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    Re: Fwd: Re: Sextant accuracy with short distance to horizon
    From: Michael Wescott
    Date: 2001 Jun 26, 2:50 PM

    Dale Tilson wrote:
    > Re: the Aussie using his fence for a horizon, several people wrote:
    > > >> That's interesting. But surely one must make an allowance for the fact
    > > >> that the fence is not at the same altitude as the horizon.
    > > >
    > > >I can't remember that much about the post, but I THINK he had some kind
    > > of HE
    > > >correction for his eye above the fence line.
    > > >
    > >He would need extremely accurate survey data if there was
    > >any meaning to be had. He would have to know the relative
    > >height of the fence and his eye, as well as the distance to
    > >the fence.
    > OK, guys, I'm new at this so somebody help me out.
    > I thought the "dip" correction was necessary because the horizon isn't
    > level with your height of eye.  Aren't we actually trying to measure
    > altitude above a line horizontal with the line-of-sight?  If so, our Aussie
    > friend would only need to find a perch so that his eye was level with the
    > fenceline.
    He'd need the fence to be level and to take his sights with the eye at the
    same height as the fence; in which case "dip" is 0. If either condition fails
    then the dip will be different depending on where on the fence the sight is
    brought down. That said, distance helps a lot. If the fence is 5 ft away, an
    inch off in the height of eye makes an error of about 58 minutes. At 20 ft
    it's 14' and at a half a mile it's 0.1'
    > If not, somebody straighten me out quick, because I was gonna set up an
    > observation post in the back yard based on that premise.
    You'll probably get better results with an artificial horizon. Say a pan
    of water or (my favorite) glycerin.
            Mike Wescott

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