<p>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.</p>

CC BY-SA 3.0
2017-08-26T09:12:13.587Z
2017-08-26T09:12:13.587Z
Dr Chuck
<p>If you look at this <a href="https://dept.astro.lsa.umich.edu/ugactivities/Labs/BrightStars/SFAStarChartsAll.pdf" rel="nofollow noreferrer">link</a> they have days realting to right acension.</p>

<p>How did they calculate nov 20th == 0h, dec 6 == 1h?</p>

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

<pre><code>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

&gt;&gt; 2017-11-21 00:00:00.000 22.505946538 34.7569396915
</code></pre>

CC BY-SA 3.0
2017-08-25T23:07:26.840Z
What are you actually trying to do?
Dr Chuck
CC BY-SA 3.0
2017-08-25T23:56:02.837Z
Want to make my own start chart, and understand how I should add a border with months around the chart.
Sam P
<amateur-observing><python><right-ascension>
How are the days relating to right ascension around a polar star chart decided?
Sam P
# How are the days relating to right ascension around a polar star chart decided?

(Asked by: username_0 on 2017-08-25)

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

**Tags:** amateur-observing, python, right-ascension

> **Comments:**
> **username_1**: What are you actually trying to do?  
> **username_0**: Want to make my own start chart, and understand how I should add a border with months around the chart.  

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## 1 answers

### Answer by username_1 on 2017-08-26. Score: 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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