NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2014 Feb 27, 12:25 -0500
Hi Bill
That was illuminating.
I saw 4 sextants with negligible backlash; 2 with a tenth of an arc minute backlash and one with an astonishing 1' 15" (12.5 tenths of an arc minute).
Then I saw your results of worm accuracy (the cyclic error as the micrometer is turned). This error is combinatorial with the backlash. Sometimes additive, sometimes subtractive. Consider the total to approach 5 tenths of an arc minute. For the average navigator, completely unknown. Could this variable and unpredictable (for the average navigator) error be considered 'operator error' or navigator bias? Maybe.
Have you performed a similar measurement on a vernier sextant? Is that even necessary? The backlash will be zero as will any cyclic worm error for a vernier sextant. But I will wait patiently for your ruling!
Brad
Brad
You wrote <<And yes, we will be able to measure gear lash, and see just how the preload reduces or eliminates it. That can happen right now. Approach some angle from one side. Align the AC and zero it. Approach that same angle (by micrometer only) from the other side. Measure deviation with the AC. Alas, I do not own a micrometer sextant. Both of my instruments are vernier type. Have you done this experiment Bill?>>
Yes indeed I have, Brad. See "Chasing tenths of an arcminute " at www.sextantbook.com and http://sextantbook.com/2010/03/
I envy you your new acquisitions.
Bill Morris
Pukenui
New Zealand
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