NavList:

Frank Reed you wrote: David Pike mentioned geographic "datums". Fortunately that's not an issue here. 

I agree with you that in the past finding the position of a point on land was obtained though celestial methods based upon a spherical Earth to give you your position on the celestial sphere.  Measurements were made with respect to the local vertical, so if that didn’t actually pass through the centre of the celestial sphere, i.e. there was a deflection of the true vertical at the measurement point, then the position measured would be in error.

However, the question that arises is: What are you going to do with the astronomical position you’ve found?  If the aim of navigation is to arrive at the correct place, at the correct time, safely, i.e. without bumping into anything on the way, then presumably you’d wish to mark the point’s position on your chart, where the topography already printed will have been marked on with respect to a particular datum.  Therefore, unless we know how the lat & longs quoted by Lecky pre-1884, have been processed, I’m still not entirely sure if we can ignore differing datums completely.

You also said:  Datums are now obsolete.

Don't tell theUK Ordnance Survey  They continue to use OSGB(1936) Datum, which is based upon the the Airy Ellipsoid (1830).  I think obsolescent is probably a better word than obsolete.  True the onwards march of digital mapping and satellite imagery is gradually taking over from traditional methods of cartography, but this has only happened over the last 60 years.  Until then, for land use, people such as explorers and the military, had to make do with whatever maps they could get hold of, often with very few technical notes on the side as to how they'd been produced.  Also, does not the World Geographic System 1984 (WGS84) contain an ellipsoid, also called WGS84, which along with other information allows WGS84 itself to be referred to as a datum, admittedly with coordinates that change with time?

One further question, does the local vertical as defined by gravity at any point on the surface of a regular homogeneous ellipsoidal star always pass through its centre?  Just wondering. DaveP