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    Re: Obscure Nautical Almanac star
    From: Robert H. van Gent
    Date: 2021 Jan 3, 13:01 +0000

    Frank Reed wrote:

    Speaking of beta Hydri... Stellarium users, have a look at the data for this star. It looks to me as if the proper motion data is incorrect. It's not really "close" but then again it's "in the same ballpark" as what I believe would be the correct numbers. Any thoughts? I encountered this by running the position of the star back to 1500 AD for some testing. It was shifted from where I expected to see it by a substantial fraction of a degree, which strikes me as quite an unusual error, if that's what it is, given that this is an exceptionally well-studied nearby star (24 lightyears distance) and an astrometric standard star.

    Did you factor in the cos δ correction?

    Some proper motion catalogues, such as FK5, give the proper motion in right ascension expressed in r.a. units so that the correction to another epoch can easily be added to the epoch value.

    Other catalogues, such as HIPPARCOS, give the actual angular displacement in arcsecs which differs by a factor 15 cos δ.

    For most stars the difference is small but near the celestial poles they can be quite different.

    Rob van Gent  


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