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Bill B.

Thanks so much for your thoughtful and stimulating analysis of the measurements I presented.

I was bothered by the difference in my measured semidiameter and the published SD and you are right it is due to my failure to properly account for the off arc readings. In theory I knew that, but I forgot to put it into practice.

Thanks also for the link to the David Burch document.  I will have to study it in detail and perhaps get ideas for further measurements.  I am interested verifying experimentally the common knowledge surrounding sextant useage.

You indicated your Astra had insensible backlash, perhaps I should verify same for mine.

I'm not a professional statistician either but I am aware that in computing a difference of two independent numbers each of which is uncertain (SD1 and SD2 ).  The computed number has an uncertainty SDc = Square Root( SD1^2 + SD2^2) = Square Root(2) x SD for SD1=SD2=SD.  This is the essential point I wanted to make concerning the overlap vs edge to edge IC measurement.  Variation is same for of each measurement but overlap method gives more precission in the result for same effort, number of measurements.