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> Convincing myself that the reflected Sun disk is indeed sitting on the
> horizon took a few seconds, which is why UT is rounded to the nearest 5 or
> 10 seconds

One thing that I started doing (which also improved my shooting accuracy)
was to position the sun so that it's just slightly above the horizon, and
then wait for the Earth's rotation to bring it down to exactly on the
horizon. Once it "kisses" the horizon (i.e. the gap between it and the
horizon disappears) I mentally call "Mark", move the sextant away from my
eye, and read the time on my Casio "atomic" watch (maybe subtracting a
second or two for the time it takes to do that - though if I remember right
you only need to be within 4 seconds of real time to get be accurate).

That's a lot easier (at least for me) than trying to bring the sun down to
exactly match a moving target (not to mention rocking the sextant back and
forth on the way down to make sure it's perpendicular) - with the Earth
doing the work for you there's never any guess as to when it's exactly on
the horizon, and it's easy enough to rock the sextant while waiting for that
to happen).

--
GregR



----- Original Message -----
From: P H
To: NavList@fer3.com
Sent: Monday, February 20, 2012 11:31 AM
Subject: [NavList] Re: Index correction sun sights


Same here with my Mark 15.  Last July I wrote:


"The main difficulty I had was establishing the index error of the
instrument. The index mirror held perpendicular to the frame very well but I
spent a lot of time tinkering with the horizon mirror. I repeatedly measured
and adjusted the index error by looking at the horizon, overlapping the two
sun disks, and also using the "Solar IC procedure" from David Burch's book
on plastic sextants. The IE tended to gravitate toward 8' to 9' off the arc
but I would have to attach an "error bar" of a few arcminutes to that value
from what I saw. Using this Solar IC method to recover the Sun's
semidiameter also showed differences of the order of 1'. Between sights I
turned my back to the Sun and shielded the sextant from direct sunlight."

The full posting is here:



http://fer3.com/arc/m2.aspx/Santa-Barbara-sights-PeterHakel-jul-2011-g16765

   

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