NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sextant accuracy with short distance to horizon
From: Dan Hogan
Date: 2001 Jun 22, 9:57 AM
From: Dan Hogan
Date: 2001 Jun 22, 9:57 AM
On 22 Jun 2001, at 10:13, Steven Wepster wrote: My reply is based on experience. I am not a mathematician. > Dear Dan and others, > > In my opinion you have hit a problem that is specific to astro on > yachts: the very small height of eye. The problem is not so much that > it makes the horizon nearby. The normal dip table should be sufficient > as long as it lists your height of eye. I don't know for shure what > Peter and Russell meant by 'dip short' tables: my 1981 Bowditch > Vol.II has a different table 14, but it has a table 22 'Dip of the Sea > Short of the Horizon'. This table gives the dip of objects _in front > of_ the horizon, so it should not be used for a normal altitude above > the horizon. Dip short tables ar for use when taking a sight SR/object distance and the horizon line you are using is short of the visible horizon. > More important: because your eyes are so low above the sea, it is most > of the time just plainly impossible to _see_ the horizon. The waves > get in between. When it is almost dead calm, there's no problem (but > it's not fun sailing then). You're lucky when the swell is running > high and regular, because then you can take a shot when you're at the > top. > > This raises another question: what is your height of eye at the top of > a wave? Do you have to add the wave height to your height above the > waterline of the vessel? Where on the swell you take the sight does not matter for HE calculation. What does matter is that you REALLY did take a horizon sight and not the top of a wave/swell. You can take a sight on land from any *level* surface of sufficient distance. I remember an "Aussie" list member that used the fence on his ranch in front of his porch. > In my opinion, the anser to that question is 'no'. I assume that waves > of appoximately the same height make up the horizon. > > _Steven Cheers -Dan-