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Am I reading this right?


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I was looking for information on the internet on formulae to calculate precession values (with a view to working out some 2050 values (en masse - i have a python prog that does it for individual co-ordinates quite well), which - I thought - should start to be more accurate than the 2000 values in the next couple of years - don't ask, I just do this sort of thing!) and came across this paper: https://adsabs.harvard.edu/full/2008ASPC..394..179W

If I am reading this correctly, it says that co-ordinates are no longer precessing as long as we use the ICRS co-ordinates. In other words, Polaris - with ICRS co-ordinates of 02-31-49.09456 +89-15-50.7923 [according to SIMBAD] - will have exactly the same co-ordinates in 12000 years when it is nearly 45° away from the NCP instead of just 45'. Is that really what it is saying? Am I really understanding it correctly ['understanding' in this instance is used in the loosest possible sense ... and then some!]? It doesn't appear to have been published on April 1st! At the moment, this concept strikes me as incredibly weird and completely unworkable, but if that is what it is saying, I will try to get my brain around it.



Edited by Demonperformer
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I think the idea is to have a universal coordinate system (ICRS) that's independent of the nuances of the earth's motion and to harmonise the disparate (and sometimes less accurate) legacy coordinate systems.

The article also seems to stress that the computers will deal with with it all, and we (plebs) don't need to worry our little heads about it. I can accept that bit, but I can see how it could cause you headaches, as you are creating a computer program to deal with this very issue!

The paper also asserts that no algorithmic changes will be necessary to existing astronomical software. From this I assume that there is a relatively straightforward  linear transform between ICRS and the other systems.

I think.

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Thanks for your comments, Paul.

Having had a bit more time to think about this, I can see that in principle a co-ordinate system that never needs updating would be a good thing, but I see two issues with it:

(1) At the moment I can look at a star's coordinates and determine how far above the horizon it will ever get for me at this latitude, but gradually that is going to become no longer true. Ultimately, in 12 millenia, stars that have an RA of about 18h hours will have a limiting declination (at my location) of about 85°S, whilst stars with an RA of about 6h will have a limiting declination of about zero (the celestial equator). Granted, this is not going to affect me a lot, but in the long term it strikes me that that would be weird (or is it just that my imagination is limited by the current system??)

(2) New co-ordinates are still going to need to be created at intervals to account for proper motion. A quick straw-poll of the 2380 stars in BD+89 to BD+85 (excluding the four for which no PM values are provided) give an average (combined) PM of 28.867mas/yr, so in 100 years that is an average of just under 2.9 arc-seconds, which is just under 0.0008°. Although there is some variation, Simbad generally lists co-ordinates to 8 decimal places of a degree, or about 12500x this rate of change.

So I'm thinking that to meaningly accommodate PM, new co-ordinates are still going to have to be produced at regular intervals (but maybe not as frequently as under the FK5 system?) and if they are going to have to do that, they may as well have stuck with a system that relates to what we actually see.

That said, I will have "fallen off the perch" long before any of this makes any significant practical difference to me, so maybe I should just accept the fact that I don't need to find a way of generating 2050 co-ordinates, giving me one less thing to worry about.


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As I understand it, up until now we have been recomputing coordinates every 25 years to establish a new epoch. For many, this was accurate enough for the next 25 years.

However, many (most?) needed the current coordinates. All telescope goto systems and planetarium Apps for instance have stored the coordinates at say J2000, and recalculate everytime a number is needed. Look up any object in a database and you would find it stored for a previous epoch.

So what they have done is remove the 25 year update, instead we all go back to ICRS (effectively J2000), and calculate forward from there. No extra calculations, just a not changing the starting point


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