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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?

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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).

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.

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  • $\begingroup$ Thank you. I found some catalog data here: 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. $\endgroup$ – Paul G. Jul 6 '15 at 17:51
  • $\begingroup$ 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.) $\endgroup$ – chrispittman Jul 6 '15 at 21:37
  • $\begingroup$ The proper motion appears to be in arcseconds per year @chrispittman. To convert to seconds of RA per year you have to divide by $15\cos(Dec)$. $\endgroup$ – Rob Jeffries Jul 6 '15 at 21:54
  • $\begingroup$ Dec=Declination? $\endgroup$ – Paul G. Jul 7 '15 at 8:56
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    $\begingroup$ Wouldn't precession be a much bigger factor, especially for a star as close to the North Pole as Polaris? $\endgroup$ – user21 Jul 7 '15 at 23:18
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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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