19th century expeditions to measure Venus transits, and with that, the absolute distances of the Solar system.

     
    Last edit: gzotti 2017-07-05


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