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David

You write (post number g31898) that you are ..." interested verifying experimentally the common knowledge surrounding sextant useage." I was too, and you can find some of my results here:  http://fer3.com/arc/m2.aspx/Index-error-Morris-jun-2008-g5300 . "Minutes" in the last sentence of the penultimate paragraph should of course read "seconds".

I later looked at backlash and micrometer error here: http://sextantbook.com/2010/03/.

Uncorrectable errors of most sextants made in the last sixty years should not exceed about 20 seconds, bearing in mind that error certificates always give reading for whole numbers of minutes, so that worm errors are neglected. Those of C Plath, the Soviet SNO-T and Tamaya should not exceed 12 seconds. British sextants, perhaps because they were made on worn-out machinery, sometimes perform much less well. For practical puposes of course, excepting lunars, most instrumental errors are swamped by other errors at sea.

I was taught (I think. I was not very attentive.) that in biological work at least, thirty results was a good number to have before attaching much significance to differences in variance, but, like you, I am not a statistician; and there are limits to everyone's patience, so sometimes I allow ten results to satisfy me.

To those owners of Tamaya "lookalikes" : I have picked apart several Tamayas and Lookalikes over the years and they all look the same to me, inside and out. I think Tamaya probably continued the widespread practice, for clocks, chronometer and sextant makers , of selling un-named instruments for other sellers to attach their names or sometimes even attaching the seller's name for them. I suggest that a zero on the calibration certificate of a micrometer sextant might correspond to Plath's "Free of error for practical use" and on a vernier instrument "less than the least reading of the vernier scale".

Bill Morris

Pukenui

New Zealand