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Thanks Wolfgang.  I was thinking of an Arduino route if nothing cheap and commercial was availiable (thanks for the otherinteresting leads too folks).

There are plenty of Arduino watch projects I have seen and ofcourse we just want something very simple.

I wondered if you just got a simple clock module with a 7 segment display if there was a way to just freeeze the display with a minimal change? An Arduino seems way more than we need for a simple clock!

Another thought was I wondered if one was gong to be bothered to make such a clock ifit was worth arranging so it could be seen in the telescope of the sextant as a head up display? Then I thought if you are doing this why dont we have an encoder on the arc to measure the angle and display that. And then I realised this was getting silly as CN is meant to be a back up for when the elctronics go wrong and I went to bed.

I would be quite happy with an economical "stop clock"! Happy to try to help if anyone wants to start a project to make one.