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Mike and Michael,

I made (at least one error).  Dip is not taken into consideration with a
bubble sextant observation, so should not be used to adjust Hc to Hs.

Also of note, since you are already observing the center of the body, no
limb corrections are made for the sun.  You can pull refraction correction
from the star/planets column (after IC correction) or add the upper and
lower limb corrections and divide by 2.  Parallax corrections should be used
for the moon.  Take the average of the upper and lower limb corrections and
subtract 15' from the altitude.

I wonder about the idea that IE taken with a natural horizon is so darned
desirable.  Given that a natural horizon is seldom razor sharp, and given
my ability to align razor sharp power line has a standard deviation of 0.1'
or greater, how good is my ability to get a dead-nuts IE from a horizon? I
have seen days on Lake Michigan with the wind out of the north for a couple
of days and 8+ ft seas, that when looking west the horizon appeared like saw
teeth to the naked eye.

Another amusing conundrum concerns the mechanical device itself.  The arc
and worm gear around the 0d mark gets a heavy workout compared to the rest
of the worm gear and arc. And what if there is eccentricity in the worm
gear?  An IE 4' off the arc may relate to the rest of the arc differently
than and IE 4' on the arc.

So we make observations for error along arc to determine that relationship,
but seldom with the worm gear/drum at the same setting used for the IE
check.

Unless I have missed something, all we can say for certain is that IE is
valid only at whatever value near 0d we found that has the mirrors parallel.

Bill


> Hi Bill,
>
> wading in here without full knowledge...however: Won't the intercept be a
> combination of all of the errors? The IE, Observational Error, Timing
> Error....etc?
>
> As an example using a standard marine sextant with known IE and known position
> of the user, once corrected for IE my intercepts represent my capacity to
> execute an accurate sight. Is it different with a bubble sextant?
>
> Regrds
>
> Mike

   

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