Fwd: Re: [Kstars-devel] problem with precession

[Jason Harris]

On Saturday 31 January 2004 06:54 am, Pablo de Vicente wrote:
> Do you think this is the correct explanation?
> 
Yes I do.  Sorry about my confusion, and thanks for looking into it.

Jason


>    Hello Jason,
>  
>    Your question puzzled me and it has taken me a couple 
> of hours, looking at KStars code, and rereading the Meeus 
> book to understand what the problem was. I think I have 
> understood it, but let us see if I got it right.  
>  
> I would say that there is no problem in KStars. I think that 
> it is normal that coordinates at J2000 do not coincide with 
> coordinates on Jan 1, 2000 at 12:00 UT. J2000 coordinates 
> are coordinates referred to a standard epoch and a _mean_ 
> equator, while apparent coordinates are referred to a given 
> epoch and the _true_ equator. The difference between the 
> true equator and the mean equator is the nutation, 
> aberration and all other effects related to the Earth as it 
> moves (except precession). 
> 
> Meeus explains in chapter 22 of his book, that the mean 
> place of a star is its apparent position as it would be seen 
> from the barycenter of the solar system, using as reference 
> system the ecliptic and the mean equator and mean equinox 
> of the date. On the other side the apparent place of a star is 
> its position as seen from the center of the moving Earth and 
> referred to the instantaneous equator, ecliptic and equinox. 
> That is why all effects related to the Earth moving (except 
> precession)  are not taken into account in the catalog 
> coordinates for a standard epoch.
> 

-- 
KStars: A KDE Planetarium
http://edu.kde.org/kstars