NavList:

Yes, holding a bubble sextant steady needs a firm hand.  Unfortunately, we’re not all like the Captain Marvel/Marlborough County figures appearing in WW2 sextant company adverts.  You need a bit of help.  British MkIX bubble sextants had a little curly pig’s tail hook on the top of the sextant and another in the top of the astrodome, but at home you can simply sit your sextant on a window ledge and level it up with playing card shims.  You end up on your knees, but so did Christopher Robin.  Now you know why Air Navigators tend to avoid the Moon unless they’re really trying to impress.  DaveP Message Ends.

After I learned from this very group that a large bubble is more "mercury rapid" my appreciation of the Mk IX bubble sextant has rocketed. I make the bubble as wide as the two vertical hairs. I then also not try to overly fixate the bubble in the center and the star in the middle of said bubble. I more or less constantly readjust the setting and by the miracle of averaging the altitude is within 1' or 2'.

There are two components of the averager, getting the bubble in the center and getting the star in the center. Even if I make excursions during the two minute averager time period, on average (ha!), these excursions cancel out very well.

Background: I first tried to be very precise, holding the sextant very still and not move the wheel setting at all much. Then any slight error in the more or less permanent setting would easily introduce a constant offset which of course was integrated by the mechanism. The results were all over the place and I could not get persistent results. By "varying wildly" that type of error is gone.