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From: Mike Hannibal (no email)
Date: Fri Oct 14 2005 - 18:22:17 EDT
For a rank beginner "rocking the sextant" is the least
well described technique in any of the literature I've
read. And, respectfully, the members of this list
haven't improved on the situation.
We have rocking happening about handles; about lines
through various mirrors; and in one of the pictures
profered around the sun itself as a fulcrum.
Please bear with my naive descriptions here: it seems
to me that we are seeking to achieve one simple
outcome and that is to ensure that the sextant is
perpendicular at the moment that we believe that we
have brought the body to the horizon.
It also seems to me that in doing that the "pivot
point" or axis for any rocking must be the line of
sight to the body. In other words we are rotating the
sextant, through a small arc, with the pivot point
being our line to the body. In so doing it is the
horizon which indeed sweeps across the field of view.
Put another way we could have the sextant at any
angle, indeed upside down and still have the body in
our line of sight. What changes is the chunk of the
horizon (or the presence of any horizon at all) that
appears directly "below" the body.
The question of perpendicularity therefore can be
posed another way "Which chunk of the horizon should
appear under this body when I'm holding this sextant
perpendicular?". We arrive at our answer to that by
gently rotating the instrument around an axis that is
from my eye to the body. What then happens is that the
body should only move off centre because we introduce
instrument movement in other planes because we are
humans not machines with defined freedoms of movement.
I'm sorry if this doesn't meet the technical needs of
the list. However I would be pleased to be corrected
if in practical terms I am wrong. I am concerned that
for the beginner this area is unnecessarily obfuscated
by less than satisfactory descriptions of what is
trying to be achieved.
I'm now heading to my boat!
Regards from the antipodes,
Mike
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