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    Re: StarFinder Epoch
    From: Frank Reed
    Date: 2021 Jul 27, 11:14 -0700

    Yes, most standard navigational star finders are drawn with star data from c.1930 --maybe a little earlier, maybe a little later. There also may be star finders that use updated coordinates, but I don't know of any. Of course, common planispheres do better. Precession shifts the positions of the stars by 50 seconds of arc annually or about 1° in 71 years. Assuming a century of precession, you can expect an error of 1.4°.

    Precession at the most basic level is a simple rotation of the celestial sphere about an axis perpendicular to the plane of the ecliptic. Picture a celestial sphere in your mind with a movable star globe floating inside a fixed transparent set of coordinates for SHA (or RA) and Dec. The ecliptic is drawn on the star globe. Now rotate the star globe about the axis perpendicular to the ecliptic. You can see that the change in SHA and Dec on the outer globe will depend on where a star is relative to the ecliptic. The largest changes are for stars that are close to the ecliptic, like Aldebaran, Pollux, Regulus, Spica, Antares, Fomalhaut, etc. Note that their ecliptic latitude does not change much. They glide along through the years on paths that parallel the ecliptic. Stars near the ecliptic poles shift less from precession. Kochab and Eltanin are near the North Ecliptic Pole in the middle of Draco, and Canopus, Miaplacidus, and Avior are near the South Ecliptic Pole in Dorado (only about 5° from the Large Magellanic Cloud).

    For sight-planning and star identification, the century out-of-date astronomical data is not a problem. If you're within a degree, you're fine. But unfortunately this error does mostly rule out other clever uses for the classic starfinder. For example, you can't use it to get azimuths for the standard intercept method. If the data were correct, it could be done by interpolation for latitude. Of course, the "classic" starfinder, known as the "HO 2102-D," is poorly designed in any case. There are many better tools for all of the imagined functions of the standard starfinder.

    Frank Reed

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