NavList:

Thanks for your further comments. I had had what I will call an anti-eureka!  moment. Rather than the concept suddenly becoming clear it suddenly became very confusing. My problem was this:

I measure altitude and record time. Then I extract GHA and dec from the NA and calculate position. I reasoned that as none of the input data depended on a datum then the position would not depend on a datum.

But.......... as Frank has pointed out altitude depends on the local vertical so the position *is* dependant on a datum.

The end of March is approaching. If I were to take my AH and sextant on the bus I could repeat Blackburn's observations on the footpath outside 56 Hawker St. The position I  obtained should be identical to Blackburne's (unless the composition of the earth beneath Wellington has changed in the past 115 years). Don't worry, I have no plans to take these sights (-;  However if I had a bubble sextant (which I do not) I would be tempted to check the 1902 position to an arc minute or two.