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Ken, Mike and Michael.

To my chagrin, it occurred to me after several (lengthy) posts that I have
no idea if a dedicated bubble sextant has the ability to observe the body,
bubble, and horizon simultaneously (as Celestaire's German build-it-yourself
cardboard unit does).

Ken's suggestion makes sense on many levels.  As the bubble is illuminated
(mine is not) Polaris will remain relatively static for all practical
purposes for a much longer period than the sun at LAN so is a better target.
Additionally, of what practical importance is it if you have an IC and
bubble error correction if you cannot view all three?  (My cardboard unit
was used for both on-the-water and bubble observations, so both were of
importance to me). What is relative is the combined affect on readings if
used in bubble mode only.

As to Mike's calibration by observation concerns, average. I have no idea
how to calibrate a sextant other than by observation.  Mike's point on
reducing variables remains valid, as does Einstein's suggestion to keep it
as simple as possible, but no simpler.  In this case, observation seems to
be about as simple as it can be.

I would still caution, subject to Ken's review, that if using a natural
horizon and the eye is not at water level, dip would have to be adjusted for
in calibration.

Bill




> On 12/10/05 4:55 PM, "Bill"  wrote:
>
>>> So, how do I determine the index error of my bubble sextant without a water
>>> horizon?
>>
>> Michael
>>
>> If your location is known, make a series of observations.  After adjusting
>> for refraction, the difference between your observation and calculated Hc
>> will be the IE.
>>
>> Bill
>>
>
> Bill and Michael
>
> A method I have used is to prop the bubble sextant up on a table outside.
> Center the bubble in the field of view more or less (it is not critical).
> Then bring Polaris down until it just winks off and on, and take the
> reading.  Then proceed to the bottom of the bubble.  Average the two
> readings, apply refraction and compare to the Hc for your position.  This
> gives you  the error of the bubble at that particular altitude.  Time
> passage while doing this is not critical for Polaris.
>
> Unlike marine sextants, bubble sextants can have different errors of up to 3
> or 4 minutes at different altitudes, due to misadjustment of the internal
> sector gear.  This makes taking sights at varying altitudes advisable to see
> if there are any changes from the accurate IC using Polaris.  But for
> goodness sake, plot an Hc curve instead of working out lots of sights.  The
> raw data you plot will not only reveal IC changes, but backlash, and even
> the personal error you may have in estimating the center of the bubble.
>
> Ken Gebhart

   

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