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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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