NavList:

First we need to distinguish between harbour marks and harbour markers.  Harbour marks are locally distant objects known and used by compass adjusters when compass swinging.  Harbour markers seem to have more than one meaning.  They might be prominent markers or transits either on land or afloat to indicate to the crew of an approaching vessel where the harbour is and how to get into it, or they might be a plaque or disc in the ground showing bearings and distances to far off places.  They might even have a lat and long written upon them, but this might be a more recent addition.  So are we looking for something of vertical extent or something with writing on or possibly a combination of both, and does it really matter?  I.e. do we need to be sitting right on top of it when we do the sextant observation?

Consider the case of an observation when the Sun is due east or west of us.  The Sun will be travelling westwards at 15 degrees per hour, or a quarter minute of arc per second of time.  Therefore, if we knew our latitude and longitude exactly, and we could achieve an overall observational accuracy of one minute of arc, we could correct our chronometer to four seconds of time.  This unknown four seconds error plus any consequential error in rating would stick with us until our next check. 

How close would we have to be to the mark when we did our observation?  Well at 60N or S, if we were 1nm E or W away, that would be two minutes of longitude, so we would add another eight seconds of timing error.  Knowing this, we don’t need to be sitting with a plumb bob right on top of an actual mark, but we must be either fairly close or make an adjustment from a chart of the harbour.  Therefore, we could probably have managed with a book of lat and longs of defined points as existed in the 18^th and 19^th centuries.  The problem being of course, the more off the beaten track places were, and therefore less frequently visited, the less accurately the lat and long was known, which worried Bligh during the launch journey, protected the mutineers on Pitcairn, and worried Worsley on Elephant Is even in the 20^th Century.  DaveP