For some reason 57 stars sticks in my mind and they were the ones in AP3270.
I remember some 12 BOAC stars, which were easily recognisable in a periscope sextant, and, ideally, one of these would be shot first as a gross error check.
This wasn't always the best method as it was/is preferable to shoot the "least angle to track" star first and the "greatest angle to track star" last.
(Thinking about it, edited to say I may have got that the wrong way round - it's obvious when you're doing it.)
This would minimise the error in running the postion line along track at the assumed ground speed.
If my memory is correct, the first shot star's position line needed running by 8 minutes of groundspeed and the second by 4 minutes.
When the fix had been obtained it told you where you were, as about a minimum of 4minutes, previously.
So one could say with navigation "you never know where you are, only where you were".
Last edited by finncapt; 9th Jan 2015 at 16:36.