Are the RA/DEC coordinates of date Geocentric or Topocentric?
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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.
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?
In Stellarium RA/DE (J2000) are: 10h08m22.1s/+11°58'02.7"
On the USNO site we can find the values here: http://aa.usno.navy.mil/data/docs/geocentric.php
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: http://vizier.u-strasbg.fr/
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: http://en.wikipedia.org/wiki/Venus_transit,_2012. Choose a location 0°E, 80°N and 80°S.
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: http://fuse.pha.jhu.edu/support/tools/precess.html
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!
I don't know the answer.
It's difficult to find out in which direction to look because USNO doesn't specify which catalogue they use.
I tried two other free planetarium programs. The topocentric positions agree with those given by Stellarium even when using an 'old' catalogue like SAO.
You can also try http://rhodesmill.org/pyephem/radec.html
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