How the epoch coordinates work, some help please.

[Stevo69]

My next question relates to how the epochs work. I want to confirm that I understand some aspects of precession and coordinates.

Where RA and DEC is quoted as epoch 2000.0 for a particular object, those coordinates are 7 years old but we all still use them until the next revision, which is, I think is every 25 years (The last epoch was 1975).

I notice on the NGC/IC Project site there are figures quoted for Annual RA Precession and Annual Declination Precession for each object. For example, NGC6352 in Lepus quotes:

Annual RA Precession: +4.54 seconds
Annual Declination Precession: +3.7"

Does that mean that for every whole year from 2000.0, you must add +4.54 seconds to the RA, and +3.7" to the DEC, if you want to be very accurate? Is this what these figures mean?

I've understand this is not really necessary to update the RA and DEC every year, but I'm just trying to make sure I'm on the right track with understanding it.

Actually there is the odd object in Burnham's books which I cannot find current (epoch 2000.0) RA and DEC for, so if I understand the above process I could adjust the 1950.0 RA and DEC accordingly by adding the appropriate annual precession figures to the 1950 figures.

Hoo roo.

[astroron (Ron)]

That is my understanding, I only thought they went every 50 years?
FOR 99% of deep sky objects it doesn't matter, but thing like stars that have proper motion it does.

[gary]

Hi Stevo,

Strictly speaking, one should always quote an RA/Dec position along with an Epoch.
For example 17h 25m 29.1s -48º 25' 22" J2000.0 once precession and nutation
is applied is equivalent to 17:26:03.295 -48:25:44.32 J2007.52 where J2007.52
is the Epoch of Date as of today 9-July-2007.

This is a shift of 341.1 arcseconds, or 5.68 arcminutes.

Indeed, if you take these co-ordinates, which are for NGC6352, it shifts
approx 4.5 seconds (not arcseconds) a year in RA and 3 arcseconds a year in Dec.

However, as long as you quote the Epoch, as one always should, then one value
is no more 'accurate' than the other.

The real question is, by what means are you locating the object in the first place?
For example, are you using a telescope with an electronic pointing capability?

The figures that you see in Bunham will be for the B1950.0 Epoch.

The B stands for Besselian whereas the J stands for Julian. The difference is
subtle and probably beyond the scope of this short response, but in a nutshell
the Besselian system was based on the length of the tropical year in 1900
and with respect when the sun was at 280 degrees longitude. Julian epochs, on the
other hand, are based on a year of 365.25 days.

Converting from B1950 to J2000 is not exactly straight forward to do by hand
on the tablecloth. I would recommend using one of the online calculators such
as this one - http://fuse.pha.jhu.edu/cgi-bin/precess_tool

Hope the above is helpful.

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place
Mount Kuring-Gai NSW 2080
Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au

[gary]

Originally Posted by astroron

That is my understanding, I only thought they went every 50 years?
FOR 99% of deep sky objects it doesn't matter, but thing like stars that have proper motion it does.

Hi Ron,

No quite ...

The effects of precession and nutation of course are to do with the wobble
of the Earth's axis. Therefore, all positions in the sky shift gradually and continually,
including nearby objects and those that are very far away, such as DSO's. In fact,
when only these effects are taken into account, the angular separation of these various objects
with respect to each other remains constant.

However, stars, a you know, also have 'proper motions' - they drift around within
galaxies and thus depending upon their velocities and whether they are moving
side-to-side with respect to us or toward or away from us and depending upon
how far from us they are will determine their relative proper motions. Different
stars will have different proper motions, hence their relative angular
separatons changes over time. As you also know, due to the expansion of the
Universe, galaxies are also moving over time but since the distances are so far
the angular change is extremely tiny.

However, with the Earth wobbling about and all these objects moving every
which away, it probably makes remembering where they all are pretty moot
if you could somehow manage to live for long enough.

Best regards

Gary

[astroron (Ron)]

Thanks Gary for putting it more succinctly