NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: sextant index error measurement
From: Frank Reed CT
Date: 2006 Nov 5, 18:52 EST
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
From: Frank Reed CT
Date: 2006 Nov 5, 18:52 EST
George H wrote:
"When you check index error with a horizon, it's true that the
direction of the horizon is displaced, due to refraction, but that
displacement is almost exactly the same for the direct view through
the horizon mirror, as it is for the reflected view through the index
mirror, from a point only a couple of inches higher up. So that any
refraction has a quite negligible effect on such error checking. For
that purpose dip tables are irrelevant."
direction of the horizon is displaced, due to refraction, but that
displacement is almost exactly the same for the direct view through
the horizon mirror, as it is for the reflected view through the index
mirror, from a point only a couple of inches higher up. So that any
refraction has a quite negligible effect on such error checking. For
that purpose dip tables are irrelevant."
Here's another fun thing to consider on this topic...
The index mirror has a higher "height of eye" than the horizon glass. So
that means that dip is a tiny bit larger for the reflected view of the horizon
than the direct view. Supposing the index mirror is three inches above the
direct sight line, how close to the surface would you have to be (how low a
height of eye) so that the difference in dip would be larger than 0.1 minutes of
arc? Looking at it another way, how close to the water would I have to be before
the visible horizon itself is too close for a proper index correction?
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---