If you look at this link they have days realting to right acension.

How did they calculate nov 20th == 0h, dec 6 == 1h?

I was trying to calculate this in astropy, but I end up with nov 20th, 8pm == 22h.

from astropy import units as u
from astropy.time import Time
from astropy.coordinates import SkyCoord, EarthLocation, AltAz

time_str =  "2017-11-20 20:00"

#Greenville SC, eclipse just went through there.
LON =  -82.3940
LAT = 34.8526
loc = EarthLocation(lat=LAT*u.deg, lon=LON*u.deg, height=0*u.m)

utcoffset = -4*u.hour  # Eastern Daylight Time
obstime = Time(time_str,scale='utc') - utcoffset

cord = SkyCoord(alt=90*u.degree,az=0*u.degree,obstime=obstime,frame = 'altaz',location = loc)
print obstime,cord.fk5.ra.hour, cord.fk5.dec.deg

>> 2017-11-21 00:00:00.000 22.505946538 34.7569396915
  • $\begingroup$ What are you actually trying to do? $\endgroup$
    – Dr Chuck
    Aug 25, 2017 at 23:07
  • $\begingroup$ Want to make my own start chart, and understand how I should add a border with months around the chart. $\endgroup$
    – Sam P
    Aug 25, 2017 at 23:56

1 Answer 1


The date on the charts shows the meridian (north south line) at 8pm local time on that date. Over a period of a month, the RA of the meridian will move 2hr (so that it gets back to where it started from after 12 months), and hence over 16 days, as you ask about, it will move 1.05 hours. So you can either say that's close enough to 1 hr, or worry about the small discrepancy - that's assuming the 1hr you cite is actually 1.00hrs.


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