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Old 20-01-2025, 08:16   #1
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Sextant problem

Hi
i have been given a Cooke sextant. I am not sure it works correctly, (or maybe it's me!). the problem is this: with the arc and micrometer both set to zero, when i look at the sun (with shades), i can adjust the side error and index error screws (both located on the horizon mirror) so that both suns appear on top of each other. however, when i simply move the sextant to look at the distant horizon, there is clearly a lot of index error present as the right half of the horizon is far higher than the left half. similarly, if i then adjust the two screws on the horizon mirror whilst looking at the horizon, and get the horizon object perfectly aligned (eg: to a distant light house), with no side/index error, but then look at the sun, the two suns (through index and horizon mirrors) are way out. i wonder what type of error this is?
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Old 20-01-2025, 09:44   #2
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Re: Sextant problem

Welcome to CruisersForum!

I would suggest updating your profile with your general location and your boat make & model or “Looking” or Looking to Crew" in the "Boat" category. This info shows up under your UserName in every post in the web view. Many questions are boat and/or location dependent and having these tidbits under your UserName saves answering those questions repeatedly. If you need help setting up your profile then click on this link: https://www.cruisersforum.com/forums...ml#post3308797

I would happily help more if the link above is not enough.




How distant a horizon? Water horizon 3+nm away or house peak, road, whatever 1/4nm away?

Consider joining NavList and asking your question there. Link to NavList in my signature below.

If you are near San Diego I would be happy to meet you here and look at what you are doing in person.
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Old 20-01-2025, 10:16   #3
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Re: Sextant problem

thankyou. the horizon i use is about 6 miles away and is a line of low hills (no visible water horizon). the index error is marked - maybe the equivalent of 6 sun diameters. i` live in Scotland - would love to visit San Diego sometime !
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Old 20-01-2025, 10:24   #4
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Re: Sextant problem

Small world, we vacationed in Scotland this year. Actually wife had a conference and me and the kids showed up at the end for another week.

Edinburgh or Glasgow?

With a target that far away my next guess would be something isn’t tight.
Index mirror mount.
Arm pivot
Micrometer attachment
Half mirror mount.
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A house is but a boat so poorly built and so firmly run aground no one would think to try and refloat it.
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Old 20-01-2025, 12:50   #5
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Re: Sextant problem

worked in Edinburgh for years. Now living bit further north in Perthshire. Thanks again for the feedback.
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Old 20-01-2025, 14:23   #6
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Re: Sextant problem

We stayed near Beattock in a place called Telford Manor. Kinda far from both G&E but dirt cheap.

Also, as a Civil engineer, it was cool to stay at a place named Telford.
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Old 20-01-2025, 14:55   #7
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Re: Sextant problem

Quote:
Originally Posted by Arniemack View Post
worked in Edinburgh for years. Now living bit further north in Perthshire. Thanks again for the feedback.

You're not Princess Anne, by any chance, are you? A Rustler? In Scotland?


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Cushion me soft . . . . rock me in billowy drowse,
Dash me with amorous wet . . . . I can repay you."
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Old 20-01-2025, 17:16   #8
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Re: Sextant problem

Have you checked first for perpendicularity?

Further reading:

- https://casualnavigation.com/sextant...d-corrections/
- https://maritimesa.org/nautical-scie...ir-correction/
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