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    Re: Skyscout: Right Ascension and Declination
    From: Antoine Couëtte
    Date: 2010 Mar 18, 13:57 -0700

    Hello Brad,


    I am very sorry that you are so disappointed with SKYSCOUT for just your star distance determination use, while on the other hand SKYSCOUT seems to have SO MANY nice features otherwise !!!

    All the details ou give on your sextant and the relative effects of the various phenomena which come to play are very right. So you really have a very clear view of all these factors (proper motion, aberration, precession ...) as well as an excellent understanding of their relative magnitudes.

    Yes, the HIPPARCOS CATALOG reference date is 1991.25, i.e. 02 Apr 1991 13h30m00s00.

    *******

    Again, this is a real pity that they did not take care of Precession at least for SKYSCOUT since in the long run, not so long anyway, precession quickly becomes THE predominant effect if you need accurate coordinates. However, as Frank just clearly pointed out, precession has no effect on the distances between stars. In this respect, the predominant effects (probably) would be aberration in a short term, then proper motion for some quick moving starts (Arcturus, Rigil Kent, and a few other Navigation stars).

    -------

    Actually, getting ANY star apparent coordinates from scratch and to a precision of at least 1" is NOT - and by far - an immense computing task.

    I remember some 25 years ago having written a quick note (not even 2 hand written pasges) on this very same subject with a few examples. If I remember correctly for approximate corrections - and sure somebody will correct me here if necessary - , you need to first perform proper motion in the Equator, then move to ecliptic for simple Precession + Aberration + Nutation (1 st part) correction then back to Equator (Nutation 2 nd part) and here you are (no requirement for yearly parallax) ! With simple approximations on all the above, you can get ANY star apparent coordinate within 1" in accuracy for a period of at least 100 years. This does not not consume very much software resource or space, anyway very much less than the huge data bases they have included in SKYSCOUT. In this respect - and only in this one - the current SKYSCOUT version makes me think of a most superb Porsche sportscar with a 2 gallon fuel tank only !!! What a pity !

    -------

    In addition to the above, meanwhile Frank has given us extremely interesting information on a new top notch "ANDROID" product, as well as clever insight on the relevant variables which come to play in a appreciable way when we deal with just star distances computation !

    Should you by chance need additionnal info, including about a such simple way to compute the Star apparent coordinates to 1" accuracy, please let me know. I will further dig into my archives. A just completed 15 minute research was no joy to-nite, but I can spend a bit more time on it. And anyway I could do it again, although I would lazily prefer recovering my former note, which I probably just need to tranlate then into American since, as I can remember, I wrote it in French.


    Good on you, mate !

    Best Regards


    Antoine M. "Kermit" Couëtte

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