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Subject: Re: Sextant accuracy with short distance to horizon
From: Steven Wepster (wepster@XXX.XXX)
Date: Fri Jun 22 2001 - 04:13:08 EDT
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.
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?
In my opinion, the anser to that question is 'no'. I assume that waves
of appoximately the same height make up the horizon.
_Steven
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