NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Suitable Sextants - Mirrors
From: Mike Hannibal
Date: 2005 Oct 15, 08:22 +1000
From: Mike Hannibal
Date: 2005 Oct 15, 08:22 +1000
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 ____________________________________________________ Do you Yahoo!? Messenger 7.0: Free worldwide PC to PC calls http://au.messenger.yahoo.com