Topocentric or Geocentric ?


  • Anonymous

    Are the RA/DEC coordinates of date Geocentric or Topocentric?

  • barrykgerdes

    Geocentric. However solar system objects like the moon and planets etc have their own special orbit calculations that apply corrections applied depending on Earth locations.

    Last edit: barrykgerdes 2014-05-17

  • Anonymous

    Thanks, Barry.
    Sounds reasonable. USNO has a website that allows one to compute topocentric
    and geocentric positions of stars and planets. I used Regulus and computed
    its apparent position for 2014 May 16 which gave both topocentric and geocentric
    coords of 10:09:08.5 in RA and +11:53:40.5 in DEC -- essentially the same to
    within the precision quoted here. However, Stellarium for the same date gave
    RA = 10:09:08 and DEC = +11:53:49, suggesting that it differs only in DEC
    from the USNO position by ~8 arcseconds. I wonder why the RA is right on
    but the DEC is that much different?

  • Kird

    In Stellarium RA/DE (J2000) are: 10h08m22.1s/+11°58'02.7"
    On the USNO site we can find the values here:
    We choose 'Position type': 'Astrometric Geocentric Right Ascension and Declination'
    The values are: 10h08m22.064s/+11°58'02.03" (they don't mention these are J2000 coördinates).
    The values used by Stellarium and USNO are not from the same catalogue.
    You can look up the coördinates of Regulus in different catalogues on this site:
    In the Hipparcos catalogue (new reduction (van Leeuwen, 2007))
    RA/DE (J2000): 10h08m22.311s/+11°58'01.95"
    On the same page let's also calculate the 'Position type': 'Apparent Geocentric Right Ascension and Declination'
    The values are: 10h09m08.598s/+11°53'40.44"

    Please read the Notes: Geocentric, Apparent position and Astrometric position

    Now for the topocentric position (of date) I used: WASHINGTON, DISTRICT OF COLUMBIA
    Location: W 77°01'48.0", N38°53'24.0", 0m
    2014 May 16 00:00:00.0 Stellarium RA/DE: 10h09m08s/+11°53'49"
    2014 May 16 00:00:00.0 USNO RA/DE: 10h09m08.615s/+11°53'40.44"

    Because stars are very far away,in most cases it's not important to calculate topocentric positions. The parallax of Regulus is 0.045".

    If you want to see a useful application of topocentric coördinates try:,_2012. Choose a location 0°E, 80°N and 80°S.

    Last edit: Kird 2014-05-19

    • Anonymous

      Thanks, Kird, for all the thought & time that went into your answer!

      I used both J2000 positions that Stellarium and USNO
      presumably used for Regulus, precessed them to the epoch of date
      and got essentially the same answer. The precession routine used was
      that of the Fuse project implemented by Ed Murphy and
      is here:

      In any case, the USNO and Stellarium J2000 positions are very close and so
      it still surprises me that USNO's Regulus WEB calculations are so
      far off in DEC compared to the Stellarium position. It could
      be that the precession routine USNO uses must be different or
      possibly have an error. I only mention DEC because the RA calculations
      could be off by a similar amount but Stellarium only tabulates RA to
      the nearest second in time which is about 15 arc seconds for Regulus. If you know
      the answer, I'd like to see it but please don't waste any more of
      your valuable time on this on my account. Stellarium is a tremendous
      asset for my backyard astronomy efforts -- and the price is just right!



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