# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Finding IC for A-12 bubble sextants
From: David Pike
Date: 2021 Oct 6, 08:05 -0700

So to summarise, work in a way to solve the problems one at a time.

1.  Get the bubble set up steady in the middle of the bubble chamber.  With an ergonomically shaped sextant like the A12, it’s difficult to hang like a MkIX, and you can’t sit it on a beanbag or wedge it level with paying card shims on a windowsill.  Ideally, you need some sort of bespoke stand.  Funnily enough I saw a large heavy camera stand in a charity shop for pennies recently, but I was in a rush, and the shop was closing, so I left it.  When I went back the next time I was passing, it’d gone.

2.  Pick a target that’s easily calculable and is hardly moving like the Sun at meridian passage or the natural horizon.  The Sun still moves in azimuth, but the natural horizon is sometimes difficult to define on a hazy day.

3.  Take numerous sights approaching from either direction and average them, but don’t have them too far apart at meridian passage (why not?).

I sometimes wonder how much error is instrument error and how much is me failing to judge when the Sun is in the centre of the bubble.  In the 1941 Edition of AP1234 this was called “Personal Error” although I know that using such a term might raise hackles in some quarters.  Has anyone tried this; assuming we can get our bubble to stop in the correct place, how about taking two observations, one of the Sun’s upper limb just touching the top of the bubble, and one of the Sun’s lower limb just touching the bottom of our bubble and taking the average?  A pencil smudge on the median drum would save the need to move the sextant to read the Vernier after the first observation.  It strikes me that for an A12, which usually ends up with a rather large bubble despite one’s efforts to make it smaller, there might be some mileage in this.  DaveP

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