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    Re: Skyscout: Right Ascension and Declination
    From: Frank Reed
    Date: 2010 Mar 19, 08:59 -0700

    Brad, you wrote:
    "So my complaint that the scientific data (RA and Dec) are about 20 years old now is a small price to pay for such convenience. It bothered me that it didn’t match the Nautical Almanac, for those 57 stars. It bothered me that it didn’t match the Astronomical Almanac, for the Bright Stars list. I do believe that Celestron has blundered. It would have been trivial to permit it to download the latest Bright Stars list from the USNO, as the Bright Stars list is available for free download. It is indeed a pity."

    Brad, I checked some of the star data that you posted (thanks for humoring me) and it appears that most of the Skyscout positions are for epoch 2000 with a second of arc or so though one appeared to be 2 arcseconds off. Epoch 2000 coordinates would be a normal and very reasonable choice. I don't think it's a blunder at all. For many astronomical applications, coordinates for a given epoch are preferred. When I was a kid in the 70s, everything was epoch 1950. There was supposed to be a switch to epoch 1975 but it never caught on. Most recent star catalogs reference epoch 2000, but a post from Dave W indicated that this would be the last such epoch and future catalogs would be referenced to a quasar grid (makes sense). For other applications like celestial navigation, the epoch of date is preferred, and in the cel nav case, required. The coordinate conversion for precession is a simple rotation about the axis perpendicular to the ecliptic. Such rotations do not change distances between stars, of course. There's no obvious reason to prefer epoch 2000 coordinates over epoch of date for a device like the Skyscout unless it's expected to function for decades.

    -FER


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