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Brad,
Perhaps Frank can explain better but my understanding is the following:
The true Lunar distance is the distance between the center of the Moon and the star.

In our case the star approached the Moon limb almost tangentially (it was just 
touching the limb if observed from Canadian boarder).
As you can see by making a picture when the star moves on a line tangent to the Moon,
than at positions close to tangency the distance from the center almost does not change.
In other words, a very small change in the distance gives big change in time.

This is why accuracy of time is lost in this situation.

Same happens with sextant observation of such configurations. About 1 hour 
before the occultation event I tried to
measure the distance with my sextant, in the usual mode as taking a lunar.
Of course I could only take the far limb. I found this extremely tricky. Despite all my efforts, the
distance was 0.6' off in comparison with Frank's calculator.

The conclusion is that neither sextant measurement nor the calculator itself 
work well near an occultation.

Alex.  

   

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