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George:

 

I agree that adjusting the sextant before measuring a second altitude can never be exact, and I'm beginning to rue that I tried to make the method easier to understand by using that example. The more I write, the more I become convinced that you are correct. For example, the absolute slope of the ascending line of altitudes is different from that of the descending one, due to Sn - d. So in my example, the time from an hs of 32°35' to maximum altitude is 27m40s, but the time from there down to the same altitude is 27m30s. An observer has no way of knowing this in real time. So, only after plotting the two runs of sights can a proper determination of  DWT be made. And I think that's what you're saying.

 

I'm still learning.

 

Jim Wilson