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Geoffrey Kolbe you wrote: Well Dave, if you had a roof-top prism to play with, what interesting property does that have....?

Ah yes Geoffrey.  Varieties of prisms are as bit above my head, but aspects of your website suggest you might have retained familiarity with them for use in spotting sights.  However, I’ve just realised that initial setting up to due south needn’t be the problem I imagined.  You just need your accurate longitude, an almanac, a radio clock with a seconds read out, and an assistant to count down the time.  Work out Mer Pass time to one second and set the equipment up on south as best you can. Then as Mer Pass approaches, keep nudging the equipment until at the moment of Mer Pass give one final nudge, if required, to leave only the white line on the screen or eyepiece.  Then tiptoe away very softly. DaveP