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    Re: Lunars with Young's formula: always off by several minutes
    From: Geoffrey Kolbe
    Date: 2018 Nov 2, 00:17 -0700

    William Porter wrote:

    "And there we have it. As confirmed by observation, your value of RA Altair is correct (19h51m41s) and mine is wrong (19h50m47s, epoch 2000). But I see mine quoted EVERYWHERE. (eg Wikipedia). I thought I had discarded the possibility of a web error, by going back to 1980 NASA docs and the like. It's not moving that fast so am I just being incredibly stupid and using the wrong epoch or something?"

    The SHA of Altair changes by about 0.73' per year, so if you use the 2000 epoch value you will be out by about 13', almost quarter of a degree, or almost a minute in time... just as you state above.

    As a shameless self-promotional plug, my book "Long Term Almanac 2000 - 2050" (published by Starpath Publications) gives the SHA and declinations of 39 'navigational stars' for each month of the year 2000. Also given are annual corrections for the SHA and declination so that current values can be derived for the rest of your lifetime!

    Geoffrey Kolbe

       
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