NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Are higher altitude sights inherently less accurate?
From: Alexandre Eremenko
Date: 2012 May 2, 23:29 -0400
From: Alexandre Eremenko
Date: 2012 May 2, 23:29 -0400
Dear John Karl, I have one remark. What you are saying is apparently applicable to observations with natural horizon, while we were discussing artificial one. This also applies to the table you attached. Actually I was wrong when I mentioned "tilt" in my explanation about why the errors increase with the angle when observing with artificial horizon. With artificial horizon there is no tilt and there is no "swinging the arc". You just touch two sun images. If you are able to do this at all, the angle that you measure is automatically in a vertical plane. Because the ray from the sun and the reflected ray are always in the same vertical plane. However I know from experience that measuring a large altitude with artificial horizon is indeed tricky. Why this is so, I am not sure. Taking a Lunar distance at 130 degrees is also hard. It is hard even to keep the two objects in the field of view when the angle is too large, because the ray hits the index glass under very small angle. Alex. On Wed, 2 May 2012, John Karl wrote: > > For once I disagree with Geoffrey. He says, "The error how vertical your sextant is becomes rapidly more important [at higher altitudes]. And in consequence, errors in high altitude sights will increase." > > The opposite is true. The error with sextant "tilt" is max at 45d altitude and decreases to zero at 90d altitude, for any given tilt angle, as the attached table shows. > > It is true that it's uncomfortable, even weird, swinging around in azimuth as you rock the sextant looking for vertical. Here's were the traditional half-silvered horizon mirror can help. Just keep the vertical division of the mirror perpendicular to the horizon (as you pivot around awkwardly on one foot) and your altitude error should be within reason, just as the attached table shows. > > Happy highs (altitudes), > > JK > ---------------------------------------------------------------- > NavList message boards and member settings: www.fer3.com/NavList > Members may optionally receive posts by email. > To cancel email delivery, send a message to NoMail[at]fer3.com > ---------------------------------------------------------------- > > > > Attached File: http://fer3.com/arc/img/119354.sextant-tilt-table.pdf > > > : http://fer3.com/arc/m2.aspx?i=119354 > > >