NavList:

Let me first quote from ’The geometrical seaman’  by E.G.R. Taylor and M.W. Richey….”owing to the precession of the equinoxes the star positions slowly change and nocturnals of different ages do not therefore correspond”

So – I am looking at a couple of nocturnals and one might have a year stamped on it or be otherwise dateable with reasonable accuracy. The other  I note, has the start date for Guards being East of the Pole Star at a slightly different date  than the first – maybe two or three days earlier or later.

Can anyone advise how I would date the second by its difference from the first? In other words, how can I convert the known value for the precession of the equinoxes (50.2” per year, I believe – or a little less precise to be on a par with how it was done 350 years ago)  into a date difference between two instruments from 17^th century – or maybe 17^th /18^th century.

Maybe I am looking at this from the wrong angle – but any indication as to how these instruments are dated by the layout (without them having maker or date marks) would be much appreciated.

Kinch.