Position of bright stars from 2025 astronomical almanac

[xdw316]

I don't seem to be able to reproduce the RA and Dec values for Bright Stars Section H of the Astronomical Almanac.
Anyone done that?
Thanks!

[brandon-rhodes]

Could you give an example of a star you are trying to locate, and the coordinates that you would expect to see as output?

[xdw316]

Thank you for the very prompt reply.

At page H2 of the 2025 AA, alpha And shows RA 00h 09m 42.7s and Dec +29d 13s 52s. I have been using NOVAS FORTRAN for some years, but the list of its errors listed in the superNOVAS fork introduction has me worried. I am teaching myself python and have written a version of the USNO Celestial Navigation Data for Assumed... using your wonderful Skyfield (for which I thank you).

Regards,
David Walden

[brandon-rhodes]

The first step I would suggest would be looking up the star in either the Hipparcos catalog. I usually step through the instructions here to find a star that I'm interested in, by finding its HIP number:

https://rhodesmill.org/skyfield/stars.html

[xdw316]

I've got the star using your example as a go by. I think my problem is in setting the epoch as the AA describes.

.......
ippList=[13847,7588,60718,33579,21421,
62956,67301,109268,26311, 46390,
76267,677,97649,2081,80763,
69673,82273,41037,25336,27989,
30438,24608,102098,57632,3419,
54061,25428,87833,107315,113368,
61084,59803,68702,9884,90185,
72607,113963,14135,68933,45238,
15863,92855,100751,37826,37279,
86032,49669,24436,71683,84012,
3179,85927,32349,65474,44816,
91262,72622 ]
#bodyradii=[695700.,1737.4,6051.8,3396.19,71492.,60268.]
print(' ')
print(' ',file=NavDat)
#for ssbody in [sun,moon,venus,mars,saturn,jupiter]:
for i in [0,1,2,3,4,5,6,7,8,9,
10,11,12,13,14,15,16,17,18,19,
20,21,22,23,24,25,26,27,28,29,
30,31,32,33,34,35,36,37,38,39,
40,41,42,43,44,45,46,47,48,49,
50,51,52,53,54,55,56]:

xvenus = bodylist[i]

star32 = Star.from_dataframe(df.loc[hippList[i]])

astrometric = earth.at(t).observe(star32)
apparent=astrometric.apparent()
ra, dec, distance = astrometric.radec(t)
xx=float(ra._degrees)
yy=float(dec._degrees)

#AA Bright star
dwt=ts.J(2025.5)

print (star32)

aa=earth.at(dwt).observe(star32)
ra, dec, dist=aa.radec(dwt)
print(starname[i],'RA; ',ra,'   Dec; ',dec)

......

[brandon-rhodes]

(Could you double-check the formatting of your code? Try looking up "github markdown" for hints on getting it to format properly. Thanks!)

[xdw316]

AstAlmBrightStar.txt
renamed .py to .txt and attached.
hope this works.
github is all new to me.
(almost the same for python.)
thanks again

[xdw316]

pics of pages from 2025 AA

[brandon-rhodes]

Well! This is a bit of a surprise for me. It turns out that Skyfield (unless I'm missing something!) doesn't provide a simple way to ask for coordinates relative to the mean equator and mean equinox of Earth. Skyfield is so focused on observational astronomy, where you always want both precession and nutation accounted for, that it doesn't provide a built-in way to turn precession "on" and nutation "off" so that mean coordinates, instead of true coordinates, are returned.

Try this.

from skyfield.api import Star, load
from skyfield.data import hipparcos

class mean_equator_and_equinox_of_date(object):
    """Coordinate frame of Earth’s mean equator and mean equinox-of-date."""
    @staticmethod
    def rotation_at(t):
        return t.P

ts = load.timescale()
t = ts.utc(2025, 7, 2.375)

planets = load('de421.bsp')
earth = planets['earth']

with load.open(hipparcos.URL) as f:
    df = hipparcos.load_dataframe(f)

𝛼_and = Star.from_dataframe(df.loc[677])

astrometric = earth.at(t).observe(𝛼_and)
dec, ra, _ = astrometric.frame_latlon(mean_equator_and_equinox_of_date)
ra.preference = 'hours'

print(ra.hstr())
print(dec.dstr())

See whether the output matches the entries in your almanac.

If this code works, I can add this mean_equator_and_equinox_of_date coordinate reference frame to the next version of Skyfield.

[xdw316]

Seems to be working just fine for a selection of examples. Although I don't quite follow how you did that.

Thanks much!

[xdw316]

Ahh.. P is the precession matrix.