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: Steven Wepster
Date: 2001 Jun 22, 2:13 AM
From: Steven Wepster
Date: 2001 Jun 22, 2:13 AM
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