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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Fwd: Re: Sextant accuracy with short distance to horizon
From: Michael Wescott
Date: 2001 Jun 26, 2:50 PM
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 Wescott_Mike@EMC.COM