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Subject: Re: Fwd: Re: Sextant accuracy with short distance to horizon
From: Michael Wescott (wescott_mike@XXX.XXX)
Date: Tue Jun 26 2001 - 16:49:58 EDT
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
Wescott_Mike@XXX.XXX
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