<p>There are several published star catalogs that list right ascensions for stars, along with a lot of other almanac-type data.  Wikipedia's a good resource for common stars (e.g. <a href="https://en.wikipedia.org/wiki/Polaris" rel="nofollow">Polaris</a>).</p>

<p>Right ascensions change over time, as stars move through space.  Star catalogs will have a year associated with them, which is the year that the measurement was taken.</p>

CC BY-SA 3.0
2016-06-30T08:52:27.540Z
Sorry I had to dig this out again: I tried proper motion RA 0.1988 arcsec/a with DEC(J2000)=89°15'50'' , then (0.1988)/15cos(89,2638°) is 0.04940.. seconds of RA per year. Now that we have ~16.5 years since 2000 I would do: 0.04940*16.5 which is 0.815 secs of RA since 2000. Polaris RA in 2000 was 02:31:48. Polaris totay's RA (stellarium) shows me 02:52:58. That are ~21 MINUTES difference. So where is my error comparing 0.815 SECONDS and 21 minutes?
Paul G.
CC BY-SA 3.0
2015-07-06T17:51:47.360Z
Thank you. I found some catalog data here: http://www.alcyone.de/SIT/mainstars/SIT000816.htm. position (J2000) is about RA: 2h 31min 48,711sec (FK4). But since J2000 15 years have passed. I just wonder at how to calculate polaris 2015 position.
Paul G.
CC BY-SA 3.0
2015-07-06T21:37:54.120Z
Look at the 'proper motion' row in your link.  Those are the number of arcseconds per year that right ascension changes by.  Multiply that number by 15 and add it to the J2000 value.  (Note that that's going to be a fraction of an arcsecond, which is probably ignorable unless you're trying for a very high degree of precision.)
chrispittman
CC BY-SA 3.0
2015-07-06T21:54:26.710Z
The proper motion appears to be in arcseconds per year @chrispittman. To convert to seconds of RA per year you have to *divide* by $15\cos(Dec)$.
ProfRob
CC BY-SA 3.0
2015-07-07T08:56:59.060Z
Dec=Declination?
Paul G.
CC BY-SA 3.0
2015-07-07T16:35:44.643Z
Yep, 'Dec' is declination.
chrispittman
CC BY-SA 3.0
2015-07-08T06:12:21.063Z
@barrycarter here are some calculation that takes precession into account: http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1959JRASC..53..249S&db_key=AST&page_ind=0&plate_select=NO&data_type=GIF&type=SCREEN_GIF&classic=YES
In the year 2100 polaris Dec. will be the largest at 90° - 0°29''31,67'
Paul G.
CC BY-SA 3.0
2015-07-06T15:25:48.037Z
2015-07-06T15:25:48.037Z
chrispittman
<p>Here I found description, formulas and an example of calculating equatorial coordinates change <a href="http://www.astronexus.com/a-a/motions-long-term" rel="nofollow">http://www.astronexus.com/a-a/motions-long-term</a>. You'll need proper motion (in Dec and RA), distance to star and star's radial velocity, all this data and Dec and RA for J2000 are available on the same page (<a href="http://www.astronexus.com/hyg" rel="nofollow">http://www.astronexus.com/hyg</a>), HYG database contains data from several star catalogues. Unfortunately I haven't tried this calculations yet so I don't know if you get same results as stated in Stellarium.</p>

CC BY-SA 3.0
2016-07-01T15:58:34.117Z
2016-07-01T15:58:34.117Z
Bartosz Kaszewski
<p>To calculate for example the polaris hour angle I use this formula to calculate polaris hour angle.</p>

<p><strong>τ = θ - α</strong></p>

<p>τ= hour angle</p>

<p>θ= sideral time</p>

<p>α= polaris RA (~ 2h32m)</p>

<p>But where does the α=2h32m come from? Is there a list? Does this change somehow over time?
Is my understanding correct that this is the time where polaris crosses the upper meridian at Greenwich?</p>

<star><positional-astronomy>
Where to get Polaris Right ascension value from
Paul G.
# Where to get Polaris Right ascension value from

(Asked by: username_0 on 2015-07-06)

To calculate for example the polaris hour angle I use this formula to calculate polaris hour angle.

**τ = θ - α**

τ= hour angle

θ= sideral time

α= polaris RA (~ 2h32m)

But where does the α=2h32m come from? Is there a list? Does this change somehow over time?
Is my understanding correct that this is the time where polaris crosses the upper meridian at Greenwich?

**Tags:** star, positional-astronomy


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## 2 answers

### Answer by username_1 on 2015-07-06. Score: 1

There are several published star catalogs that list right ascensions for stars, along with a lot of other almanac-type data. Wikipedia's a good resource for common stars (e.g. [Polaris](https://en.wikipedia.org/wiki/Polaris)).

Right ascensions change over time, as stars move through space. Star catalogs will have a year associated with them, which is the year that the measurement was taken.

> **Comments:**
> **username_0**: Thank you. I found some catalog data here: http://www.alcyone.de/SIT/mainstars/SIT000816.htm. position (J2000) is about RA: 2h 31min 48,711sec (FK4). But since J2000 15 years have passed. I just wonder at how to calculate polaris 2015 position.  
> **username_1**: Look at the 'proper motion' row in your link.  Those are the number of arcseconds per year that right ascension changes by.  Multiply that number by 15 and add it to the J2000 value.  (Note that that's going to be a fraction of an arcsecond, which is probably ignorable unless you're trying for a very high degree of precision.)  
> **username_2**: The proper motion appears to be in arcseconds per year @username_1. To convert to seconds of RA per year you have to *divide* by $15\cos(Dec)$.  
> **username_0**: Dec=Declination?  
> **username_1**: Yep, 'Dec' is declination.  
> **username_0**: @barrycarter here are some calculation that takes precession into account: http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1959JRASC..53..249S&db_key=AST&page_ind=0&plate_select=NO&data_type=GIF&type=SCREEN_GIF&classic=YES In the year 2100 polaris Dec. will be the largest at 90° - 0°29''31,67'  
> **username_0**: Sorry I had to dig this out again: I tried proper motion RA 0.1988 arcsec/a with DEC(J2000)=89°15'50'' , then (0.1988)/15cos(89,2638°) is 0.04940.. seconds of RA per year. Now that we have ~16.5 years since 2000 I would do: 0.04940*16.5 which is 0.815 secs of RA since 2000. Polaris RA in 2000 was 02:31:48. Polaris totay's RA (stellarium) shows me 02:52:58. That are ~21 MINUTES difference. So where is my error comparing 0.815 SECONDS and 21 minutes?  

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### Answer by username_3 on 2016-07-01. Score: 0

Here I found description, formulas and an example of calculating equatorial coordinates change <http://www.astronexus.com/a-a/motions-long-term>. You'll need proper motion (in Dec and RA), distance to star and star's radial velocity, all this data and Dec and RA for J2000 are available on the same page (<http://www.astronexus.com/hyg>), HYG database contains data from several star catalogues. Unfortunately I haven't tried this calculations yet so I don't know if you get same results as stated in Stellarium.


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