# why are my Sha and Declination calculations not the same as the answers from the Nautical Almanac?

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

achernar_star=Star.from_dataframe(df.loc[7588])
earth = planets['earth']
t = ts.now()
astrometric = earth.at(t).observe(achernar_star)
f = float(ra._degrees)
f1= float(dec.degrees)
print('achernar')
print('Sha',360-round(f,2))
print('Dec', round(f1,2))
print('')

achernar
Sha 335.57
Dec -57.24


From Nautical Almanac

achernar
Sha 335.36
Dec -57.13

• Did you use another Epoche? My guess : Almanac will use current while hipparcos is meanwhile a bit dated Dec 3, 2021 at 7:17
• Thanks. I didn't quite understand the 'epoch', isn't that determined by the timescale import? It put me on rereading the doc's and the difference between 'astrometric and 'apparent' radec. Although I don't quite understand the difference, changing to 'apparent' radec seems to have worked. If you agree i'll close out this question. Thanks n Rgds Sybe
– sybe
Dec 3, 2021 at 13:14
• The “epoch” is the moment for which the positions are valid. Most times, positions are referred to the “J2000.0” epoch, which is January 0, 2000 (or December 31, 1999, if you prefer, but in order to keep it “year 2000,” astronomers have created this fictitious date of January 0). Stars will have some proper motion between J2000.0 and now, and there will also be the effects of precession, aberration, and nutation, for which you need to correct. Only then will you be able to compare apples with apples. Dec 3, 2021 at 23:05