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WDS Identifier and Precession...

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#1 Cotts

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Posted 03 December 2019 - 11:28 AM

What happens to WDS identifiers when precession significantly alters the actual RA and Dec of the pair?

 

Example:  STF 2614 in Ursa Major ......  WDS 18059+8815   Has it always been 18059+8815?  Or was this determined at some date in the past?

 

I assume the ID stays the same while the RA and Dec change (most rapidly near the poles....)?

 

Position in Epoch 1950.0 was 18h 39m 25.7s  +88º 13' 24.3"...   which is rather off for the identifier..

 

Position in Epoch 2000.0 is 18h 06m 05.92s    +88º 15' 17.6"  which matches reasonably well to  the identifier.

 

Position in Epoch 2050.0 will be 17h 31m 52.8s    +88º 14' 28.3".   The RA is off by a whopping 36minutes!

 

Since we will be moving to epoch 2050 relatively soon, will the WDS identifier be adjusted?  I would think not....  

 

So will WDS identifiers become further and further from the 'current' position, making life more difficult for future astronomers?

 

And what of my Digital Setting Circles?  Will they be rendered less accurate?  Will the switch to Epoch 2050.0 in a few years be a problem?

 

Duh.

 

Dave



#2 catalogman

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Posted 03 December 2019 - 03:37 PM

Where did you get the position from?

According to other catalogues, the designation WDS 18059+8815 = CCDM J18059+8815AB is correct.
It comes from the position of the bright star at

 

  18 05 54.1400 +88 15 11.900 (WDS)
  18 05 54.4635 +88 15 12.088 (CCDM)

 

Even if a more accurate position is known, the IAU rule is that the positional name should not
be changed.

 

Your second point about changing epoch is the same point that Dixon and Sonneborn strongly argued
back in 1980. Their advice against positional designators has been completely ignored by modern
cataloguers and the IAU.

 

--catalogman



#3 Cotts

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Posted 03 December 2019 - 05:28 PM

I got the positions at the three epochs in my post from Sky Safari......

 

I'm not suggesting that the WDS identifier for STF 2614 is incorrect - that's its "Name" 18059+8815

 

What is interesting is that the two positions that you gave for it differ from one another,

 

18 05 54.1400 +88 15 11.900 (WDS)
  18 05 54.4635 +88 15 12.088 (CCDM)

 

Are these from J2000 Epoch?  If so, why do they differ, not only from each other but from the J2000 position given by Sky Safari?   18h 06m 05.92s    +88º 15' 17.6"....

 

Bottom line - will my digital setting circles become obsolete for positions given in the J2050 epoch which is only a half dozen years away from being the default Epoch...

 

and poor old STF 2614 will be called  WDS 18059+8815  forever, even when Vega is the pole star again??

 

Dave



#4 catalogman

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Posted 03 December 2019 - 09:02 PM

My 2000.0 positions were from SIMBAD.

 

In VizieR they are reported as

 

18 05 54.240   +88 15 12.24    (CCDM)   *differs from SIMBAD*
18 05 54.1400 +88 15 11.900  (WDS)      *same as SIMBAD*

 

with regard to FK5 (equinox J2000.0 at epoch 2000.0, with proper motions taken into account).

 

Maybe the differences among catalogues are just a natural source of error when different observers determine

the centroid of the disk near the pole.

 

Only one outlier catalogue comes close to (but does not match) the Sky Safari value:

 

18 06 15.24452 +88 15 13.5096 (AGK3U)
18 06 15.240     +88 15 13.51     (AGK3U)  *ICRS frame*

 

but I don't think Sky Safari would be using this catalogue from 1992.

 

---

 

The point that Dixon and Sonneborn were making (in their famous DSO catalogue of 1980)

is that if you call the star STF 2614 and don't get the position from the name, you would

never have to worry about your digital setting circles becoming obsolete. 

 

--catalogman 



#5 c2m2t

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Posted 04 December 2019 - 01:25 PM

Hi Dave!

I had this very discussion with Brian Mason about 18 months ago. I had coincidently come across a system for which the precise co-ordinates were significantly off and to which Brian agreed that an update was warranted. The discussion led naturally to the WDS identifiers and the logistical problem of trying to update them. His position is that so much literature and scientific papers exist that it would be impossible/ impractical at this time, and I suspect any time in the future, to begin a process to update identifiers. The present RA and DEC information, will over time, become more and more a cataloguing tool and that the precise co-ordinates column will be the location that future users will go to, to confirm location.

 

Best regards,

Chris Thuemen

Pembroke, Ontario.



#6 WRAK

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Posted 04 December 2019 - 05:20 PM

The WDS ID's are generally J2000 and given only once when objects are included into the WDS catalog. Any corrections of position J2000 afterwards are reflected in the precise position field but the WDS ID remains as it is. Positions in new observations are also given at the J2000 reference frame (taking into account precession) but might differ from the J2000 position due to proper and/or orbital motion leading to changes of position angle and angular separation over time



#7 3c_273

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Posted 04 December 2019 - 06:50 PM

What is needed, of course, is positional identifiers that don't change over millennia.

 

Galactic coordinates, anyone?



#8 catalogman

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Posted 05 December 2019 - 11:04 AM

But identifiers based on galactic coordinates do change when the galactic center/poles are
redefined:  M57 = PK 63+13.1 in (li,bi) became G63.1+13.9 in (lii,bii). Although it's
probably safe to say that galactic coordinates won't be redefined every 25 years. smile.gif

 

The first problem with positional identifiers is that they don't always agree (see the
OP's example).

 

Another problem with all types of positional identifiers is that in larger catalogues, the
needed precision makes the name too long: for example, charts made with the HMQ

(Half-Million Quasars catalogue) are useless unless the labels are turned off.

 

Uranographers prefer to use the shortest acronym wherever possible. For instance, the
ESO/Uppsala (B) Atlas (1982) recommended to always use a positional identifier, but
no one ever did (ESO 149-013, never ESO 000013-5302.8).

 

Including the discovery year, per SIMBAD, means you won't run out of acronyms:

 

  [BBW55] = Bok, Bester, Wade (1955)
  [BBW86] = Brand, Blitz, Wouterloot (1986)

 

Including the publication year of each version (or a brief, standard code for it) also lets
you insert new objects:

 

  STF 2614 = [WDS2010]74723 = [WDS2015]91784 (from 2010-12-19 and 2015-08-30 versions)

 

For large running numbers, divide the catalogue into fields. (For example, ESO 149-013
above is object number 13 in Field 149.)

 

This was the best solution offered by Dixon and Sonneborn in the introduction to
A Master List of Nonstellar Optical Astronomical Objects in 1980. They concluded
that this system is more convenient than the current IAU practice of confirming the

accuracy of, and then updating, billions of positional names every 25 years.

 

--catalogman


Edited by catalogman, 05 December 2019 - 11:05 AM.


#9 Ed Wiley

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Posted 11 December 2019 - 12:18 AM

I spent 40 years writing about the theory and practice of biological classification with considerable discussion of the three International Rules of Biological Nomenclature and have always viewed with awe the way astronomers name their individual objects. Not that my opinion matters, but I am totally comfortable with the WDS identifiers remaining as they are and becoming, as Chris says, cataloging tools. We biologists went through this back in the 19th Century with cases like one biologist or another who wanted to change the species epithet (e.g. sapiens in the name Homo sapiens) because they discovered that the worm was green in life rather than brown in the museum bottle. The Rules put and end to that chaos. As to all the names applied to individual stars that appear in various surveys; it would be a massive undertaking to adopt anything like "one star one name" similar to the biological world, I think astronomers are on the right track and thank the sky gods for the CDS.

 

Ed


Edited by Ed Wiley, 11 December 2019 - 12:20 AM.



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