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Örjan Sandström wrote:
> Today I tried to cobble together a long term almanac for stars using 2008 as 
base, this as 2008-2011 was closest quadrennial I had covered by NA (For 
Aries table).

Örjan, accuracy is a little better if you use barycentric star
coordinates with respect to the mean equator. The almanac coordinates 
(geocentric apparent place with respect to the true equator) are not the 
best basis for long term corrections because they include the periodic 
effects of aberration and nutation.

For example, let's look at the effect of aberration on the geocentric 
apparent coordinates of Vega:

80°42.23' +38°47.36' 2008 Jan 0.0 TT
80°42.16' +38°47.21' 30
80°42.00' +38°47.11' 60
80°41.78' +38°47.08' 90
80°41.56' +38°47.12' 120
80°41.41' +38°47.23' 150
80°41.35' +38°47.38' 180
80°41.40' +38°47.52' 210
80°41.54' +38°47.63' 240
80°41.75' +38°47.68' 270
80°41.97' +38°47.65' 300
80°42.15' +38°47.54' 330
80°42.23' +38°47.39' 360
80°42.23' +38°47.36' 365.25

The January coordinates are almost identical to your table. But SHA is 
maximum in that month. By July it's .88' less, then it returns to 
maximum at the end of the year. On the other hand, declination is near 
its mean at Jan 0, minimum at day 90, and maximum at 270. In the almanac 
that's not so easy to see because precession and nutation also affect 
the coordinates. At .01' precision, even proper motion is noticeable in 
one year. To eliminate those factors in the above table, Vega, the 
equator, and the equinox were fixed at their true orientation at Jan 0.

Because the maximum aberration of any star is only about .3' (great 
circle), it's reasonable for a long term almanac to ignore aberration. 
That is, use barycentric or heliocentric coordinates instead of 
geocentric apparent coordinates.

It also helps to refer the coordinates to the mean equator, not the true 
equator. Tha latter causes a periodic variation of SHA and declination 
due to nutation. For example, here is the barycentric SHA of Vega with 
respect to both the mean equator and true equator. The difference is due 
to nutation.

     mean      true  nut.
80°41.85' 80°41.78' -.07 2008 Jan 0 0h TT
80°41.34' 80°41.22' -.12 2009
80°40.84' 80°40.68' -.16 2010
80°40.33' 80°40.15' -.18 2011
80°39.82' 80°39.64' -.18 2012
80°39.31' 80°39.15' -.16 2013
80°38.80' 80°38.68' -.12 2014
80°38.30' 80°38.23' -.07 2015
80°37.79' 80°37.78' -.01 2016
80°37.28' 80°37.32' +.04 2017
80°36.77' 80°36.87' +.10 2018
80°36.26' 80°36.40' +.14 2019
80°35.76' 80°35.92' +.16 2020
80°35.25' 80°35.41' +.16 2021
80°34.74' 80°34.89' +.15 2022
80°34.23' 80°34.35' +.12 2023
80°33.72' 80°33.79' +.07 2024
80°33.21' 80°33.23' +.02 2025

The table above includes the proper motion of Vega, which amounts to 
.08' SHA in the 17 years. Nutation affects SHA by about plus or minus 
.17' over this period. The dominant term has a 19-year cycle, but a 
30-day table interval would show high frequency nutation components too.

If you don't remove aberration and nutation from the star coordinates, 
the error is only a few tenths of a minute (great circle distance). And 
it's a constant offset - does not increase with time - so perhaps for 
you this is not significant. It is more important to have the correct 
rate of change. In the case of Vega, my table (left column) shows that 
your rate of -.51' / year is correct. I have not checked any other 
values in your table, however.

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