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Subject: Re: Sextant accuracy with short distance to horizon
From: Dan Hogan (dhhogan@XXX.XXX)
Date: Fri Jun 22 2001 - 11:57:06 EDT
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-
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